Ornithine aminotransferase : Purification and Properties from Beef Kidney and Molecular Weight from Rat Liver

Abstract

Ornithine aminotransferase was purified from beef kidney 100-fold by acetone fractionation and DEAE-cellulose chromatography. Heat, pH, and ammonium sulfate treatments were found to be impractical as purification procedures. The enzyme from beef kidney was less stable to temperature and pH changes than it is from other sources. The specific activity of the enzyme in the medulla was about six times greater than it is in the cortex. The pH optimum was found to be 8.0. The Michaelis constants for L-ornithine and -ketoglutarate were 2.0 mM and 7.1 mM respectively. The enzyme was inhibited by para-chloromercuribenzoic acid and iodoacetamide showing that sulfhydryl groups are required for enzyme activity. A metal ion is not required for activity as evidenced by stability to EDTA. L-norvaline inhibited the reaction by 43%, but glutamate, a product of the reaction, did not inhibit significantly. Pure rat liver ornithine aminotransferase was used for molecular weight studies. The molecular weight of the whole enzyme was determined to be 203,000 ± 13,000 by gel filtration chromatography using Sephadex G-200. The enzyme has four sub-units and the molecular weight of each was determined to be 52,000 ± 2000 by polyacrylamide gel electrophoresis.