Properties of the insulin receptor of rat pancreatic islet

Rat pancreatic islets have been shown to possess specific binding sites for 125I-labeled insulin. Enzymatic and chemical modification of islets are used to reveal important structures and chemical groups for insulin binding. pretreatment with trypsin, neuraminidase, 1-ethyl-3-(3-dimethylamino)carbodiimide (a carboxyl reagent), tetranitromethane (a tyrosyl and thiol reagent), and 1,3-difluoro-4,6-dinitrobenzene (modification of protein functional groups) decreased binding of insulin. This was due to the diminuation of the receptor number; in the case of trypsin-pretreatment also the receptor affinity was decreased. Inhibition of insulin binding was in each case associated with a decrease of the inhibitory effect of exogenous insulin on glucose-induced insulin secretion (not measured in the case of difluorodinitrobenzene and tetranitromethane). Phospholipase A2, (cleavage of phospholipids) did not affect these parameters. 5,5'-dithiobis(2-nitrobenzoic acid) (Ellman's reagent) and possibly p-chloromercuribenzoate (both thiol reagents) increased the number of receptors and decreased receptor affinity, but did not influence the inhibitory effect of insulin on insulin release. It is concluded that protein functional groups, sialic acid, carboxyl and tyrosyl groups, but no phospholipids and probably not sulfhydryl groups are important for the interaction of insulin with insulin receptors of rat pancreatic islets.