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Mitochondrial aspartate aminotransferase from beef kidney.

Mitochondrial aspartate aminotransferase has been purified in a very simple way by taking advantage of its high isoelectric point (9.8). The beef kidney homogenate was fractionated between 60–90% saturation with ammonium sulfate, dialyzed and percolated through a DEAE-cellulose column at pH 9.5, then through a Sephadex G-100 column. 
 
 
 
The enzyme was shown to be pure by ultracentrifugation and starch electrophoresis: polyacrylamide gel electrophoresis revealed slower components. The molecular weight was found to be 93000: by use of sodium dodecylsulphate, the enzyme can be resolved into two subunits of 46000 molecular weight, probably with different amino-terminals one of which is serine. Fourteen cysteine residues have been detected per molecule of protein. The holoenzyme can be resolved easily to the apoenzyme and this latter titrated with pyridoxal-5′-phosphate: with this method and by chemical procedure, 2 mol coenzyme per mol protein were found. 
 
 
 
The enzyme has a high specificity for the substrate-pair aspartate 2-oxoglutarate, but shows also an affinity for glutamate-phenylpyruvate and glutamate-hydroxyphenylpyruvate: the apparent Km values for keto acids were found to be respectively 2.5, 33 and 35 mM. The affinity for these aromatic keto acids, though modest, seems peculiar to the mitochondrial aminotransferases, whereas the soluble enzymes, such as that from pig heart, have an extremely low or no affinity for these substrates.