Metabolism of cytokinin: ribosylation of cytokinin bases by adenosine phosphorylase from wheat germ.

As part of the study of cytokinin metabolic pathways, an enzyme, adenosine phosphorylase (EC 2.4.2.-), which catalyzed the ribosylation of N(6)-(Delta(2)-isopentenyl)adenine, N(6)-furfuryladenine, and adenine to form the corresponding nucleosides, was partially purified from wheat (Triticum aestivum) germ. The pH optimum for the ribosylation of the cytokinins and adenine was from 6.5 to 7.8; for guanine and hypoxanthine it was from 7.0 to 8.5 At pH 7.2 (63 millimolar N-2-hydroxyethyl piperazine-N'-ethanesulfonic acid) and 37 C the K(m) for N(6)-(Delta(2)-isopentenyl)adenine was 57.1 micromolar; N(6)-furfuryladenine, 46.5 micromolar; adenine, 32.2 micromolar; and the V(max) for N(6)-(Delta(2)-isopentenyl)adenine, N(6)-furfuryladenine, and adenine were 134.7, 137.1, and 193.1 nanomoles per milligram protein per minute, respectively. The equilibrium constants of the phosphorolysis of N(6)-(Delta(2)-isopentenyl)adenosine and adenosine by this enzyme indicated that the reaction strongly favored nucleoside formation. This enzyme was shown to be distinct from inosine-guanosine phosphorylase based on the differences in the Sephadex G-100 gel filtration behaviors, pH optima, and the product and p-hydroxymercuribenzoate inhibitor studies. These results suggest that adenosine phosphorylase may play a significant role in the regulation of cytokinin metabolism.