Formation of Isodityrosine by Peroxidase Isozymes

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Abstract
Fry, S. C. 1987. Formation of isodityrosine by peroxidase isozymes.—J. exp. Bot. 38: 853–862. Tyrosine residues of extensin are oxidatively coupled in vivo to form isodityrosine bridges, whereas treatment of purified extensin with H2O2+ peroxidase in vitro yields only dityrosine. Two explanations for the correct mode of coupling in vivo were tested. The first, that the pH of the cell wall is lower than that (pH 9-0) at which in vitro experiments have been conducted, provided part of the answer since treatment of L-tyrosine with H2O2+peroxidase in vitro at pH 37–5 yielded some isodityrosine. The second, that the wall contains other isozymes of peroxidase than the basic isozyme usually studied in vitro, appeared unlikely because several sharply contrasting isozymes yielded similar isodityrosine: dityrosine ratios from L-tyrosine+ H2O2 at any given pH. The isozymes were also similar in their ability to oxidize tyrosine-dimers further to higher polymers. It is concluded that the formation of isodityrosine in vivo is dictated by neighbouring wall molecules, possibly ionically-bound pectins, which modify the local environment of the tyrosine residues of extensin.