Synthesis of Fatty Acids in Animal Tissues II. THE OCCURRENCE AND BIOSYNTHESIS OF CJS-VACCENIC ACID* P. W. HOLLOWAY AND SALIH .I'. WAKIL

The occurrence of cis-vaccenic acid (cis-11-octadecenoic acid) in animal tissues was first reported by Morton and Todd (l), who were able to isolate this acid from horse brain lipids. The extent to which this acid was present in brain was estimated to be about 0.025% wet weight and was thought to be a minor constituent of horse brain fatty acids. The trans isomer, however, was isolated earlier by Bertram (2) from margarine and butterfats and by Nath, Barki, Elvehjem, and Hart (3) from sheep and beef lipids. The origin of the trans-vaccenic acid was obscure but was thought to be derived from oleic acid by a process of cis-truns isomerization in addition to a selective shift of the 9, lo-double bond to the 11 , 12-position. A more thorough study of the occurrence of cis-vaccenic acid in nature was reported by Hofmann, Lucas, and Sax (4-6) and Hofmann and Tausig (7), who isolated this acid from various microorganisms and characterized it as cis-vaccenic acid by its melting point, oxidative cleavage to C&dicarboxylic acid, and by its conversion to various other derivatives. These workers also found that cis-vaccenic acid is a major constituent of the fatty acids of these organisms and in some cases is the only Cl*unsaturated fatty acid present (5). Since intestinal bacteria were found to be a rich source of cis-vaccenic acid, Hofmann and Tausig (5) pointed to them as a likely source of the trans-vaccenic acid in the lipid of higher animals. These workers suggested that the cis isomer synthesized by these organisms may undergo cis-truns isomerization during its absorption or transport to the tissues. Recently, Harlan and Wakil (8, 9) studied the synthesis of long chain fatty acids in subcellular particles derived from livers of various animals and found that 14C-acetyl coenzyme A was incorporated into various saturated and unsaturated fatty acids. Addition of avidin to a soluble extract of this system resulted in the inhibition of the de novo process of fatty acid synthesis (yielding Cl6 and Cls acids) but not the elongation pathway of fatty acid synthesis (9, 10). In our attempt to identify the various unsaturated fatty acids that were synthesized by this system, we were surprised to discover that cis-vaccenic acid constituted a relatively large proportion of the C1sZ1 acids present in these preparations. A closer examination of the occurrence of cisvaccenic acid in animal tissues showed that this acid is a normal constituent of the fatty acids of these tissues and sometimes it