Biological properties of chemically modified insulins. I. Biological activity of proinsulin and insulin modified at A1-glycine and B29-lysine.

Beef insulin, pork proinsulin and four derivatives of beef insulin modified at the A1-B29 site on the molecular surface have been studied. Three derivatives had a synthetic crosslink between the A and B chains. Previous studies with these materials [2, 3 and 5] had demonstrated in vivo bioactivities which were much higher than those displayed in vitro. This paper reports experiments which explain this discrepancy. The analogues were administered at equimolar rates to anaesthetised greyhounds by a priming-dose constant infusion technique and the plasma concentrations achieved were estimated by radioimmunoassay. Proinsulin and the modified insulins were metabolised more slowly than insulin. Biopotency values, which related fall in plasma glucose concentration to the total administered dose of analogue, agreed broadly with published results of conventional in vivo bioassays. On the other hand, calculation of potency in relation to the serum concentration of analogue actually achieved, yielded results which agreed more closely with in vitro assay data. We conclude that for these analogues, reported discrepancies between in vitro and in vivo biopotencies can be largely explained by the different rates at which these materials are metabolised.