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

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 K(m) 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 K(i) values for these nucleotides are 3.4 muM, 1.6 muM, 26 muM, and 9.1 muM, 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.