An enzyme mediating the conversion of zeatin to dihydrozeatin in phaseolus embryos.

A reductase catalyzing the conversion of zeatin to dihydrozeatin was detected in soluble fractions of immature Phaseolus vulgaris embryos. The enzyme was partially purified by ammonium sulfate fractionation and affinity, gel filtration, and anion exchange chromatography. NADPH was the only cofactor required for enzyme activity, and the pH optimum was 7.5 to 8.0. The enzyme did not recognize compounds closely related to zeatin, such as ribosylzeatin, cls-zeatin, O-xylosylzeatin, N(6)-(Delta(2)-isopentenyl)adenine, or N(6)-(Delta(2)-isopentenyl)adenosine. No conversion of dihydrozeatin to zeatin by the enzyme was observed. Two forms of the reductase could be separated by either gel filtration or anion exchange high performance liquid chromatography. The high molecular weight isozyme (M(r) 55,000 +/- 5,000) eluted as the second peak from the anion exchange column, while the low molecular weight isozyme (M(r) 25,000+/- 5000) was less negatively charged. The results suggest that side chain reduction occurs at the free base level. In addition, Phaseolus embryos are useful for the detection of zeatin-specific metabolic enzymes.