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In the trypsin-mediated semisynthetic transformation of pig insulin into an ester of insulin of human sequence, the B30 alanine residue of the pig hormone is replaced by an ester of threonine. The mechanism of this reaction was investigated by carrying out the transformation in a medium containing water enriched with 18O. Subsequent analysis by combined g.l.c.-mass spectrometry demonstrated that the oxygen isotope, is incorporated into the B29 carbonyl group of the insulin ester product. This result, together with those of appropriate controls, supports the conclusion that the transformation occurs, in the system studied, by a mechanism involving hydrolysis followed by coupling, and not by direct transpeptidation as has been previously found the case for another such system [Markussen &amp; Schaumberg (1983) in Peptides 1982 (Bláha &amp; Malon, eds.), pp. 387-394, W. de Gruyter, Berlin and New York].

A mass-spectrometric investigation of the mechanism of the semisynthetic transformation of pig insulin into an ester of insulin of human sequence