KINETICS AND MECHANISM OF THE FACILE CYCLIZATION OF HISTIDYL-PROLINEAMIDE TO CYCLO (HIS-PRO) IN AQUEOUS-SOLUTION AND THE COMPETITIVE INFLUENCE OF HUMAN PLASMA

Abstract— A crucial point in the biosynthesis of cyclo (His‐Pro), an endogenous and biologically active cyclic dipeptide, is the spontaneous cyclization of its precursor L‐histidyl‐L‐prolineamide (His‐ProNH2). In this study the kinetics and mechanism of the cyclization process has been investigated. His‐ProNH2 was found to be converted quantitatively to cyclo(His‐Pro) in aqueous solution at pH 2–10 and 37°C, the rate of cyclization being maximal at pH 6–7. Buffer substances such as phosphate (pH 6–7.4) were found to catalyse the cyclization. The bell‐shaped pH‐rate profile observed was accounted for by assuming spontaneous and specific acid‐ and base‐catalysed reactions of the His‐ProNH2 species in which the imidazole group is protonated and the primary amino group unprotonated. The much more rapid rate of cyclization of His‐ProNH2 (t 1/2 of 140 min at pH 6–7 and 37°C) relative to other proline‐containing di‐ and tripeptides studied was suggested to be due to an intramolecular general acid catalytic effect by the protonated imidazole group. In the presence of human plasma enzymatic hydrolysis of His‐ProNH2 competed with the cyclization and predominated greatly at 80% plasma concentration. 1990 Royal Pharmaceutical Society of Great Britain