Abstract
The effect of bovine proinsulin and related factors on the glucose metabolism of isolated, rat, fat cells was studied. The dose-response curve of proinsulin was parallel to that of insulin and the action of the two proteins showed an identical time course. The activity of single chain proinsulin was about 0.5 U/mg (as estimated from its ability to increase the conversion of14C-glucose to14CO2 or to14C-glycerides by fat cells) and rose to about 17 U/mg after treatment with trypsin. — The activity of insulin was quantitatively recovered in the presence of proinsulin. Split chain proinsulin showed an activity of 5 U/mg, which rose to about 18 U/mg after treatment with trypsin. Connecting peptide did not influence the glucose metabolism in the absence or presence of insulin. — There was no conversion of proinsulin in the isolated cell incubation medium to insulin or a similar molecule with high biological activity. The activity was the same on rat epididymal fat pads as on isolated fat cells, and there was no significant suppression by Kunitz pancreatic trypsin inhibitor in either system. — The following was concluded: the biological activity of proinsulin on rat isolated fat cells and epididymal fat pads is about 2 per cent of that of insulin, and the effect is caused by the proinsulin molecule itself. The reason for the low biological activity is presumably a smaller affinity for insulin receptors.
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