Stimulatory action of insulin on leucine uptake and metabolism in adipose tissue

Epididymal fat incubated in the presence of glucose and dl-leucine-2-C 14 incorporated C 14 into fatty acids and CO 2 . Insulin at physiological concentrations increased leucine uptake and conversion to fatty acids and usually decreased oxidation to C 14 O 2 . In the absence of glucose, lipogenesis was sharply curtailed, but oxidation of leucine to C 14 O 2 was unchanged. In this situation, insulin failed to increase lipogenesis from leucine, although its stimulatory action on leucine uptake remained. Virtually all the leucine C 14 taken up under the influence of insulin now appeared as C 14 O 2 . Insulin similarly increased the uptake and conversion of acetate-2-C 14 to fatty acids and decreased C 14 O 2 production in the presence of glucose but not in its absence. Addition of α-aminoisobutyric acid to the medium blocked the stimulatory effect of insulin on leucine uptake. From these observations it is suggested that insulin may affect the synthesis of fatty acids from leucine in two ways: directly by increasing the transport of leucine into the cell and indirectly by increasing the availability of glucose and hence channeling the acetate derived from leucine into fatty acids.