Demonstration of a Specific Serum Carrier Protein of Nonsuppressible Insulin-like Activity in Vivo*

The fate of nonsuppressible insulin-like activity (NSILA-S) was studied by injecting a tracer of 125I-NSILA-S iv into rats. Ten minutes after injection of 125I-NSILA-S alone, 20% of the label is found in serum, whereas after the injection of 125-I-insulin or 125I-NSILA-S together with an excess of cold NSILA-S only 8% of the label are recovered. Sephadex G-200 chromatography at neutral pH of serum after injection of 125I-NSILA-S reveals 2 peaks of radioactivity in the high molecular weight region at 67 and 47% bed volume. Five minutes after injection the peak at 67% starts to disappear, whereas the one at 47% persists with a half-life of 3 h. The latter peak was submitted to Sephadex G-200 chromatography at acidic pH which dissociates NSILA-S from its binding protein. The labeled material obtained by this procedure still exhibits the same binding characteristics to chick embryo fibroblasts as standard 125 I-NSILA-S. A third peak at 90% bed volume corresponding to low molecular NSILA-S is no longer detectable 20 min after injection. A fourth peak at 100% bed volume becomes apparent after 1 h. The half-life and chromatographic pattern of iv injected 125 I-NSILA-S are not changed by the simultaneous administration of insulin or growth hormone. These findings confirm the existence of a specific serum carrier protein for NSILA-S and may explain why endogenous NSILA-S does not exert insulin-like effects under physiological conditions in vivo.