Evidence for a biologically active, carrier-bound form of non-suppressible insulin-like activity (NSILA) in human serum.

1. Quantitative gel filtration of Cohn fraction IV-1 has shown that high MW NSILA recovered under "neutral" conditions (pH 5.5) was greater in magnitude (approximately X 2) than acid-stable high MW NSILA obtained under acid conditions. 2. Recombination, at neutral poH, of high and low MW NSILA, obtained by previous acid gel filtration, gave quantitative recovery of activity which was solely of high MW. 3. These data indicate that a biologically active, and dissociable high MW form of NSILA exists in serum. 4. Con A-Sepharose affinity chromatography gave one unbound NSILA fraction and two bound fractions, which both gave high and low MW activity after acid chromatography. 5. This study provides the first evidence for a carrier-bound form of low MW NSILA which retains biological activity.