The effect of insulin on the conversion of pyruvate into fatty acids in the presence and in the absence of glucose was studied in epididymal adipose tissue of the rat. 1. In adipose tissue from the normal rat, conversion of pyruvate into fatty acids is directly related to its concentration, the maximal rates occurring with 40mm- and the half-maximal rates with approx. 4mm-pyruvate. Insulin treatment did not greatly influence the maximal rates, but the half-maximal rates were at much lower pyruvate concentrations. This effect of insulin could be seen with physiological concentrations of this hormone (50–100μunits/ml). 2. In adipose tissue from acute-alloxan-diabetic and 36h-starved rats the conversion of pyruvate into fatty acids was almost zero until its concentration exceeded 3mm and then increased markedly as the concentration of pyruvate was increased. The lag phase of this S-shaped curve was decreased but not eliminated when insulin was present. This could account for the very low rates of glucose conversion into fatty acids in these metabolic states. Maximum rates of fatty acid synthesis were similar in the presence and in the absence of insulin, but only when 30–40mm-pyruvate was employed. Re-feeding of the starved rats or insulin treatment of the diabetic rats in vivo for several days restored these patterns to normal.

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