Phosphorylation of Cytokinin by Adenosine Kinase from Wheat Germ

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Abstract
Adenosine kinase was partially purified from wheat germ. This enzyme preparation, which was devoid of adenine phosphoribosyltransferase and nearly free of adenosine deaminase but contained adenylate kinase, rapidly phosphorylated adenosine and a cytokinin, N6-(.DELTA.2-isopentenyl)adenosine. Electrophoretic analysis indicated that only N6-(.DELTA.2-isopentenyl)AMP was formed from the cytokinin while about 55% AMP, 45% ADP and a trace of ATP were formed from adenosine. The biosynthesized nucleoside monophosphates were quantitatively hydrolyzed to the corresponding nucleosides by 5''-nucleotidase and the isopentenyl side chain of the phosphorylated cytokinin was not cleaved. The enzyme did not catalyze phosphorylation of inosine. The phosphorylation of the cytokinin and adenosine required ATP and Mg2+. The pH optimum was from 6.8-7.2 for both the cytokinin and adenosine. At pH 7 and 37.degree. C the Km and Vmax for the cytokinin were 31 .mu.M and 8.3 nmol/mg protein per min, and the values for adenosine were 8.7 .mu.M and 46 nmol/mg protein per min. Crude enzyme preparations from tobacco callus tissue and wheat germ phosphorylated N6-(.DELTA.2-isopentenyl)adenosine. These preparations also phosphorylated N6-(.DELTA.2-isopentenyl)adenine when 5-phosphorylribose-1-pyrophosphate was present.