Fatty acid elongation and desaturation enzyme activities of bovine liver and subcutaneous adipose tissue microsomes.

Assay conditions were established for the fatty acid elongation and the delta 9 desaturase enzyme systems of bovine liver and adipose tissue microsomes; rat liver microsomes were used as a reference. Overall fatty acid elongation was determined by measuring the incorporation of [2-14C]malonyl-coenzyme A (CoA) to 14C-labeled stearate. Rat liver elongation activity was .50 +/- .02 nmol.min-1.mg protein-1; bovine liver microsomal elongation activity was substantially lower (P less than .05), with a mean value of .15 +/- .02 nmol.min-1.mg protein-1. The elongation activity of bovine s.c. adipose tissue microsomes (.42 +/- .10 nmol.min-1.mg protein-1) was not different (P greater than .05) from the activity observed in rat liver microsomes. To determine the fatty acid delta 9 desaturase activity, microsomes were incubated in the presence of [1-14C]stearoyl-CoA and nicotinamide adenine dinucleotide (reduced form) (NADH), and the production of radioactively labeled oleate was quantified. Microsomal delta 9 desaturase activity was similar in rat liver and bovine s.c. adipose tissue microsomes with rates of .15 +/- .04 and .21 +/- .05 nmol.min-1.mg protein-1, respectively. However, no desaturase activity was detected in bovine liver microsomes, indicating that the liver is not a major site of oleate synthesis in this species. To investigate differences in fatty acid metabolism relative to breed type, eight Angus and seven Braford heifers were slaughtered at approximately 12 mo of age. Subcutaneous fat thickness over the 12th-13th thoracic vertebrae was greater in the Angus heifers than in the Braford heifers. However, no differences (P greater than .05) were observed in mean adipocyte size or number of cells per gram of adipose tissue between the Angus and Braford heifers. Similarly, there were no significant differences between the Angus and Braford s.c. adipose tissues for microsomal fatty acid elongation or delta 9 desaturation, or for nicotinamide adenine dinucleotide phosphate (NADP)-malate dehydrogenase, fatty acid synthetase, or the pentose cycle reductases. The inability of bovine liver to convert stearate to oleate was in agreement with the fatty acid composition of the liver lipid, which had a smaller percentage of oleate and a higher percentage of stearate than s.c. adipose tissue.