Purification and some properties of the histidyl-tRNA synthetase from the cytosol of rabbit reticulocytes

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Abstract
The histidyl-tRNA synthetase of rabbit reticulocyte cytosol was purified 84,000-fold to apparent homogeneity with a specific activity of 687 nmol of histidyl-tRNA formed/min per mg of protein. Ten to 15% of the enzyme activity is sedimented with the ribosomes while the remainder is in the cytosol. The purified enzyme has a MW of 122,000 as determined by sucrose density gradient centrifugation. Gel electrophoresis in the presence of 0.1% sodium dodecyl sulfate suggests that it is composed of 2 similar subunits with a MW of .apprx. 64,000. The enzyme has a Mg optimum of 45 mM; however, this is reduced to 5 mM in the presence of an intracellular K concentration (160 mM). The enzyme acylates the 2 histidine tRNA isoacceptors of rabbit reticulocytes with similar Km values and at similar rates.