Polypeptides with nonsuppressible insulin-like and cell-growth promoting activities in human serum: isolation, chemical characterization, and some biological properties of forms I and II.

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Abstract
Serum contains a polypeptide with insulin-like activity not suppressible by insulin antibodies (NSILA). A large-scale isolation procedure for NSILA is described, starting from an acid ethanol extract of a Cohn fraction (precipitate B) obtained from human plasma. Two homogenous polypeptides with insulin-like and cell-growth promoting activities could be isolated by gel filtration, ion exchange chromatography and preparative polyacrylamide gel electrophoresis. Both components are slightly basic polypeptides with a minimal MW of 5800 .+-. 400. Both are single-chain molecules with 2 intrachain disulfide bridges each and no free sulfhydryl groups. NSILA I and II differ in their amino acid compositions. The N-terminal amino acid sequences are Gly-Pro-Glu- in NSILA I and Ala-Tyr-Arg- and Tyr-Arg- in NSILA II. Both NSILA I and II enhance net gas exchange in adipose tissue with a specific activity 60 times lower than that of insulin. In the range of 1-50 ng/ml, both substances stimulate [3H]thymidine incorporation into DNA of chick embryo fibroblasts. The same effect can be obtained with insulin but only at concentrations 50-100 times higher than those of NSILA. NSILA I and II are probably 2 forms of an insulin-like hormone with predominating effects on cell and tissue growth parameters.