CYTOKININ OXIDASE - BIOCHEMICAL FEATURES AND PHYSIOLOGICAL SIGNIFICANCE

The catabolism of cytokinin in plant tissues appears to be due, in large part, to the activity of a specific enzyme, cytokinin oxidase. This enzyme catalyses the oxidation of cytokinin substrates bearing unsaturated isoprenoid side chains, using molecular oxygen as the oxidant. In general, substrate specificity is highly conserved and cytokinin substrates bearing saturated or cyclic side chains do not serve as substrates for most cytokinin oxidases tested to date. Despite variation in molecular properties of the enzyme from a number of higher plants, oxygen is always required for the reaction. Cytokinin oxidases from several sources have been shown to be glycosylated. Cytokinin oxidase activity appears to be universally inhibited by cytokinin-active urea derivatives. Auxin has been reported to act as an allosteric regulator which increases activity of the enzyme. Cytokinin oxidase activity is subject to tight regulation. Levels of the enzyme are controlled by a mechanism sensitive to cytokinin supply. The up-regulation of cytokinin oxidase expression in response to exogenous application of cytokinin suggests that the metabolic fate of exogenously applied cytokinins may not accurately mimic that of the endogenous compounds. Cytokinin oxidase is believed to be a copper-containing amine oxidase (EC 1.4.3.6). Considerable evidence strongly supports a common mechanism for amine oxidases. It is possible that advances in understanding of other amine oxidases could be extrapolated to increase our understanding of cytokinin oxidase at the molecular level. This is discussed with reference to what is currently known about the catalytic mechanism of the enzyme. The possibility of pyrroloquinoline quinone, or a closely related compound, as a redox cofactor of cytokinin oxidase is considered, as are the implications of the glycosylated nature of the enzyme for its regulation and compartmentalisation within the cell.