Lipid Metabolism  

Lipid Metabolism


Fatty Acid Synthesis

One might predict that the pathway for the synthesis of fatty acids would be the reversal of the oxidation pathway. However, this would not allow distinct regulation of the two pathways to occur even given the fact that the pathways are separated within different cellular compartments. The pathway for fatty acid synthesis occurs in the cytoplasm, whereas, oxidation occurs in the mitochondria. The other major difference is the use of nucleotide co-factors. Oxidation of fats involves the reduction of FADH+ and NAD+. Synthesis of fats involves the oxidation of NADPH. However, the essential chemistry of the two processes are reversals of each other. Both oxidation and synthesis of fats utilize an activated two carbon intermediate, acetyl-CoA. However, the acetyl-CoA in fat synthesis exists temporarily bound to the enzyme complex as malonyl-CoA. The synthesis of malonyl-CoA is the first committed step of fatty acid synthesis and the enzyme that catalyzes this reaction, acetyl-CoA carboxylase (ACC), is the major site of regulation of fatty acid synthesis. Like other enzymes that transfer CO2 to substrates, ACC requires a biotin co-factor.

The rate of fatty acid synthesis is controlled by the equilibrium between monomeric ACC and polymeric ACC. The activity of ACC requires polymerization. This conformational change is enhanced by citrate and inhibited by long-chain fatty acids. ACC is also controlled through hormone mediated phosphorylation (see below).
The acetyl groups that are the products of fatty acid oxidation are linked to CoASH. As you should recall, CoA contains a phosphopantetheine group coupled to AMP. The carrier of acetyl groups (and elongating acyl groups) during fatty acid synthesis is also a phosphopantetheine prosthetic group, however, it is attached a serine hydroxyl in the synthetic enzyme complex. The carrier portion of the synthetic complex is called acyl carrier protein, ACP. This is somewhat of a misnomer in eukaryotic fatty acid synthesis since the ACP portion of the synthetic complex is simply one of many domains of a single polypeptide. The acetyl-CoA and malonyl-CoA are transferred to ACP by the action of acetyl-CoA transacylase and malonyl-CoA transacylase, respectively. The attachment of these carbon atoms to ACP allows them to enter the fatty acid synthesis cycle.
The synthesis of fatty acids from acetyl-CoA and malonyl-CoA is carried out by fatty acid synthase, FAS. The active enzyme is a dimer of identical subunits.
All of the reactions of fatty acid synthesis are carried out by the multiple enzymatic activities of FAS. Like fat oxidation, fat synthesis involves 4 enzymatic activities. These are, b-keto-ACP synthase, b-keto-ACP reductase, 3-OH acyl-ACP dehydratase and enoyl-CoA reductase. The two reduction reactions require NADPH oxidation to NADP+.
The primary fatty acid synthesized by FAS is palmitate. Palmitate is then released from the enzyme and can then undergo separate elongation and/or unsaturation to yield other fatty acid molecules.
Origin of Cytoplasmic Acetyl-CoA

Acetyl-CoA is generated in the mitochondria primarily from two sources, the pyruvate dehydrogenase (PDH) reaction and fatty acid oxidation. In order for these acetyl units to be utilized for fatty acid synthesis they must be present in the cytoplasm. The shift from fatty acid oxidation and glycolytic oxidation occurs when the need for energy diminishes. This results in reduced oxidation of acetyl-CoA in the TCA cycle and the oxidative phosphorylation pathway. Under these conditions the mitochondrial acetyl units can be stored as fat for future energy demands.
Acetyl-CoA enters the cytoplasm in the form of citrate via the tricarboxylate transport system (see Figure). In the cytoplasm, citrate is converted to oxaloacetate and acetyl-CoA by the ATP driven ATP-citrate lyase reaction. This reaction is essentially the reverse of that catalyzed by the TCA enzyme citrate synthase except it requires the energy of ATP hydrolysis to drive it forward. The resultant oxaloacetate is converted to malate by malate dehydrogenase (MDH).

Pathway for the movement of acetyl-CoA units from within the mitochondrion to the cytoplasm for use in lipid and cholesterol biosynthesis. Note that the cytoplasmic malic enzyme catalyzed reaction generates NADPH which can be used for reductive biosynthetic reactions such as those of fatty acid and cholesterol synthesis.

The malate produced by this pathway can undergo oxidative decarboxylation by malic enzyme. The co-enzyme for this reaction is NADP+ generating NADPH. The advantage of this series of reactions for converting mitochondrial acetyl-CoA into cytoplasmic acetyl-CoA is that the NADPH produced by the malic enzyme reaction can be a major source of reducing co-factor for the fatty acid synthase activities.
Regulation of Fatty Acid Metabolism

One must consider the global organismal energy requirements in order to effectively understand how the synthesis and degradation of fats (and also carbohydrates) needs to be exquisitely regulated. The blood is the carrier of triacylglycerols in the form of VLDLs and chylomicrons, fatty acids bound to albumin, amino acids, lactate, ketone bodies and glucose. The pancreas is the primary organ involved in sensing the organisms dietary and energetic states via glucose concentrations in the blood. In response to low blood glucose, glucagon is secreted, whereas, in response to elevated blood glucose insulin is secreted.
The regulation of fat metabolism occurs via two distinct mechanisms. One is short term regulation which is regulation effected by events such as substrate availability, allosteric effectors and/or enzyme modification. ACC is the rate limiting (committed) step in fatty acid synthesis. This enzyme is activated by citrate and inhibited by palmitoyl-CoA and other long chain fatty acyl-CoAs. ACC activity also is affected by phosphorylation. The primary phosphorylation of ACC occurs through the action of AMP-activated protein kinase, AMPK (this is not the same as cAMP-dependent protein kinase, PKA). Glucagon stimulated increases in PKA activity result in phosphorylation and inhibition of ACC. Additionally, glucagon activation of PKA leads to phosphorylation and activation of phosphoprotein phosphatase inhibitor-1, PPI-1 which results in a reduced ability to dephosphorylate ACC maintaining the enzyme in a less active state. On the other hand insulin leads to activation of phosphatases, thereby leading to dephosphorylation of ACC that results in increased ACC activity. These forms of regulation are all defined as short term regulation.
Control of a given pathways' regulatory enzymes can also occur by alteration of enzyme synthesis and turn-over rates. These changes are long term regulatory effects. Insulin stimulates ACC and FAS synthesis, whereas, starvation leads to decreased synthesis of these enzymes. Adipose tissue lipoprotein lipase levels also are increased by insulin and decreased by starvation. However, in contrast to the effects of insulin and starvation on adipose tissue, their effects on heart lipoprotein lipase are just the inverse. This allows the heart to absorb any available fatty acids in the blood in order to oxidize them for energy production. Starvation also leads to increases in the levels of fatty acid oxidation enzymes in the heart as well as a decrease in FAS and related enzymes of synthesis.
Adipose tissue contains hormone-sensitive lipase, that is activated by PKA-dependent phosphorylation leading to increased fatty acid release to the blood. The activity of hormone-sensitive lipase is also affected positively through the action of AMPK. Both of these effects lead to increased fatty acid oxidation in other tissues such as muscle and liver. In the liver the net result (due to increased acetyl-CoA levels) is the production of ketone bodies. This would occur under conditions where insufficient carbohydrate stores and gluconeogenic precursors were available in liver for increased glucose production. The increased fatty acid availability in response to glucagon or epinephrine is assured of being completely oxidized since both PKA and AMPK also phosphorylate (and as a result inhibits) ACC, thus inhibiting fatty acid synthesis.
Insulin, on the other hand, has the opposite effect to glucagon and epi leading to increased glycogen and triacylglyceride synthesis. One of the many effects of insulin is to lower cAMP levels which leads to increased dephosphorylation through the enhanced activity of protein phosphatases such as PP-1. With respect to fatty acid metabolism this yields dephosphorylated and inactive hormone sensitive lipase. Insulin also stimulates certain phosphorylation events. This occurs through activation of several cAMP-independent kinases. Insulin stimulated phosphorylation of ACC activates this enzyme.
Regulation of fat metabolism also occurs through malonyl-CoA induced inhibition of carnitine acyltransferase I. This functions to prevent the newly synthesized fatty acids from entering the mitochondria and being oxidized.
Elongation and Desaturation

The fatty acid product released from FAS is palmitate (via the action of palmitoyl thioesterase) which is a 16:0 fatty acid, i.e. 16 carbons and no sites of unsaturation. Elongation and unsaturation of fatty acids occurs in both the mitochondria and endoplasmic reticulum (microsomal membranes). The predominant site of these processes is in the ER membranes. Elongation involves condensation of acyl-CoA groups with malonyl-CoA. The resultant product is two carbons longer (CO2 is released from malonyl-CoA as in the FAS reaction) which undergoes reduction, dehydration and reduction yielding a saturated fatty acid. The reduction reactions of elongation require NADPH as co-factor just as for the similar reactions catalyzed by FAS. Mitochondrial elongation involves acetyl-CoA units and is a reversal of oxidation except that the final reduction utilizes NADPH instead of FADH2 as co-factor.
Desaturation occurs in the ER membranes as well and in mammalian cells involves 4 broad specificity fatty acyl-CoA desaturases (non-heme iron containing enzymes). These enzymes introduce unsaturation at C4, C5, C6 or C9. The electrons transferred from the oxidized fatty acids during desaturation are transferred from the desaturases to cytochrome b5 and then NADH-cytochrome b5 reductase. These electrons are un-coupled from mitochondrial oxidative-phosphorylation and, therefore, do not yield ATP.
Since these enzymes cannot introduce sites of unsaturation beyond C9 they cannot synthesize either linoleate (18:2D9, 12) or linolenate (18:3D9, 12, 15). These fatty acids must be acquired from the diet and are, therefore, referred to as essential fatty acids. Linoleic is especially important in that it required for the synthesis of arachidonic acid. As we shall encounter later, arachindonate is a precursor for the eicosanoids (the prostaglandins and thromboxanes). It is this role of fatty acids in eicosanoid synthesis that leads to poor growth, wound healing and dermatitis in persons on fat free diets. Also, linoleic acid is a constituent of epidermal cell sphingolipids that function as the skins water permeability barrier.
Synthesis of Triglycerides

Fatty acids are stored for future use as triacylglycerols in all cells, but primarily in adipocytes of adipose tissue. Triacylglycerols constitute molecules of glycerol to which three fatty acids have been esterified. The fatty acids present in triacylglycerols are predominantly saturated. The major building block for the synthesis of triacylglycerols, in tissues other than adipose tissue, is glycerol. Adipocytes lack glycerol kinase, therefore, dihydroxyacetone phosphate (DHAP), produced during glycolysis, is the precursor for triacylglycerol synthesis in adipose tissue. This means that adipoctes must have glucose to oxidize in order to store fatty acids in the form of triacylglycerols. DHAP can also serve as a backbone precursor for triacylglycerol synthesis in tissues other than adipose, but does so to a much lesser extent than glycerol.

The glycerol backbone of triacylglycerols is activated by phosphorylation at the C-3 position by glycerol kinase. The utilization of DHAP for the backbone is carried out through the action of glycerol-3-phosphate dehydrogenase, a reaction that requires NADH (the same reaction as that used in the glycerol-phosphate shuttle). The fatty acids incorporated into triacylglycerols are activated to acyl-CoAs through the action of acyl-CoA synthetases. Two molecules of acyl-CoA are esterified to glycerol-3-phosphate to yield 1,2-diacylglycerol phosphate (commonly identified as phosphatidic acid). The phosphate is then removed, by phosphatidic acid phosphatase, to yield 1,2-diacylglycerol, the substrate for addition of the third fatty acid. Intestinal monoacylglycerols, derived from the hydrolysis of dietary fats, can also serve as substrates for the synthesis of 1,2-diacylglycerols.
Phospholipid Structures

Phospholipids are synthesized by esterification of an alcohol to the phosphate of phosphatidic acid (1,2-diacylglycerol 3-phosphate). Most phospholipids have a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2 of the glycerol backbone. The most commonly added alcohols (serine, ethanolamine and choline) also contain nitrogen that may be positively charged, whereas, glycerol and inositol do not. The major classifications of phospholipids are:

Phosphatidylcholine (PC)
Phosphatidylethanolamine (PE)
Phosphatidylserine (PS)
Phosphatidylinositol (PI)
Phosphatidylglycerol (PG)
Diphosphatidylglycerol (DPG)
Phospholipid Synthesis

Phospholipids can be synthesized by two mechanisms. One utilizes a CDP-activated polar head group for attachment to the phosphate of phosphatidic acid. The other utilizes CDP-activated 1,2-diacylglycerol and an inactivated polar head group.

PC:This class of phospholipids is also called the lecithins. At physiological pH, phosphatidylcholines are neutral zwitterions. They contain primarily palmitic or stearic acid at carbon 1 and primarily oleic, linoleic or linolenic acid at carbon 2. The lecithin dipalmitoyllecithin is a component of lung or pulmonary surfactant. It contains palmitate at both carbon 1 and 2 of glycerol and is the major (80%) phospholipid found in the extracellular lipid layer lining the pulmonary alveoli.
Choline is activated first by phosphorylation and then by coupling to CDP prior to attachment to phosphatidic acid. PC is also synthesized by the addition of choline to CDP-activated 1,2-diacylglycerol. A third pathway to PC synthesis, involves the conversion of either PS or PE to PC. The conversion of PS to PC first requires decarboxylation of PS to yield PE; this then undergoes a series of three methylation reactions utilizing S-adenosylmethionine (SAM) as methyl group donor.

PE:These molecules are neutral zwitterions at physiological pH. They contain primarily palmitic or stearic acid on carbon 1 and a long chain unsaturated fatty acid (e.g. 18:2, 20:4 and 22:6) on carbon 2.
Synthesis of PE can occur by two pathways. The first requires that ethanolamine be activated by phosphorylation and then by coupling to CDP. The ethanolamine is then transferred from CDP-ethanolamine to phosphatidic acid to yield PE. The second involves the decarboxylation of PS.

PS:Phosphatidylserines will carry a net charge of -1 at physiological pH and are composed of fatty acids similar to the phosphatidylethanolamines.
The pathway for PS synthesis involves an exchange reaction of serine for ethanolamine in PE. This exchange occurs when PE is in the lipid bilayer of the a membrane. As indicated above, PS can serve as a source of PE through a decarboxylation reaction.

PI:These molecules contain almost exclusively stearic acid at carbon 1 and arachidonic acid at carbon 2. Phosphatidylinositols composed exclusively of non-phosphorylated inositol exhibit a net charge of -1 at physiological pH. These molecules exist in membranes with various levels of phosphate esterified to the hydroxyls of the inositol. Molecules with phosphorylated inositol are termed polyphosphoinositides. The polyphosphoinositides are important intracellular transducers of signals emanating from the plasma membrane.
The synthesis of PI involves CDP-activated 1,2-diacylglycerol condensation with myo-inositol. PI subsequently undergoes a series of phosphorylations of the hydroxyls of inositol leading to the production of polyphosphoinositides. One polyphosphoinositide (phosphatidylinositol 4,5-bisphosphate, PIP2) is a critically important membrane phospholipid involved in the transmission of signals for cell growth and differentiation from outside the cell to inside.

PG:Phosphatidylglycerols exhibit a net charge of -1 at physiological pH. These molecules are found in high concentration in mitochondrial membranes and as components of pulmonary surfactant. Phosphatidylglycerol also is a precursor for the synthesis of cardiolipin.
PG is synthesized from CDP-diacylglycerol and glycerol-3-phosphate. The vital role of PG is to serve as the precursor for the synthesis of diphosphatidylglycerols (DPGs).

DPG:These molecules are very acidic, exhibiting a net charge of -2 at physiological pH. They are found primarily in the inner mitochondrial membrane and also as components of pulmonary surfactant.
One important class of diphosphatidylglycerols is the cardiolipins. These molecules are synthesized by the condensation of CDP-diacylglycerol with PG.
The fatty acid distribution at the C-1 and C-2 positions of glycerol within phospholipids is continually in flux, owing to phospholipid degradation and the continuous phospholipid remodeling that occurs while these molecules are in membranes. Phospholipid degradation results from the action of phospholipases. There are various phospholipases that exhibit substrate specificities for different positions in phospholipids.
In many cases the acyl group which was initially transferred to glycerol, by the action of the acyl transferases, is not the same acyl group present in the phospholipid when it resides within a membrane. The remodeling of acyl groups in phospholipids is the result of the action of phospholipase A1 and phospholipase A2.

Sites of action of the phospholipases A1, A2, C and D.

The products of these phospholipases are called lysophospholipids and can be substrates for acyl transferases utilizing different acyl-CoA groups. Lysophospholipids can also accept acyl groups from other phospholipids in an exchange reaction catalyzed by lysolecithin:lecithin acyltransferase (LLAT).
Phospholipase A2 is also an important enzyme, whose activity is responsible for the release of arachidonic acid from the C-2 position of membrane phospholipids. The released arachidonate is then a substrate for the synthesis of the prostaglandins and leukotrienes.
Plasmalogens

Plasmalogens are glycerol ether phospholipids. They are of two types, alkyl ether (-O-CH2-) and alkenyl ether (-O-CH=CH-). Dihydroxyacetone phosphate serves as the glycerol precursor for the synthesis of glycerol ether phospholipids. Three major classes of plasmalogens have been identified: choline, ethanolamine and serine plasmalogens. Ethanolamine plasmalogen is prevalent in myelin. Choline plasmalogen is abundant in cardiac tissue.
One particular choline alkyl ether plasmalogen (1-O-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine) has been identified as an extremely powerful biological mediator, capable of inducing cellular responses at concentrations as low as 10-11 M. This molecule is called platelet activating factor, PAF. PAF functions as a mediator of hypersensitivity, acute inflammatory reactions and anaphylactic shock. PAF is synthesized in response to the formation of antigen-IgE complexes on the surfaces of basophils, neutrophils, eosinophils, macrophages and monocytes. The synthesis and release of PAF from cells leads to platelet aggregation and the release of serotonin from platelets. PAF also produces responses in liver, heart, smooth muscle, and uterine and lung tissues.

Platelet activating factor

Metabolism of the Sphingolipids

The sphingolipids, like the phospholipids, are composed of a polar head group and two nonpolar tails. The core of sphingolipids is the long-chain amino alcohol, sphingosine. Amino acylation, with a long chain fatty acid, at carbon 2 of sphingosine yields a ceramide.

Top: Sphingosine
Bottom: Ceramide

The sphingolipids include the sphingomyelins and glycosphingolipids (the cerebrosides, sulfatides, globosides and gangliosides). Sphingomyelins are the only sphingolipid that are phospholipids. Sphingolipids are a component of all membranes but are particularly abundant in the myelin sheath.

Sphingomyelins are sphingolipids that are also phospholipids. Sphingomyelins are important structural lipid components of nerve cell membranes. The predominant sphingomyelins contain palmitic or stearic acid N-acylated at carbon 2 of sphingosine.
The sphingomyelins are synthesized by the transfer of phosphorylcholine from phosphatidylcholine to a ceramide in a reaction catalyzed by sphingomyelin synthase.

A sphingomyelin

Defects in the enzyme acid sphingomyelinase result in the lysosomal storage disease known as Niemann-Pick disease. There are at least 4 related disorders identified as Niemann-Pick disease Type A and B (both of which result from defects in acid sphingomyelinase), Type C1 and a related C2 and Type D. Types C1, C2 and D do not result from defects in acid sphingomyelinase. More information on Niemann-Pick sub-type C1 is presented below in the section on Clinical Significances of Sphinoglipids.

Glycosphingolipids, or glycolipids, are composed of a ceramide backbone with a wide variety of carbohydrate groups (mono- or oligosaccharides) attached to carbon 1 of sphingosine. The four principal classes of glycosphingolipids are the cerebrosides, sulfatides, globosides and gangliosides.

Cerebrosides have a single sugar group linked to ceramide. The most common of these is galactose (galactocerebrosides), with a minor level of glucose (glucocerebrosides). Galactocerebrosides are found predominantly in neuronal cell membranes. By contrast glucocerebrosides are not normally found in membranes, especially neuronal membranes; instead, they represent intermediates in the synthesis or degradation of more complex glycosphingolipids.
Galactocerebrosides are synthesized from ceramide and UDP-galactose. Excess accumulation of glucocerebrosides is observed in Gaucher's disease.

A Galactocerebroside

Sulfatides: The sulfuric acid esters of galactocerebrosides are the sulfatides. Sulfatides are synthesized from galactocerebrosides and activated sulfate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Excess accumulation of sulfatides is observed in sulfatide lipidosis (metachromatic leukodystrophy).

Globosides: Globosides represent cerebrosides that contain additional carbohydrates, predominantly galactose, glucose or GalNAc. Lactosyl ceramide is a globoside found in erythrocyte plasma membranes. Globotriaosylceramide (also called ceramide trihexoside) contains glucose and two moles of galactose and accumulates, primarily in the kidneys, of patients suffering from Fabry's disease.

Gangliosides: Gangliosides are very similar to globosides except that they also contain NANA in varying amounts. The specific names for gangliosides are a key to their structure. The letter G refers to ganglioside, and the subscripts M, D, T and Q indicate that the molecule contains mono-, di-, tri and quatra(tetra)-sialic acid. The numerical subscripts 1, 2 and 3 refer to the carbohydrate sequence that is attached to ceramide; 1 stands for GalGalNAcGalGlc-ceramide, 2 for GalNAcGalGlc-ceramide and 3 for GalGlc-ceramide.

Deficiencies in lysosomal enzymes, which normally are responsible for the degradation of the carbohydrate portions of various gangliosides, underlie the symptoms observed in rare autosomally inherited diseases termed lipid storage diseases, many of which are listed below.
Clinical Significances of Sphingolipids

One of the most clinically important classes of sphingolipids are those that confer antigenic determinants on the surfaces of cells, particularly the erythrocytes. The ABO blood group antigens are the carbohydrate moieties of glycolipids on the surface of cells as well as the carbohydrate portion of serum glycoproteins. When present on the surface of cells the ABO carbohydrates are linked to sphingolipid and are therefore of the glycosphingolipid class. When the ABO carbohydrates are associated with protein in the form of glycoproteins they are found in the serum and are referred to as the secreted forms. Some individuals produce the glycoprotein forms of the ABO antigens while others do not. This property distinguishes secretors from non-secretors, a property that has forensic importance such as in cases of rape.

Structure of the ABO blood group carbohydrates

R represents the linkage to protein in the secreted forms
sphingolipid in the cell-surface bound form.
open square = GlcNAc, open diamond = galactose, filled square = fucose, filled diamond = GalNAc, filled diamond = sialic acid (NANA)

A significant cause of death in premature infants and, on occasion, in full term infants is respiratory distress syndrome (RDS) or hyaline membrane disease. This condition is caused by an insufficient amount of pulmonary surfactant. Under normal conditions the surfactant is synthesized by type II endothelial cells and is secreted into the alveolar spaces to prevent atelectasis following expiration during breathing. Surfactant is comprised primarily of dipalmitoyllecithin; additional lipid components include phosphatidylglycerol and phosphatidylinositol along with proteins of 18 and 36 kDa (termed surfactant proteins). During the third trimester the fetal lung synthesizes primarily sphingomyelin, and type II endothelial cells convert the majority of their stored glycogen to fatty acids and then to dipalmitoyllecithin. Fetal lung maturity can be determined by measuring the ratio of lecithin to sphingomyelin (L/S ratio) in the amniotic fluid. An L/S ratio less than 2.0 indicates a potential risk of RDS. The risk is nearly 75-80% when the L/S ratio is 1.5.
The carbohydrate portion of the ganglioside, GM1, present on the surface of intestinal epithelial cells, is the site of attachment of cholera toxin, the protein secreted by Vibrio cholerae.
These are just a few examples of how sphingolipids and glycosphingolipids are involved in various recognition functions at the surface of cells. As with the complex glycoproteins, an understanding of all of the functions of the glycolipids is far from complete.

Disorders Associated with Abnormal Sphingolipid Metabolism

Disorder Enzyme Deficiency Accumulating Substance Symptoms
Tay-Sachs disease
see below table		 HEXA GM2 ganglioside rapidly progressing mental retardation, blindness, early mortality
Sandhoff-Jatzkewitz disease
see below table		 HEXB Globoside, GM2 ganglioside same symptoms as Tay-Sachs, progresses more rapidly
Tay-Sachs AB variant
see below table		 GM2 activator (GM2A) GM2 ganglioside same symptoms as Tay-Sachs
Gaucher's disease Glucocerebrosidase Glucocerebroside hepatosplenomegaly, mental retardation in infantile form, long bone degeneration
Fabry's disease a-Galactosidase A Globotriaosylceramide; also called ceramide trihexoside (CTH) kidney failure, skin rashes
Niemann-Pick disease, more info below
Types A and B
Type C1
Type C2
Type D

Sphingomyelinase
see info below
see info below

Sphingomyelin
LDL-derived cholesterol
LDL-derived cholesterol 		all types lead to mental retardation, hepatosplenomegaly, early fatality potential
Krabbe's disease; globoid leukodystrophy Galactocerebrosidase Galactocerebroside mental retardation, myelin deficiency
GM1 gangliosidosis GM1 ganglioside:b -galactosidase GM1 ganglioside mental retardation, skeletal abnormalities, hepatomegaly
Sulfatide lipodosis;
metachromatic leukodystrophy		 Arylsulfatase A Sulfatide mental retardation, metachromasia of nerves
Fucosidosis a-L-Fucosidase Pentahexosylfucoglycolipid cerebral degeneration, thickened skin, muscle spasticity
Farber's lipogranulomatosis Acid ceramidase Ceramide hepatosplenomegaly, painful swollen joints

The GM2 gangliosidoses include Tay-Sachs disease, the Sandhoff diseases and the GM2 activator deficiencies. GM2 ganglioside degradation requires the enzyme b-hexosaminidase and the GM2 activator protein (GM2A). Hexosaminidase is a dimer composed of 2 subunits, either a and/or b. The HexS protein is aa, HexA is ab and HexB is bb. It is the a-subunit that carries out the catalysis of GM2 gangliosides. The activator first binds to GM2 gangliosides followed by hexosaminidase and then digestion occurs.
Based upon genetic linkage analyses as well as enzyme studies and the characterization of accumulating lysosomal substances, Niemann Pick disease should be divided into type I and type II; type I has 2 subtypes, A and B (NPA and NPB), which show deficiency of acid sphingomyelinase. Niemann Pick disease type II likewise has 2 subtypes, type C1 and C2 (NPC) and type D (NPD). It is obviously confusing to use the abbreviation NPD for Niemann Pick disease in some cases and for subtype D of Niemann Pick disease in other cases.
Recent studies (Science vol. 277 pp. 228-231 and 232-235: July 11, 1997) identified the gene for NPC1. This gene contains regions of homology to mediators of cholesterol homeostasis suggesting why LDL-cholesterol accumulates in lysosomes of afflicted individuals. The encoded protein product of NPC1 gene is 1278 amino acids long. Within the protein are regions of homology to the transmembrane domain of the morphogen receptor patched (of Drosophila melanogaster), the sterol-sensing domain (SSD) of SREBP (sterol regulated element binding protein) cleavage-activating protein, SCAP and HMG-CoA reductase.
Metabolism of the Eicosanoids

The eicosanoids consist of the prostaglandins (PGs), thromboxanes (TXs) and leukotrienes (LTs). The PGs and TXs are collectively identified as prostanoids. Prostaglandins were originally shown to be synthesized in the prostate gland, thromboxanes from platelets (thrombocytes) and leukotrienes from leukocytes, hence the derivation of their names.

Structures of Representive Clinically Relevant Eicosanoids
PGE2
TXA2
LTA4

The eicosanoids produce a wide range of biological effects on inflammatory responses (predominantly those of the joints, skin and eyes), on the intensity and duration of pain and fever, and on reproductive function (including the induction of labor). They also play important roles in inhibiting gastric acid secretion, regulating blood pressure through vasodilation or constriction, and inhibiting or activating platelet aggregation and thrombosis.
The principal eicosanoids of biological significance to humans are a group of molecules derived from the C20 fatty acid, arachidonic acid. Minor eicosanoids are derived from eicosopentaenoic acid which is itself derived from a-linolenic acid obtained in the diet. The major source of arachidonic acid is through its release from cellular stores. Within the cell, it resides predominantly at the C-2 position of membrane phospholipids and is released from there upon the activation of phospholipase A2 (see diagram above). The immediate dietary precursor of arachidonate is linoleic acid. Linoleic acid is converted to arachidonic acid through the steps outlined in the figure below. Linoleic acid (arachidonate precursor) and a-linolenic acid (eicosapentaenoate precursor) are essential fatty acids, therefore, their absence from the diet would seriously threaten the body's ability to synthesize eicosanoids.

Pathway from linoleic acid to arachidonic acid. Numbers in parentheses refer to the fatty acid length and the number and positions of unsaturations.

All mammalian cells except erythrocytes synthesize eicosanoids. These molecules are extremely potent, able to cause profound physiological effects at very dilute concentrations. All eicosanoids function locally at the site of synthesis, through receptor-mediated G-protein linked signaling pathways leading to an increase in cAMP levels.
Two main pathways are involved in the biosynthesis of eicosanoids. The prostaglandins and thromboxanes are synthesized by the cyclic pathway, the leukotrienes by the linear pathway.

Synthesis of the clinically relevant prostaglandins and thromboxanes from arachidonic acid. Numerous stimuli (e.g. epinephrine, thrombin and bradykinin) activate phospholipase A2 which hydrolyzes arachidonic acid from membrane phospholipids. The prostaglandins are identified as PG and the thromboxanes as TX. Prostaglandin PGI2 is also known as prostacyclin. The subscript 2 in each molecule refers to the number of -C=C- present.

Synthesis of the clinically relevant leukotrienes from arachidonic acid. Numerous stimuli (e.g. epinephrine, thrombin and bradykinin) activate phospholipase A2 which hydrolyzes arachidonic acid from membrane phospholipids. The leukotrienes are identified as LT. The leukotrienes, LTC4, LTD4, LTE4 and LTF4 are known as the peptidoleukotrienes because of the presence of amino acids. The peptidoleukotrienes, LTC4, LTD4 and LTE4 are components of slow-reacting substance of anaphylaxis The subscript 4 in each molecule refers to the number of -C=C- present.

The linear pathway is initiated through the action of lipoxygenases. It is the enzyme, 5-lipoxygenase that gives rise to the leukotrienes.
The cyclic pathway is initiated through the action of prostaglandin G/H synthase, PGS (also called prostaglandin endoperoxide synthetase). This enzyme possesses two activities, cyclooxygenase (COX) and peroxidase. There are 3 forms of the COX activity. COX-1 (PGS-1) is expressed constitutively in gastric mucosa, kidney, platelets, and vascular endothelial cells. COX-2 (PGS-2) is inducible and is expressed in macrophages and monocytes in response to inflammation. The primary trigger for COX-2 induction in monocytes and macrophages is platelet-activating factor, PAF and interleukin-1, IL-1. Both COX-1 and COX-2 catalyze the 2-step conversion of arachidonic acid to PGG2 and then to PGH2. Most recently a splice variant of COX-1 mRNA has been found in many tissues such as heart, kidney and several neuronal tissues. This variant mRNA retains intron 1 from the COX-1 gene and the encoded protein has been termed COX-3. In vitro evidence indicates that the canine COX-3 enzyme is inhibited by acetominophen. However, data also shows that in humans the consequence of inclusion of intron 1 sequences leads to synthesis of a truncated protein with no significant homology to COX-1 or COX-2 and that this protein appears unresponsive to acetominophen action.
A widely used class of drugs, the non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, indomethacin, naproxen, phenylbutazone and aspirin, all act upon the cyclooxygenase activity, inhibiting both COX-1 and COX-2. Because inhibition of COX-1 activity in the gut is associated with NSAID-induced ulcerations, pharmaceutical companies have developed drugs targeted exclusively against the inducible COX-2 activity [e.g. Celebrex (celecoxib), Prexige (lumiracoxib) and the recently removed Vioxx (rofecoxib) and Bextra (valdecoxib)].
Another class, the corticosteroidal drugs, act to inhibit phospholipase A2, thereby inhibiting the release of arachidonate from membrane phospholipids and the subsequent synthesis of eicosinoids.
Properties of Significant Eicosanoids

Eicosanoid Major site(s) of synthesis Major biological activities
PGD2 mast cells inhibits platelet and leukocyte aggregation, decreases T-cell proliferation and lymphocyte migration and secretion of IL-1a and IL-2; induces vasodilation and production of cAMP
PGE2 kidney, spleen, heart increases vasodilation and cAMP production, enhancement of the effects of bradykinin and histamine, induction of uterine contractions and of platelet aggregation, maintaining the open passageway of the fetal ductus arteriosus; decreases T-cell proliferation and lymphocyte migration and secretion of IL-1a and IL-2
PGF2a kidney, spleen, heart increases vasoconstriction, bronchoconstriction and smooth muscle contraction
PGH2   precursor to thromboxanes A2 and B2, induction of platelet aggregation and vasoconstriction
PGI2 heart, vascular endothelial cells inhibits platelet and leukocyte aggregation, decreases T-cell proliferation and lymphocyte migration and secretion of IL-1a and IL-2; induces vasodilation and production of cAMP
TXA2 platelets induces platelet aggregation, vasoconstriction, lymphocyte proliferation and bronchoconstriction
TXB2 platelets induces vasoconstriction
LTB4 monocytes, basophils, neutrophils, eosinophils, mast cells, epithelial cells induces leukocyte chemotaxis and aggregation, vascular permeability, T-cell proliferation and secretion of INF-g, IL-1 and IL-2
LTC4 monocytes and alveolar macrophages, basophils, eosinophils, mast cells, epithelial cells component of SRS-A, microvascular vasoconstrictor, vascular permeability and bronchoconstriction and secretion of INF-g
LTD4 monocytes and alveolar macrophages, eosinophils, mast cells, epithelial cells predominant component of SRS-A, microvascular vasoconstrictor, vascular permeability and bronchoconstriction and secretion of INF-g
LTE4 mast cells and basophils component of SRS-A, microvascular vasoconstrictor and bronchoconstriction

**SRS-A = slow-reactive substance of anaphylaxis