Carbohydrate sparing of fatty acid oxidation. I. The relation of fatty acid chain length to the degree of sparing. II. The mechanism by which carbohydrate spares the oxidation of palmitic acid

Abstract

1. I. Carboxyl-labeled fatty acids of the even series (C 8C 16) and the triglycerides of carboxyl-labeled palmitic and capric acids were injected into fasted and carbohydrate-fed rats. The expired CO 2 was collected at various intervals for 24 hr., and the cumulative C 14O 2 and the specific activity-time curves were derived for each rat. 1.1. 1. The conversion of the C 14 of the injected fatty acids to CO 2 was lower in the fed than in the fasted rat. This sparing action of carbohydrate was more pronounced in the case of the longer-chain fatty acids. Administered carbohydrate failed, under the conditions employed, to spare the oxidation of acetate to CO 2. 1.2. 2. The sparing action of carbohydrate lasted several hours, and was followed by a secondary rise in the specific activity of the expired CO 2. This secondary rise, suggestive of an increased breakdown of labeled fatty acid, was more pronounced in the case of the longer-chain fatty acids. 2. II. To study the mechanism of the action of carbohydrate in sparing the oxidation of palmitic acid to CO 2, the triglyceride and the nonlipide fractions were isolated from carbohydrate-fed and fasted rats 5 hr. after the injection of palmitic acid-1-C 14. The C 14 contents of the carboxyl carbon and decarboxylated moieties of the triglyceride 16-` and 18-carbon fatty acids and the C 14 contents of the nonlipides were determined. 2.1. 1. The results obtained indicate that the extent of utilization of acetyl-S-CoA, derived from palmitic acid breakdown, for fatty acid synthesis is far greater in the carbohydrate-fed than in the fasted rat. But diversion of the acetyl-S-CoA from an oxidative to a synthetic fate can account for only a small part of the difference in the C 14O 2 recoveries observed in the fed and fasted rats. 2.2. 2. The results obtained for the C 14 nonlipides in the carbohydrate-fed and fasted rats indicate that carbohydrate feeding did not bring about a diversion of C 14 into this fraction. 2.3. 3. The conclusion was drawn that the principal action of carbohydrate in sparing the oxidation of palmitic acid is in restricting its breakdown. 2.4. 4. In both the carbohydrate-fed and fasted rats, the injected palmitic acid was recovered after 5 hr. chiefly as 16-carbon fatty acids. It was found, however, that a much greater proportion of the unoxidized palmitic acid was converted to 18-carbon fatty acids in the carbohydrate-fed than in the fasted rats. 2.5. 5. It is suggested that the degree of sparing, by carbohydrate, of the oxidation of fatty acids of chain lengths less than 16 carbons is related to the capacity of the organism to convert a particular fatty acid directly to 16- and 18-carbon fatty acids.