Kinetics and mechanism of the facile cyclization of histidyl-prolineamide to cyclo (His-Pro) in aqueous solution and the competitive influence of human plasma.
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 degrees 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 (t1/2 of 140 min at pH 6-7 and 37 degrees 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.
Møss
J
J
Royal Danish School of Pharmacy, 2 Universitetsparken, Copenhagen, Denmark.
Bundgaard
H
H
D002021:Buffers; D002851:Chromatography, High Pressure Liquid; D003500:Cyclization; D004355:Drug Stability; D006207:Half-Life; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010456:Peptides, Cyclic / Q000097:blood / Q000138:chemical synthesis*; D010879:Piperazines / Q000097:blood / Q000138:chemical synthesis*; D012996:Solutions
D002021:Buffers; D010456:Peptides, Cyclic; D010879:Piperazines; D012996:Solutions; C015924:histidyl-proline diketopiperazine