Metabolism of Cytokinin

Two forms (F-I and F-II) of 5'-nucleotidases (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) which catalyze the dephosphorylation of N(6)-(Delta(2)-isopentenyl)adenosine 5'-monophosphate and AMP to form the corresponding nucleosides were partially purified from the cytosol of wheat (Triticum aestivum) germ. Both the F-I (molecular weight, 57,000) and F-II (molecular weight, 110,000) 5'-nucleotidases dephosphorylate the ribonucleotides at an optimum pH of 7. The K(m) values for the cytokinin nucleotide are 3.5 micromolar (F-I enzyme) and 12.8 micromolar (F-II enzyme) in 100 millimolar Tris-maleate buffer (pH 7) at 37 C. The F-I enzyme is less rapidly inactivated by heating than is the F-II enzyme. Both nucleotidases hydrolyze purine ribonucleoside 5'-phosphates, AMP being the preferred substrate. N(6)-(Delta(2)-isopentenyl)Adenosine 5'-monophosphate is hydrolyzed at a rate 72 and 86% that of AMP by the F-I and F-II nucleotides, respectively. Phenylphosphate and 3'-AMP are not substrates for the enzymes. It is proposed that dephosphorylation of cytokinin nucleotide by cytosol 5'-nucleotidases may play an important role in regulating levels of "active cytokinin" in plant cells.