The Metabolism 14c-labeled Cis and Trans Isomers of Octadecenoic and Octadecadienoic Acids.

The physical properties of common unsaturated fatty acids are altered when they are converted to their geometric isomers. For example, isomerization of the naturally occurring cis acid, oleic acid, to its trans form, elaidic acid, raises its melting point by 30”, changing it from a liquid to a solid at body temperature. Theoretically, such isomeric pairs might be expect.ed to differ in their metabolism, a possibility having considerable practical implications since cis to trans conversion occurs on a large scale in commercial hydrogenation of food fats (1). Metabolic comparisons among isomers have been limited mainly to long term feeding experiments. These have shown that in rats only the cis,cislinoleic acid possessed essential fatty acid activity (2). The effect of isomerization on many important metabolic processes involving fatty acids has yet to be observed, however, including some which can be examined only through use of labeled compounds. For this purpose, 1-14C-labeled oleic, elaidic, cis,cis-linoleic, and trans, trans-linoleic acids have been synthesized. The isomeric pairs were administered as nonesterified fatty acids and then systematically compared in the rat and dog, with the use of simultaneously administered 3H-palmitic acid as a “metabolic marker.” The nonesterified fatty acids have been studied with respect to rates of intestinal absorption, transport from plasma and subsequent oxidation to carbon dioxide, and transfer across the placental membrane. The retention of configuration during several of these processes was also determined.