Fatty acids elongation/monounsaturation

Intestinal Handling of FFA In the cells of the small intestine, several enzymes can act on free but not on esterified fatty acids. In contrast to MAGs, FFA are diluted with fatty acids originating from the plasma free fatty acid pool (44). After activation they can be oxidized, elongated, chain desaturated, and converted into complex lipids (45, 46). The relative rates depend on the nature of the fatty acids and on the presence of other components in the intestinal cells (47). Conversion of saturated fatty acids in monounsaturated ones when they are absorbed as FFA, i.e., when they were present in the outer position of the dietary TAGs, could... 

As the name anaerobic implies, the double bond of the fatty acid is inserted in the absence of oxygen. Biosynthesis of monounsaturated fatty acids follows the pathway described previously for saturated fatty acids until the intermediate /3-hydroxydecanoyl-ACP is reached (fig. 18.15). At this point, a new enzyme, /3-hydroxydecanoyl-ACP dehydrase, becomes involved. This dehydrase can form the a-j8 trans double bond, and saturated fatty acid synthesis can occur as previously discussed. In addition, this dehydrase is capable of isomerization of the double bond to a cis /3-y double bond as shown in figure 18.15. The /3-y unsaturated fatty acyl-ACP is subsequently elongated by the normal enzymes of fatty acid synthesis to yield pal-mitoleoyl-ACP (16 1A9). The conversion of this compound to the major unsaturated fatty acid of E. coli, cA-vacccnic acid (18 1A11), requires a condensing enzyme that we have not previously discussed, /3-ketoacyl-ACP synthase II, which shows a preference for palmitoleoyl-ACP as a substrate. The subsequent conversion to vaccenyl-ACP is cata-... 

Oleic acid (18 1 tjj9), a monounsaturated fatty acid, is the major fatty add in human milk. Oleic acid, as well as the products resulting from its further desaturation and elongation, are called the omegit-9 fatty adds. The omega-9 fatty acid 24 l(u9 is a major fatty acid in the membranes of nerves. The omega-9 fatty add 20 3to9 is called the Mead acid. Mead acid accumulates in the body in response to a defi-... 

One explanation is that the Cie and Cig fatty acids are synthesized in a manner similar to other plant systems, whereas the monounsaturated C20 and C22 acids result from a chain elongation system similar to that shown by Wakil and Mead in animal tissues. The proposed system would involve oleic acid as the precursor and the addition of one acetate to the carboxylic end of the fatty acid molecule to form eicosenoic acid and two acetates to form erucic acid. 

Time course of formation of very long chain monounsaturated fatty acids from [1- C]oleoyl-CoA or [2- C]malonyl-CoA by the 15 000 X g particulate fractions from S. alba, L. annua, and T. majus [11] corroborates the pathways involving successive condensations [1,3,4,8,9] in a similar manner as observed in the elongation of stearoyl-CoA [12]. 

Fatty acid desaturation, a second major source of variation in the phylogenetic distribution of fatty acids, has been reviewed in detail (294-302). Some bacteria have a unique anaerobic system for production of monounsaturated fatty acids. This mechanism is involved in elongation of medium-chain length c/.r-3-unsaturated fatty acyl intermediates, and functions via P,y-dehydration of P-OH intermediates. It should be noted that this process cannot generate methylene-interrupted polyunsaturated fatty acids. 

The cis-monounsaturated acids might also resnlt from the desaturation of a fatty acid with 16 carbons which would undergo desaturation prior to the elongation by acetyl-CoA. 

Some natural edible oils contain, in addition to the widely occurring monounsaturated fatty acid, oleic acid, appreciable quantities of monounsaturated acids with 20 or 22 carbon atoms. Biochemically, these are usually formed by elongation of oleic acid, so that the double bonds are in positions 11 and 13 respectively. Examples are gadoleic acid cis-11-20 1) in certain fish oils and erucic acid (cw-13-22 l. Table 3.2), characteristic of seed oils in the plant family Cruciferae. Herring oil also... 

---