Incorporation of H-3-leucine into proinsulin and insulin and insulin secretion were determined in isolated pancreatic islets of mice. Batches of forty islets were incubated for three hours at 0, 100 or 300 mg. per cent glucose alone or in the presence of glucagon, theophylline or cyclic AMP. Proinsulin and insulin were separated by gel filtration on columns of Sephadex C50fine in 1 M. acetic acid.

In the absence of glucose, no stimulation of insulin biosynthesis as judged from the incorporation of H-3-leucine was observed with glucagon, cyclic AMP or dibutyryl cyclic AMP; insulin, secretion was not significantly stimulated. At 100 and 300 mg. per cent glucose, both leucine incorporation and insulin secretion were potentiated by the substances tested; the effect of glucagon was pronounced mainly at high concentrations of glucose. However, leucine incorporation and insulin secretion were not stimulated to the same extent in all cases.

Biosynthesis of insulin thus appears to be not always directly linked to insulin secretion. It is suggested that glucagon and the adenylcyclase system, besides regulating insulin secretion, are involved in insulin biosynthesis.

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