The following proteins were analyzed for insulin-like behavior in the rat hemidiaphragm, rat fat pad, and two antibody immunoassay systems: ribonuclease A, deoxyribo-nuclease I, alpha-chymotrypsin, chymotrypsinogen A, tryp-sin, and trypsinogen. Chymotrypsin stimulated glucose uptake in the rat hemidiaphragm and behaved like insulin in the immunoassay system when the enzyme concentration was 7 × 10-6 M. No similar activity was found for any of the other proteins. Chymotrypsin was inactive at lower concentrations.

The stimulation of glucose uptake in the rat hemidiaphragm by chymotrypsin was not prevented by the presence of guinea pig anti-insulin serum. This and other evidence suggests that the enhanced uptake of sugar is not due to insulin contamination and is probably a result of partial proteolysis of cell membrane with resulting increase in permeability.

Mixtures of chymotrypsin with guinea pig anti-insulin serum and 1-131-labeled insulin under the conditions of the immunoassay were analyzed electrophoretically, and the results indicate partial hydrolysis of the labeled insulin. This hydrolysis probably accounts for the apparent insulin response produced by chymotrypsin in the two antibody immunoassay system.

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