Metabolism of Fatty Aldehydes and Alcohols
The fundamental biochemical defect in SLS is an inability to oxidize fatty aldehyde, which is derived from the metabolism of fatty alcohol and related lipids. Since fatty aldehydes are intermediates in the synthesis and oxidation of fatty alcohols, it is relevant to review both fatty alcohol and aldehyde metabolism as a framework for understanding SLS.
Fatty aldehydes and alcohols are aliphatic lipids that exist as free molecules or as components of other lipids (Fig. 98-6). They tend to be saturated or monounsaturated straight chain molecules that range from 14 to 24 carbons.57,58 Most fatty aldehyde in mammalian tissues is derived from the metabolism of aldehydogenic lipids such as ether glycerolipids and sphingolipids. Free fatty aldehydes appear to be short-lived metabolites that generally do not accumulate in mammalian tissues to any appreciable extent. Fatty alcohols, on the other hand, serve as precursors for biosynthesis of wax esters and ether lipids and are found in most tissues at very low concentrations.58-60
Chemical structures of fatty alcohol, fatty aldehyde, wax ester, and neutral ether glycerolipid. The brackets outline the portions of wax ester and ether glycerolipid derived from fatty alcohol incorporation.
As shown in Fig. 98-6, wax esters are composed of fatty alcohols esterified to long-chain fatty acids. Variation in the chain length and degree of unsaturation of the aliphatic components typically results in structural heterogeneity of wax esters. Ether lipids possess even greater diversity than wax esters. These lipids are almost exclusively glycerolipids in which a fatty alcohol is incorporated in an ether linkage at the 1-position of the glycerol backbone (1-O-alkylglycerol)61 (Fig. 98-6). In most cases, the 2-position of glycerol is esterified with fatty acid. In mammalian tissues, most ether glycerolipids have a 1-O-alkyl chain that is unsaturated at the 1,2 position (alk-1-enyl ether); these are commonly known as plasmalogens. These plasmalogens are usually found as phospholipids, in which the 3-position of the glycerol backbone is occupied by a phosphatidyl diester with ethanolamine, choline, or serine.
It is noteworthy that fatty alcohol-derived lipids are particularly abundant in the skin and brain. Wax esters and alkyldiacylglycerol, which are primarily synthesized by the sebaceous glands, compose up to 24 percent62 and 15 percent,63 respectively, of skin surface lipids. In human brain myelin, phosphatidylethanolamine (PE) accounts for 11–14 percent of the total lipids, and almost all of it is in the ether plasmalogen form.64
The metabolic pathways for fatty alcohol and fatty aldehyde are shown in Fig. 98-7. The pathways leading to the synthesis and oxidation of fatty alcohol seem to operate as a biochemical cycle.65 The concept of a “fatty alcohol cycle” is supported by evidence that 1) fatty alcohol is simultaneously synthesized from fatty acid and recycled back to fatty acid by distinct enzymes within the intact cell and 2) the rates at which these two reactions occur far exceed the cell's biosynthetic requirements for fatty alcohol. Studies on mutant Chinese hamster ovary cells66 and on cells from patients who are genetically deficient in specific steps in fatty alcohol metabolism9,67,68 have been instrumental in providing experimental evidence for the cycle. It is not known whether the fatty alcohol cycle has an ancillary function besides supplying precursors for lipid biosynthesis or recycling fatty alcohol. In cells that have a low biosynthetic capacity for synthesizing alcohol-derived lipids, fatty alcohol metabolism resembles a “futile cycle”69 that may function to exchange reducing equivalents between NADPH and NADH for other biosynthetic reactions.67
Metabolic pathways for fatty aldehyde and fatty alcohol. Brackets surrounding fatty aldehyde indicate enzyme-bound intermediates, whereas a rectangle surrounding fatty aldehyde indicates free fatty aldehyde. 1. Acyl-CoA synthetase. 2. Acyl-CoA reductase. 3. Fatty alcohol:NAD+ oxidoreductase. 3a. Fatty alcohol dehydrogenase. 3b. Fatty aldehyde dehydrogenase. 4. Alkyl-DHAP synthase. 5. Acyl-CoA:fatty alcohol acyl transferase. Figure modified from Rizzo.67
Fatty alcohols are predominantly synthesized from fatty acids by reduction of their corresponding acyl-CoA esters.70 The initial activation of free fatty acids requires ATP and is catalyzed by acyl-CoA synthetase. The subsequent reduction of acyl CoA to fatty alcohol is catalyzed by acyl-CoA reductase and appears to proceed via an aldehyde intermediate.71 This enzyme is membrane-bound, utilizes NADPH as nucleotide cofactor, and has a relatively narrow chain-length specificity that determines the profile of alcohols made.72-74 A cytosolic protein that binds its acyl CoA substrate stimulates the activity of the enzyme.75 Acyl CoA-reductase is not subject to product inhibition,71 and studies on intact human fibroblasts indicate that the addition of exogenous fatty alcohol does not decrease endogenous alcohol synthesis.65 The acyl-CoA reductase enzyme responsible for fatty alcohol synthesis has not been purified from mammalian tissue, and it may consist of more than one protein. Evidence suggests that the reduction of the fatty aldehyde intermediate requires NADPH-cytochrome-c reductase or a closely related protein.76 A distinct cytosolic acyl-CoA reductase that produces hexadecanal and uses NADH as cofactor has been identified in bovine heart,77 but the role of this enzyme in fatty alcohol metabolism is not known. Fatty alcohol may also be derived from hydroxylation of environmental alkanes;78 this pathway is undoubtedly minor and may contribute only a minuscule amount of fatty alcohol to the metabolic pool.
The primary function of fatty alcohol in intermediary metabolism is to provide a substrate for the synthesis of wax esters and ether glycerolipids. Wax ester synthesis requires long-chain acyl CoA73,79 or an acyl chain derived from phosphatidylcholine as a cosubstrate.80 The incorporation of fatty alcohol into ether glycerolipids is catalyzed by alkyl dihydroxyacetone phosphate (DHAP) synthase.81 This reaction exchanges the acyl group of 1-acyl-DHAP for fatty alcohol and results in the formation of 1-O-alkyl-DHAP, which is a necessary substrate for subsequent reactions in ether phospholipid synthesis.82
Fatty alcohol is oxidized back to fatty acid by fatty alcohol:NAD+ oxidoreductase (FAO),65,83 a complex enzyme consisting of at least two separate proteins that catalyze the sequential oxidation of fatty alcohol to fatty aldehyde and fatty acid.84 This reaction resembles the more widely known oxidation of ethanol, but the enzymes composing FAO are clearly distinct from those involved in short-chain alcohol oxidation. The initial oxidative step is catalyzed by an NAD-dependent alcohol dehydrogenase83,84 and is probably rate-limiting,10 but the alcohol dehydrogenase isozyme responsible for this reaction has not yet been identified. The subsequent oxidation of fatty aldehyde to fatty acid is catalyzed by FALDH.84-86 This enzyme acts on medium- and long-chain aliphatic aldehydes, including branched chain substrates.86 Although FAO has not yet been purified as an intact protein complex, in vitro studies suggest that its two components are in physical contact, since the aldehyde intermediate is not freely accessible.83 Purified FALDH from rabbit intestine has been reconstituted with partially purified fatty alcohol dehydrogenase to restore the complete oxidation of fatty alcohol to fatty acid.87 In addition to its role in fatty alcohol oxidation, FALDH is capable of oxidizing free fatty aldehydes.
Apart from fatty alcohol metabolism, fatty aldehyde is also generated by the catabolism of ether lipids88,89 and sphingolipids.90,91 The ether bond of 1-O-alkyl glycerolipids is cleaved by the action of alkylglycerol monooxygenase, a microsomal enzyme that requires molecular oxygen and tetrahydropteridine.88,89 The product of this reaction is a fatty aldehyde corresponding to the alkyl chain length of the ether lipid substrate. This fatty aldehyde is oxidized to fatty acid via an NAD(P)-dependent enzyme(s).88 Sphingolipid catabolism generates dihydrosphingosine, which is further cleaved in a pyridoxal-dependent reaction to yield ethanolamine and a fatty aldehyde (hexadecanal) that is 2 carbons shorter.90,91 The hexadecanal is either oxidized to fatty acid or reduced to fatty alcohol and preferentially channeled into ether lipid biosynthesis.92 In the catabolic pathways for both ether lipids and sphingolipids, the enzyme(s) responsible for oxidation of the fatty aldehydes—FALDH, another aldehyde dehydrogenase isozyme, or perhaps aldehyde oxidase—has not been positively identified.
It has been shown that the α-oxidation of phytanic acid (3,7,11,15-tetramethylhexadecanoic acid), a branched chain fatty acid of dietary origin, produces a fatty aldehyde product (pristanal) that is one carbon shorter.93,94 Pristanal is subsequently oxidized to fatty acid by FALDH prior to further β-oxidation.95 This raises the possibility that the α-oxidation of other fatty acids, perhaps of endogenous origin, may also be a source of fatty aldehydes.
In cultured cells, the flow of fatty alcohol into biosynthetic reactions occurs simultaneously with its oxidative metabolism. The relative flux of fatty alcohol through these pathways is undoubtedly dependent on the wax ester and ether lipid synthetic rates, which may vary between tissues. Cultured human fibroblasts oxidize most fatty alcohol to fatty acid and incorporate only 1–2 percent of the fatty alcohol into the 1-O-alkyl ether bond of plasmalogen PE, the major ether lipid in these cells.65 In contrast, cultured human AB589 breast cancer cells incorporate about 20 percent of octadecanol into ether lipids, and the remaining fatty alcohol is largely oxidized to fatty acid.96 Furthermore, the flow of fatty alcohol through its metabolic pathways may be subject to regulation. The addition of fatty acid to the medium of cultured fibroblasts decreases fatty alcohol oxidation, a process that may be mediated in part by long-chain acyl CoA, which inhibits FAO activity.65 In the presence of 20 μM palmitate in the medium, the turnover rate of the hexadecanol pool in cultured human fibroblasts is estimated to be about 1 hour.
Enzymes involved in the metabolism of fatty alcohol and ether lipids appear to be coordinately regulated to modulate fatty alcohol availability for tissue ether lipid synthesis. In cells or tissues that have a high ether lipid content, the activities of acyl-CoA reductase and alkyl DHAP synthase are increased,83,97,98 whereas FAO activity is low.83,98 Cells or tissues with a low ether lipid content show the opposite trend in enzyme activities. Interestingly, alkyl-DHAP synthase, the key enzyme responsible for incorporation of fatty alcohol into ether lipids, is not considered an inducible enzyme,81 and it may not determine the rate of ether lipid synthesis alone. The tissue ether lipid content may be regulated in part by activity of alkylglycerol monooxygenase.82
Fatty alcohol is a substrate for ether lipid synthesis, but it is not clear whether an elevated fatty alcohol concentration results in more ether lipid synthesis. In cultured cells, the addition of increasing amounts of fatty alcohol does not seem to cause elevation of cellular ether lipids, since the excess fatty alcohol is preferentially oxidized to fatty acid.65,99 However, in vivo studies indicate that animals fed large amounts of fatty alcohol accumulate ether phospholipids in liver.100
Several enzymatic steps in fatty alcohol metabolism occur within specific subcellular compartments, which may be important for regulating substrate for biosynthetic needs. Activation of fatty acid to acyl CoA occurs in several subcellular locations, including microsomes, mitochondria, and peroxisomes.101 Acyl-CoA reductase, previously thought to be a microsomal enzyme,71,73,74,79,102 has now been shown to reside on the outer peroxisomal membrane in guinea pig intestine.103 This location ensures ready availability of fatty alcohol for entry into the peroxisome, where two key enzymes for ether lipid synthesis reside: alkyl-DHAP synthase, which incorporates fatty alcohol into the ether bond, and DHAP-acyl transferase, which catalyzes the formation of its cosubstrate (1-acyl DHAP).104-106 The other biosynthetic pathway for fatty alcohol, wax ester synthesis, is reported to be active in microsomes,73,79 but rigorous subcellular fractionation studies using density gradient separations have not been done to confirm this localization. In contrast to the biosynthetic enzymes, FAO and FALDH in human liver have a microsomal localization,86 which ensures that fatty alcohol inside peroxisomes is protected from oxidation. Similarly, alkylglycerol monooxygenase is a microsomal enzyme88,89 and therefore separated from the peroxisomal location where ether lipids are made.
There is evidence that fatty alcohol metabolism may be modified by clofibrate, a drug that induces peroxisome proliferation in rodents. Rats treated with clofibrate have been reported to develop peroxisomal FAO activity in liver.107 In clofibrate-treated rats, de novo synthesis of hexadecanol from acetate appears to occur in liver peroxisomes.108,109 Whether clofibrate has a similar effect in humans is not known, but clofibrate and other related drugs are generally ineffective in inducing peroxisome proliferation in humans.
Dietary Origin of Fatty Alcohols and Aldehydes
Fatty alcohols and fatty aldehydes may also be derived from dietary lipids, either directly or indirectly, from the metabolism of wax esters and ether lipids. The major dietary sources of free fatty alcohols and aldehydes are probably fruits and vegetables. Fatty alcohols and wax esters are abundant lipids in the cell walls of leafy plants.110 Free aliphatic aldehydes 8–11 carbons in length are particularly abundant in the peels of oranges, limes, and tangerines,111 but it is not known to what extent longer-chain aldehydes are present in fruits. Ether lipids and wax esters are major lipid components of certain marine microorganisms and the fish that consume them.112 Certain cartilaginous fish have a high content of ether lipids, particularly alkyldiacylglycerol, in their oils. A survey of dietary meats, poultry, and fish found that ether phospholipids were present in significant amounts.113 Among mammals, ether phospholipids are relatively abundant in brain and heart muscle and less so in most other organs, including skeletal muscle.82 Despite these dietary sources, the consumption and digestibility of aldehydogenic lipids by humans is not well established.
Studies in animals, however, clearly indicate that fatty alcohols,100,114-116 wax esters,117 and neutral ether glycerolipids118-120 are absorbed from the diet. Rats fed fatty alcohol have been shown to incorporate it into 1-O-alkyl ether lipids and wax esters in the small intestine116 and the liver100 but showed no change in brain lipids.100 This suggests that fatty alcohol does not readily cross the blood-brain barrier. Octadecanol is absorbed via the lymphatic system, but most of it is oxidized to fatty acid prior to entry.121 In addition, dietary phytol (3,7,11,15-tetramethyl-2-hexadecen-1-ol), a plant-derived branched chain fatty alcohol, is absorbed by humans and rodents.122
Dietary wax esters may be less digestible than fatty alcohol, in part due to their purgative effect. Evidence suggests that rats fed the wax oleoyl palmitate hydrolyzed much of it in the intestine and reutilized the fatty alcohol for wax ester synthesis.117
Neutral ether glycerolipids are also absorbed from the diet. In rodents, dietary neutral alkylglycerol lipids are digested with the 1-O-alkyl bond intact and subsequently incorporated into the ether phospholipids of various tissues.118-120 Humans show a similar capacity for utilizing dietary ether lipids for biosynthesis. The oral administration of 1-O-alkylglycerol to patients with Zellweger syndrome, who are genetically deficient in alkyl-DHAP synthase and have impaired ether lipid synthesis, caused an increase in erythrocyte plasmalogen levels.120,123
In summary, aldehydogenic lipids are absorbed from the diet and are metabolized or incorporated into new lipids. It seems reasonable to conclude that dietary lipids are an indirect source of fatty aldehyde that is subject to oxidation by FALDH. Nevertheless, it is not known to what extent the diet contributes to the total fatty aldehyde pool in humans.