Hydroxylation of Proline in vitro I. Hydroxyproline Formation in the System Containing Hydrogen Peroxide

Abstract

The conversion of proline into hydroxyproline in vitro was studied. The hydroxylation system consists of bivalent Fe bound in a complex, ascorbic acid and hydrogen peroxide. In total, 4 isomers of hydroxyproline (4-cis and 4-trans, 3-cis- and 3-trans-) were isolated. It was found that hydroxylation takes place also in the absence of metal ions. Effective components in this case are probably free radicals formed by the reduction of hydrogen peroxide by ascorbic acid. Ascorbic acid increases the conversion of proline into hydroxyproline, but is not an essential part of the reaction system and can be replaced by an increased amount of Fe. There is a non-specific hydroxylation caused by free radicals. The Fe ions can be replaced by Cu+ or Cr3+. In the model mixture hydroxylation of peptide-bound proline also takes place. The amount of hydroxyproline formed depends on the position of the proline residue and on the length of the chain. Hydroxyproline is also formed in aqueous solution of proline which has been irradiated by a Co bomb.