Fatty acid chain elongation microsomal

Figure 21-S. Microsomal elongase system for fatty acid chain elongation. NADH is also used by the reductases, but NADPH is preferred.

Liver microsomes were the site of fatty acid chain-elongation activity (Guchhait et al., 1966 Nugteren, 1965) and desaturation (Marsh and James, 1962). Aeberhard and Menkes (1968) reported that brain microsomes were additionally capable of synthesis de novo under certain conditions. The microsomes are probably not responsible for the net synthesis of a substantial quantity of fatty acids, but their functional importance appears to be as modifier of the products of synthesis. 

Figure 7. Enzymatic steps in long-chain fatty acid elongation. Enzymatic steps of microsomal fatty acyl chain elongation. ELOVL, elongation of very-long-chain fatty acids KAR, 3-ketoacyl-CoA reductase HADC, 3-hydroxyacyl-CoA dehydratase TER, /rowi-2,3-enoyl-CoA reductase [108].

Figure 23-3. Biosynthesis of the co9, co6,and co3 families of polyunsaturated fatty acids. Each step is catalyzed by the microsomal chain elongation or desaturase system 1,elongase 2,A desaturase 3,A desaturase 4,A desaturase. ( .Inhibition.)...

One of the sex pheromone components of the housefly, Musca domestica, is Z9-21 H that is found on the cuticular surface of the fly. This compound is formed by the elongation of Z9-18 CoA using malonyl-CoA and NADPH to Z15-24 CoA which is decarboxylated to form Z9-21 Hc (Fig. 3) [78-80]. Other pheromone components include an epoxide and ketone that are produced from Z9-21 Hc by a cytochrome P450 [81,82] and methyl-branched alkanes that are produced by the substitution of methylmalonyl-CoA in place of malonyl-CoA at specific points during chain elongation [83,84]. A novel microsomal fatty acid synthase is involved in production of methyl-branched alkanes in most insects [85-87]. This fatty acid synthase is different from the ubiquitous soluble fatty acid synthase that produces saturated straight chain fatty acids in that it is found in the microsomes and prefers methylmalonyl-CoA. The amino acids valine and isoleucine can provide the carbon skeletons for methylmalonyl-CoA as well as propionate [83]. 

The FAS multi-enzyme complex synthesizes saturated C16 fatty acids, but cells and tissues need unsaturated and longer chain fatty acids. The palmitoyl-CoA can be modified by either chain elongation and/or oxidation in order to produce different fatty acid molecules. Both elongation and desaturation occur within the smooth endoplasmic reticulum (SER, microsomal fraction) of the cell. 

The biosynthesis of hydrocarbons occurs by the microsomal elongation of straight chain, methyl-branched and unsaturated fatty acids to produce very long-chain fatty acyl-CoAs (Figure 11.1). The very long chain fatty acids are then reduced to aldehydes and converted to hydrocarbon by loss of the carboxyl carbon. The mechanism of hydrocarbon formation has been controversial. Kolattukudy and coworkers have reported that for a plant, an algae, a vertebrate and an insect, the aliphatic aldehyde is decarbonylated to the hydrocarbon and carbon monoxide, and that this process does not require cofactors (Cheesbrough and Kolattukudy, 1984 1988 Dennis and Kolattukudy, 1991,1992 Yoder et al., 1992). In contrast, the Blomquist laboratory has presented evidence that the aldehyde is converted to hydrocarbon and carbon dioxide in a process that... 

One molecule of oxygen accepts two pairs of electrons, one from palmitoyl-CoA and the other from NADPH or NADH. The electrons NAD(P)H are transported via cytochrome-bs reductase to cytochrome bs (microsomal electron transport Chapter 14). An enzyme-bound superoxide radical is responsible for the oxidation of acyl-CoA. Four desaturases specific for introducing cis double bonds at C9, Ca, C5, and C4, respectively, are known. If the substrate is saturated, the first double bond introduced is C9. With an unsaturated substrate, other double bonds are introduced between the carboxyl group and the double bond nearest the carboxyl group. Desaturation yields a divinylmethane arrangement of double bonds (—CH=CH—CH2—CH=CH—). Usually desaturation alternates with chain elongation. Desaturation is inhibited by fasting and diabetes. The oxidation of unsaturated fatty acids occurs in mitochondria. 

The desaturation of fatty acids is usually assayed by incubating radioactive fatty acids with microsomes in the presence of appropriate cofactors. This general protocol has been used by many investigators to assay A9, A6, and A5 desaturase activities. It was thus assumed that 7,10,13,16-22 4 and 7,10,13,16,19-22 5 were desaturated by a microsomal A4 desaturase. In 1991, we showed that when rat liver microsomes were incubated with [1- C]7,10,13,16,19-22 5, it was not desaturated to 4,7,10,13,16,19-22 6. However, when malonyl-CoA was included in the incubation, the substrate was chain elongated to 9,12,15,18,21-24 5, which was then desaturated, at position-6, to yield 6,9,12,15,18,21-24 6. When [1- C]7,10,13,16,19-22 5 and the two [3- C]-labeled 24-carbon acids were incubated with hepatocytes, all three acids were metabolized to esterified [1- C]4,7,10, 13,16,19-22 6 (19). The findings implied that 4,7,10,13,16, 19-22 6 was made from 9,12,15-18 3 as follows ... 

Nugteren, D.H. (1965) The Enzymatic Chain Elongation of Fatty Acids by Rat-Liver Microsomes, fiioc/u tn. Biophys. Acta 106,280-290. 

The results presented here support the hypothesis that the 22 6n-3 synthetic pathway involves elongation and desaturation of 18 3n-3 to 24 6n-3, processes most likely carried out in microsomes then 24 6n-3 is transported to peroxisomes for one cycle of fatty acid P-oxidation to yield 22 6n-3. We showed that peroxisomal straight-chain acyl-Co A oxidase (AOxl), DBF, and 3-oxoacyl-CoA thiolase or sterol carrier protein X are involved in the P-oxidation of 24 6n-3 to 22 6n-3. 

Fig. 2. Pathway for the microsomal chain elongation of fatty acids.

Rates of Component Reactions in the Microsomal Chain Elongation of Fatty Acids Using Livers from Rats Raised on a Normal Chow or Fat Free Diet ... 

Under normal dietary conditions liver lipids contain high levels of linoleate and arachidonate but only trace or negligible amounts of other acids in this metabolic sequence. We have measured rates of both desaturation and chain elongation reactions in the linoleate sequence to determine what relationship exists between rates of conversion and the type of fatty acid found in liver lipids (Bernert Sprecher, 1975). Dietary linoleate is either incorporated into liver lipids, or as shown in Figure 1, it is desaturated to 6,9,12-18 3 at a rate of 1.0 nmole/min/mg microsomal protein. Liver lipids never contain measureable amounts of 6,9,12-18 3 even when this acid is added directly to the diet... 

FIGURE 1. Rates of desaturation and chain elongation for acids in the linoleate sequence. Rates expressed as nmoles product/ min/mg microsomal protein. Each incubation for desaturation contained 10 ymoles ATP, 2 ymoles NADH, 0.3 ymole CoA, 150 ymoles potassium phosphate buffer, pH 7.4, 150 nmoles radioactive fatty acid, 5 mg liver microsomal protein from rats raised on a fat-free diet and 5 mg bovine serum albumin. Chain elongation conditions were the same except 2.0 ymoles of NADPH were used instead of NADH and 0.3 ymole of malonyl-CoA was added. All incubations were run in 1.5 ml for 3 minutes at 37°C. (Bernert Sprecher, 1975)... 

Microsomal elongating systems. These subcellular particles are able to synthesize long chain fatty acids (9) the very long chain fatty acids found in myelin are synthesized in these organelles (4). Ammonium sulphate precipitation leads to the separation of elongating systems from microsomal de novo synthesis (10, 11). 

---