Metabolism of Cytokinin: Deribosylation of Cytokinin Ribonucleoside by Adenosine Nucleosidase from Wheat Germ Cells

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Abstract
Adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) which catalyzes the deribosylation of N6-(Δ2-isopentenyl)adenosine and adenosine to form the corresponding bases was partially purified from wheat germ. This enzyme (molecular weight 59,000 ± 3,000) deribosylates the ribonucleosides at an optimum pH of 4.7 Km values for the cytokinin nucleoside and adenosine are 2.38 and 1.43 micromolar, respectively, in 50 millimolar Tris-citrate buffer (pH 4.7) at 30 C. The presence of adenosine and other cytokinin nucleosides inhibited the hydrolysis of N6-(Δ2-isopentenyl)adenosine but this reaction was insensitive to guanosine, uridine, or 3′-deoxyadenosine. It is hypothesized that an adequate level of “active cytokinin” in plant cells may be provided through the deribosylation of cytokinin riboside in concert with other cytokinin metabolic enzymes.