Current methods used in the preparation of modified insulins for investigating structure-activity relations are briefly discussed. Several chemical degradation and semisynthetic procedures are outlined which were used in the authors' laboratory for the preparation of molecularly homogeneous insulin analogs and derivatives. Des-PheB1 insulin was obtained through selective Edman degradation of insulin, Arg-GlyAl insulin was synthesized by elongation of A-chain S-sulfonate, and this was followed by combination of Arg-GlyA1 chain with natural B chain S-sulfonate. The preparation of [p-iodo-PheB1] insulin was accomplished by removal and replacement of PheB1. Monoacetyl insulins, substituted at GlyA1 or LysB29, respectively, were obtained via partial acetylation of insulin as well as by acetylation of the separate chains. The five compounds could be crystallized in the presence of zinc.
Two insulin derivatives with split A7-B7 disulfide bond, [di(S-carboxymethylcysteine)A7,B7] insulin and [di(S-sulfocysteine)A7,B7] insulin, were obtained through selective electrolytical reduction or partial sulfitolysis.
Some biological and immunological properties of these and seven other modified insulins are presented. The N-terminus of the A chain is highly sensitive to chemical alterations, with the biological activity decreasing in the following order: substitution > removal of the NH2 group > removal of Gly > removal of Gly and lie. The deletion of PheB1 causes no activity loss; the A7-B7 bond can be cleaved and full or at least partial activity be retained.
In view of the scope and limitations of both semisynthetic and synthetic methods, both approaches will be required in future work to prepare potentially important analogs based upon predictions from the three-dimensional structure of insulin as determined by x-ray analysis.