FUNCTIONAL IMPORTANCE OF THE A-CHAIN LOOP IN RELAXIN AND INSULIN

Relaxin and insulin are disulfide homologues with divergent functions and antigenicity. We have synthesized human relaxin II and porcine ins ulin and several A chain loop variants of each and measured the effect of substitutions in vivo and in vitro. Substitution of Ile(A10) in in sulin with glycine reduced the receptor binding ability by 2 orders of magnitude. Conversely, exchange of the glycine A14 residue in relaxin , which corresponds to the insulin position A10 for isoleucine, reduce d the bioactivity and the receptor binding capacity of relaxin about 1 00-fold. Substitution of L-Ala in insulin as well as relaxin represent ed a compromise that allowed both hormones to recover about 30% of the native potency. X-ray analysis and computer derived energy calculatio n confirm our receptor binding and biological potency studies, which s uggest that the functional difference be tween derivatives and native hormones is based upon a structural change introduced into the A chain loop by substitution of the penultimate, intrachain loop residue. In order to achieve a conformation that favors dynamic or passive interac tion with the receptor, insulin and relaxin require a different A chai n loop structure in spite of the striking overall similarity.