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

¹ Pratt Clinic-New England Center Hospital, Department of Medicine, Tufts University School of Medicine, and Department of Physiology, Harvard Medical School, Boston, Massachusetts

Epididymal fat incubated in the presence of glucose and dl-leucine-2-C¹⁴ incorporated C¹⁴ into fatty acids and CO₂. Insulin at physiological concentrations increased leucine uptake and conversion to fatty acids and usually decreased oxidation to C¹⁴O₂. In the absence of glucose, lipogenesis was sharply curtailed, but oxidation of leucine to C¹⁴O₂ was unchanged. In this situation, insulin failed to increase lipogenesis from leucine, although its stimulatory action on leucine uptake remained. Virtually all the leucine C¹⁴ taken up under the influence of insulin now appeared as C¹⁴O₂. Insulin similarly increased the uptake and conversion of acetate-2-C¹⁴ to fatty acids and decreased C¹⁴O₂ production in the presence of glucose but not in its absence. Addition of -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.

Key Words: acetate metabolism, adipose tissue • acetate metabolism, insulin • alpha-amino isobutyric acid, competition with leucine • fatty acid synthesis from leucine • leucine conversion to fatty acids • leucine transport, insulin • amino acid metabolism • rat epididymal fat

Submitted on July 19, 1963