Conversion of first and sixth carbons of stearic acid to carbon dioxide by rats.

An earlier study from this laboratory dealt with the rates of U402 expiration when palmitic acid labeled with Cl4 was injected intravenously into rats (1). It was shown that, regardless of which carbon atom of this fatty acid was labeled (the carboxyl, the 6th, or the 11th) the amounts of Cl402 expired and the rates of its elimination were quite similar. It was therefore postulated that, once the breakdown of palmitic acid is initiated in the intact animal, a palmitic acid molecule is disrupted in such a manner that all of its carbons are converted to COz at about the same time. Even though it has been clearly demonstrated that myristic and lauric acids can be derived directly from palmitic acid (2), the small amounts of these acids present in the body, as compared with those of palmitic acid, are in line with the view that palmitic acid is predominantly oxidized to a small fragment. Since physiologically occurring fatty acids have a chain length principally of 16 to 18 carbons, it became of interest to determine whether the conclusions drawn for palmitic acid applied also to the oxidation of stearic acid. It is conceivable that considerable amounts of palmitic acid are derived directly from stearic acid. If this were the case, the carboxyl and a-carbons of stearic acid would appear in COz more rapidly than would its other carbons. In the present investigation we used stearic acids in which either the carboxyl or the 6th carbon atom was labeled with C14. These were administered to rats intravenously as tristearin, and the elimination of the Cl4 in the expired CO2 was studied. In addition, in the experiment with tristearin-6-C14, the fatty acids of the rats were isolated and the distribution of the radioactivity in 16-carbon and 18-carbon fatty acids was determined.