Sera were treated with Dowex-50 W resin and the fraction which escaped adsorption, and that which was adsorbed and subsequently eluted, were assayed for insulin-like activity (ILA) using the rat epididymal fat pad technic. Previous investigators have stated that these two fractions contain, respectively,“free” and “bound” insulin.
In individual normal and diabetic sera assayed by thismethod, the “free” fraction accounted for all of the ILA present in the original whole serum. “Bound” insulin was detectable in small quantities when measured by incorporation of glucose-1-C-14 into C-14-O2, but not by incorporation into glycogen. The addition of an adipose tissue extract did not increase the ILA of the eluate nor modify it so that it was neutralized by an anti-insulin serum.
A lyophilized resin eluate from pooled fasting normal human sera, stated to contain “bound” insulin, also had a low level of ILA. This activity did not increase after acid alcohol extraction or at pH 9.8. It was not neutralized by anti-insulin serum either before or after these treatments.
These experiments do not support the concept of a “bound” insulin.