| Abstract: | Rat liver cytoplasmic tyrosine:tRNA ligase (tyrosine:tRNA ligase, EC 6.1.1.1) was purified by ultracentrifugation, DEAE-cellulose chromatography and repeated phosphocellulose chromatography by more than 1500-fold. The molecular weight of the enzyme was approx. 150 000 as determined by Sephadex G-200 gel filtration. On the basis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the enzyme consisted of two subunits, each of 68 000 daltons. We found the following Km values for the enzyme: 13 micrometer for tyrosine and 1.7 mM for ATP in the ATP:PPi exchange reaction and 13 micrometer for tyrosine, 210 micrometer for ATP and 0.14 micrometer for tRNATyr in the aminoacylation reaction. The rate of tyrosyl-tRNA synthesis was 50-fold lower than that of ATP:PPi exchange. Addition of a saturating amount of tRNA did not affect the rate of ATP:PPi exchange. |
