Metabolism of leucine by bovine adipose tissue: Effects of media supplementation and insulin

Leucine metabolism in comparison to glucose, and with substrate and insulin supplementation, were studied in bovine adipose tissue slices obtained from the tailhead region. In addition, leucine metabolism by isolated adipocytes in a Krebs-Ringer bicarbonate (KRB) buffer was compared to metabolism in Medium-199. Slices oxidized leucine and incorporated the amino acid into cellular protein and lipid, though at much lower rates than for glucose. Glucose addition increased leucine oxidation and its incorporation into lipid but did not affect protein synthesis. Insulin, up to 100 ng/ml, had no effect. Isolated cells convened a higher proportion of the leucine utilized to lipid, and less to protein, than did slices. Absolute rates of oxidation and lipid synthesis were lower, and protein synthesis higher, for Medium-199 than for KRB. Of the total leucine utilized, conversion to lipid represented the largest percentage in both buffers. Insulin had no effect in either buffer system. Bovine adipose tissue, the major site of fatty acid synthesis in this species, was found to both oxidize leucine, and utilize the amino acid for synthesis of cellular components. The isolated adipocyte, free of connective tissue, directs this "ketogenic" amino acid primarily towards lipid synthesis, by mechanisms which appear to be insulin insensitive in the adult bovine, as studied under short-term, in-vitro conditions.