Proinsulin, like insulin, has a biphasic effect on the lipolysis of isolated fat cells. Low concentrations of the peptides inhibit the response to submaximal doses of epinephrine, while higher concentrations of the peptides enhance the effect of supramaximal doses of epinephrine. Half-maximal doses are: (a) inhibition of lipolysis: insulin, 2.3 × 10−11M; proinsulin, 6.9 × 10−10M; (b) enhancement of lipolysis: insulin, 3.1 × 10−9M; proinsulin, 8.1 × 10−8M. The ratios of the potencies of the peptides are the same for both effects. These results suggest that both the inhibitory and the lipolytic effects of insulin are not caused by contaminants, and are mediated by similar receptors.

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