Formation of auxin from tryptophan through action of polyphenols

Abstract

Phenols, under conditions leading to their oxidation, react with tryptophan to form the auxin indoleacetic acid. The action is catalyzed by polyphenolase enzymes of mung bean, oat, and sunflower, and occurs spontaneously at alkaline pH. Various orthodihydric phenols, or phenols readily oxidized to that form, are effective. The reaction is inhibited by replacement of air with N2, and by reducing and chelating agents. It can be duplicated by mushroom tyrosinase, by exposure of tryptophan to ethereal solutions of o-benzo-puinone, or by adding tryptophan to a prolyl-catechol pigment formed via either ferricyanide or enzymatic oxidation. It is shown experimentally that indolepyruvic acid and not tryptamine or indoleacetaldehyde is the probable intermediate in the conversion of tryptophan to indoleacetic acid by the enzyme system. These observations are consistent with current concepts of oxidatlve deamination of amino acids by quinonimines. The pertinence of this new pathway of auxin biosynthesis is discussed in regard to normal auxin biogenesis, wound regeneration, and radiation-induced hyperplasia.