Methionyl-tRNA Synthetase from Sheep Mammary Gland. Purification of a Fully Active Monomeric Enzyme Derived from High-Molecular-Weight Complexes by Controlled Proteolysis

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Abstract
Methionyl-tRNA synthetase from sheep lactating mammary gland is found predominantly in the form of high MW complexes. Controlled proteolysis of these aggregates generates a low-MW species of the enzyme with full maintenance of activity as assessed by the rate of aminoacylation of tRNA. The product of proteolysis, which was purified to homogeneity with a yield of 23%, is a monomeric enzyme of MW 78,000. It has a specific activity of 405 units/mg at 24.degree. C. These findings clearly demonstrate that the aggregated state of methionyl-tRNA synthetase is not a prerequisite for full expression of catalytic activity. The results emphasize the need to provide effective protection against proteolytic damage in studies dealing with the characterization of high MW complexes of aminoacyl-tRNA synthetases.