Structure-function relationships of des-(B26-B30)-insulin.

In order to study the role of the amino acid in position B25 and its environment in shortened insulins, a series of analogues was prepared with the following modifications: 1, Stepwise shortening of the B-chain including replacements of TyrB26 and ThrB27 by glycine; 2, substitutions at the carboxamide nitrogen of des-(B26-B30)-insulin-B25-amide by apolar, polar or charged residues of various chain lengths; 3, replacement of PheB25 by asparagine-amide, phenylalaninol or a series of alkyl and aralkyl residues. Trypsin-catalyzed semisyntheses were performed with Boc-protected or unprotected des-octapeptide-(B23-B30)-insulin and synthetic peptides. Relative receptor binding and in vitro bioactivity of [AsnB25]-des-(B26-B30)-insulin-B25-amide was 227 and 292% (on insulin), other activities ranged between 1 and ca. 200%. We make the following conclusions. An L-amino acid is essential in position B25. The B25-carbonyl and NH groups favour high binding and "superpotency", but are not indispensible for receptor contacts. For high affinity receptor interaction, the planarity at the C gamma-atom and the distance of B25-side-chain branching in position B25 are important, but an aromatic ring is not necessary.