Enzymes of Nucleic Acid Metabolism from Mung Bean Sprouts
II. THE SPECIFICITY OF 3'-NUCLEOTIDASE ACTIVITY AND GENERAL PROPERTIES OF THE 3'-NUCLEOTIDASE-RIBONUCLEASE M2 FRACTION
Hubert S. Loring 1, J. E. McLennan 1, and Tom L. Walters 1
From the
1 From the Department of Chemistry, Stanford University, Stanford, California 94305
The 3'-nucleotidase present in mung bean sprouts was found to hydrolyze the 3'-monophosphates of adenosine, guanosine, uridine, and cytidine in the order 3'-AMP > 3'-GMP > 3'-UMP > 3'-CMP, and also to hydrolyze the 3'-phosphate group of coenzyme A, but showed no significant activity for 2'- or 5'-ribonucleotides or for several non-nucleotide phosphomonoesters. pH optima and the respective Km values for 3'-AMP, 3'-GMP, 3'-CMP, and 3'-UMP were determined. The 3'-nucleotidase and ribonuclease M2 were inactivated reversibly at pH 5 and by dialysis and irreversibly by ethylenediaminetetraacetate. Inactivation of both enzymes at pH 5 could be largely prevented by the presence of Zn++, while several other metal ions were ineffective. The optimal concentrations of Zn++ for preventing inactivation were considerably different for the 3'-nucleotidase and RNase M2. The two enzyme activities were similarly inactivated by heat at pH 5 and 7.5. The pH optimum of RNase M2 was pH 5 with no Zn++ present, but dropped to pH 4.5 in the presence of 5 x 10-5 m Zn++, which also greatly increased its activity in the pH range 3.5 to 4.5. Several lines of evidence suggesting that the 3'-nucleotidase and RNase M2 activities may be due to the same protein are presented.
Submitted on December 8, 1965
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