CYTOKININ OXIDASE - BIOCHEMICAL FEATURES AND PHYSIOLOGICAL SIGNIFICANCE

The catabolism of cytokinin in plant tissues appears to be due, in lar ge part, to the activity of a specific enzyme, cytokinin oxidase. This enzyme catalyses the oxidation of cytokinin substrates bearing unsatu rated isoprenoid side chains, using molecular oxygen as the oxidant. I n general, substrate specificity is highly conserved and cytokinin sub strates bearing saturated or cyclic side chains do not serve as substr ates for most cytokinin oxidases tested to date. Despite variation in molecular properties of the enzyme from a number of higher plants, oxy gen is always required for the reaction. Cytokinin oxidases from sever al sources have been shown to be glycosylated. Cytokinin oxidase activ ity appears to be universally inhibited by cytokinin-active urea deriv atives. Auxin has been reported to act as an allosteric regulator whic h increases activity of the enzyme. Cytokinin oxidase activity is subj ect to tight regulation. Levels of the enzyme are controlled by a mech anism sensitive to cytokinin supply. The up-regulation of cytokinin ox idase expression in response to exogenous application of cytokinin sug gests that the metabolic fate of exogenously applied cytokinins may no t accurately mimic that of the endogenous compounds. Cytokinin oxidase is believed to be a copper-containing amine oxidase (EC 1.4.3.6). Con siderable evidence strongly supports a common mechanism for amine oxid ases. It is possible that advances in understanding of other amine oxi dases could be extrapolated to increase our understanding of cytokinin oxidase at the molecular level. This is discussed with reference to w hat is currently known about the catalytic mechanism of the enzyme. Th e possibility of pyrroloquinoline quinone, or a closely related compou nd, as a redox cofactor of cytokinin oxidase is considered, as are the implications of the glycosylated nature of the enzyme for its regulat ion and compartmentalisation within the cell.