Insulin analog possessing higher in vitro biological activity than receptor binding affinity. [21-Proline-B]insulin

To study the effect of an increase in the potential for beta-turn formation of the B20-B23 segment of the B-chain moiety on the biological behavior of insulin, the [21-proline-B]insulin [( Pro21-B]insulin) was synthesized. The in vitro biological activity and the receptor binding affinity of this analogue were compared with that of insulin. In stimulating labeled glucose incorporation into lipids in rat fat cells, the analogue displayed 33.2% potency relative to insulin; receptor binding affinity for the analogue was 15.9% in rat liver membranes and 17.8% in isolated fat cells. [Pro21-B]insulin is thus the first example of a modified insulin for which the biological activity exceeds the receptor binding potency. The secondary structure of this analogue was investigated by circular dichroism studies. Although no significant differences in the conformation of monomeric insulin and analogue could be discerned, their difference in behavior with respect to dimerization and biological properties indicates that these forms are not equivalent. We suggest that the intrinsic activity of receptor-bound [Pro21-B]insulin is greater than that of insulin, although the receptor displays greater affinity for insulin than for the analogue. We consider a model for the interaction between insulin and its receptor that accommodates our findings.