Insulin-like activity (ILA) and nonsuppressible insulinlike activity (NSILA) of fasting normal human serum have been studied following fractionation by cation resin chromatography, or extraction with acid-ethanol or ethanol. Studies of solubility, heat stability and immunosuppressibility suggest three different fractions, none of which is immunoassayable insulin. The total activity of the fractions was usually several times that of the original serum.

“Bound” NSILA, which binds to a cation resin column, appears to be a smaller molecule than “free insulin” NSILA found in the column effluent. Our studies suggest that the column effluent NSILA is converted to the smaller “bound” NSILA by acid-ethanol extraction. Although the column effluent NSILA is partially suppressible with guinea pig antiserum, only a trace amount of immunoassayable insulin was found in this fraction.

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