Purification and Characterization of Tyrosyl-RNA Synthetase of Baker's Yeast*

Abstract

Tyrosyl-tRNA synthetase [EC 6. 1. 1. 1] was purified from baker's yeast. The purified preparation had no cross reaction with 19 other amino acids. Three kinds of assay involving hydroxamate formation, ATP-PP₁ exchange and tyrosyl-tRNA formation were adopted to characterize the enzyme. It was observed that the K_m values for tyrosine obtained with the above three methods were not the same. Among many tyrosine analogues only 3-iodo-tyrosine and D-tyrosine stimulated the ATP-PP₁ exchange reaction, while tyrosol and tyramine were competitive inhibitors. Based on these results it was concluded that the hydroxyl group at the para-position and the α-amino group of the substrate have an essential role in binding to the enzyme. The effects of several metal ions upon enzymatic activity were studied. The effects differed remarkably depending upon the assay methods.