Isolation and partial purification of an enzyme catalyzing the formation of O-xylosylzeatin in Phaseolus vulgaris embryos

An enzyme catalyzing the formation of a cytokinin metabolite, an O-pentosyl derivative of zeatin [Lee, Y. H., Mok, M. C., Mok, D. W. S., Griffin, D. A. & Shaw, G. (1985) Plant Physiol. 77, 635-641], was isolated from Phaseolus vulgaris embryos. Of all the potential pentose donors tested, UDP-xylose was the only substrate recognized by the enzyme. This indicates that the O-pentosyl derivatives previously obtained are O-xylosylzeatin and its ribonucleoside. The enzyme (UDP-xylose:zeatin O-xylosyltransferase, EC 2.4.2.-) has high affinity for trans-zeatin (K_m 2 μM) and dihydrozeatin (K_m 10 μM) but does not recognize cis-zeatin or ribosylzeatin. The molecular weight of the enzyme is approximately 50,000 and the pH optimum of the reaction is 8-8.5. Under comparable isolation and reaction conditions, similar enzyme activity could not be detected in P. lunatus embryos, confirming the genetic differences observed in vivo.