Regulation of a Plant 5′(3′)-Ribonucleotide Phosphohydrolase by Cyclic Nucleotides and Pyrimidine, Purine, and Cytokinin Ribosides

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A highly specific 5′(3′)-ribonucleotide phosphohydrolase has been purified extensively from wheat seedling leaves. While catalyzing the hydrolysis of a wide variety of phosphomonoesters, the enzyme has Km values for adenosine 5′-monophosphate and adenosine 3′-monophosphate in the micromolar range and appears highly specific for 5′- and 3′-ribonucleoside monophosphates as substrates at pH 5.0. The cyclic nucleotides adenosine 3′:5′-cyclic monophosphate, guanosine 3′:5′-cyclic monophosphate, and 8-bromoadenosine 3′:5′-cyclic monophosphate, as well as 8-bromoadenosine 5′-monophosphate, are powerful competitive inhibitors of the enzyme; the apparent Ki values for these nucleotides are 3.4 μM, 1.6 μM, 26 μM, and 9.1 μM, respectively. The enzyme is inhibited noncompetitively by a variety of pyrimidine and purine (including cytokinin) ribosides and 2′-deoxyribosides. Since the cyclic nucleotide competitive inhibitors are also active growth promoters of higher plants, and, furthermore, since elevation of cyclic AMP levels appears to be a consequence of some phytohormone treatments that promote plant growth, it is suggested that negative control of this 5′(3′)-ribonucleotide phosphohydrolase may be a significant component of growth regulation through the maximizing of the levels of nucleotide precursors for RNA and DNA synthesis.

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