Effect of Insulin on the Synthesisin Vitroof Hexokinase in Rat Epididymal Adipose Tissue

The ability of insulin to increase the hexokinase content of epididymal adipose tissue was studied in vitro, using glucose, pyruvate, or alanine as energy source. Glucose alone caused an increase in hexokinase content which was maximal at 1–10 mm glucose when compared to controls incubated with no energy source. Insulin by itself was ineffective but, in the presence of 10 HIM glucose, caused a significant stimulation at 5×1O-4 U/ml. The effect of insulin with glucose present was significant when expressed per mg extract protein, per g tissue, or per fat pad. Pyruvate alone caused a significant increase in hexokinase activity /mg protein. Insulin led to a significant further increase. Insulin is thus, itself, a stimulator of hexokinase content when a suitable energy source is available. The presence of glucose, the substrate for the enzyme, is not necessary. The stimulation of hexokinase activity by insulin with glucose or pyruvate as energy source was blocked by the addition of actinomycin D to the incubation medium. By a double labeling technique it was demonstrated that insulin stimulates incorporation of histidine into hexokinase II protein. Insulin did not reduce the rate at which histidine was released from hexokinase II protein during the 16-hr incubation. The results are consistent with the view that the insulin stimulation of hexokinase II activity in adipose tissue is accompanied by de novo synthesis of hexokinase II protein. (Endocrinology 86: 57, 1970)