STUDIES ON THE STATE OF INSULIN IN BLOOD:“FREE” INSULIN AND INSULIN COMPLEXES IN HUMAN SERA AND THEIRIN VITROBIOLOGICAL PROPERTIES1

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Abstract
Studies are presented on the relative amounts of the protein-bound and the “free” forms of insulin in the sera of non-diabetic subjects. The “bound” form of insulin was extracted from sera by a cationic exchange resin (Na+ cycle) which adsorbed only the “bound” form of insulin. The insulin-protein complexes (basic protein-insulin complexes) in turn were eluted from the resin by alkali. The resin eluates, representing the “bound” form of insulin, were examined for insulin activity by the rat diaphragm assay in the presence of adipose tissue extracts which, as it has been shown, dissociate insulin from its complex. The insulin-like activity present in the whole serum is characterized as the “free” portion of insulin. This characterization resulted from earlier studies demonstrating that insulin in its “bound” form is devoid of insulin activity when examined under specified conditions. It is shown further that incubation of whole diluted serum with adipose tissue extract results in a significant increase of glucose uptake by the intact rat diaphragm and the rat adipose tissues. The increase of glucose uptake under these conditions is attributed to the dissociation of the “bound” insulin of serum by a factor or factors present in the adipose tissue extracts. These studies indicate again that insulin circulates in the blood in “free” and “bound” forms. These two forms of insulin differ in their biological properties in that the “bound” insulin exerts its biological activity only after dissociation of the insulin complex.