Abstract
In order to investigate the base specificity of the RNase from Rhizopus sp. (RNase Rh), its kinetic parameters were measured with 16 dinucleoside phosphates (XpY, where X or Y represents one of adenosine, guanosine, uridine and cytidine) as substrates at pH 5.5 and 25°. The average Km values of ApY, GpY, CpY and UpY increased in the order A, G, C and U. The average Km values of ApY, GpY, CpY and UpY increased in the order A, G, U and C. The average Vmax values of ApY, GpY, CpY and UpY were larger for dinucleoside phosphates having A and G at X. However, the average Vmax values of XpA, XpG, XpC and XpU were relatively constant. It was concluded that the X base in XpY contributes mainly to the specificity of the enzyme and the Y base may modify this somewhat. The K1 values of various nucleotides towards RNase Rh were measured at pH 5.5. These compounds inhibited RNase Rh competitively. Although the inhibitory effect depends on the base, sugar and location of the phosphate moiety, for a given location of phosphate on the sugar, the K1 values of ribonucleotides decreased in the order U, C, G and A and those of deoxyribonucleotides decreased in the order T, C, G and A. These data also show that purine bases, especially adenine, have high affinities for RNase Rh.
