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Binding of Proinsulin and Proinsulin Conversion Intermediates to Human Placental Insulin-Like Growth Factor I Receptors^*

PHILIP A. GRUPPUSO, BRUCE H. FRANK and ROBERT SCHWARTZ

Division of Pediatric Metabolism, Rhode Island Hospital, Providence Rhode Island 02903
Lilly Research Laboratories Indianapolis, Indiana 46285

Address requests for reprints to: Philip A. Gruppuso, Division of Pediatric Endocrinology and Metabolism, Rhode Island Hospital, 593 Eddy Street, Providence, Rhode Island 02903.

Insulin-like growth factor I (IGF-I) and proinsulin share similarities in both primary and tertiary structure. Proinsulin, endogenously secreted or exogenously administered, would, therefore, be expected to interact with IGF-I receptors. We determined the relative activities of IGF-I, insulin, proinsulin, and the proinsulin conversion intermediates in IGF-I radioreceptor assays using term human placental membranes. Insulin was approximately 0.5% as potent as IGF-I, and proinsulin was only 2% as potent as insulin. The six major proinsulin conversion intermediates were studied; all had activities intermediate between those of insulin and proinsulin. We conclude that the binding of proinsulin and the proinsulin conversion intermediates to IGF-I receptors is not of physiological significance at the concentrations occurring endogenously or after exogenous administration of proinsulin.

^* This work was supported by a subproject of Perinatal Emphasis Research Center Grant HD-11343 from the NIH (to R.S.), March of Dimes Grant 5-570 (to P.A.G.), the Rhode Island Hospital Research Fund, and Eli Lilly Co.

Recipient of Special Emphasis Research Career Award AM-01755 from the NIH.

Received December 28, 1987.