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Articles

Phosphorylation of Cytokinin by Adenosine Kinase from Wheat Germ ¹

^a Science Division, University of Wisconsin-Parkside, Kenosha, Wisconsin 53140

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-(²-isopentenyl)adenosine. Electrophoretic analysis indicated that only N⁶-(²-isopentenyl)adenosine-monophosphate 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 to 7.2 for both the cytokinin and adenosine. At pH 7 and 37 C the Km and V_max for the cytokinin were 31 µM and 8.3 nmoles per mg protein per minute, and the values for adenosine were 8.7 µM and 46 nmoles per mg protein per minute. Crude enzyme preparations from tobacco callus tissue and wheat germ phosphorylated N⁶-(²-isopentenyl)adenosine. These preparations also phosphorylated N⁶-(²-isopentenyl)adenine when 5-phosphorylribose-1-pyrophosphate was present.

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