Ornithine Transcarbamylase from Streptococcus faecalis and Bovine Liver: I. ISOLATION AND SUBUNIT STRUCTURE

Abstract

Abstract Procedures are presented for the isolation of ornithine transcarbamylase from Streptococcus faecalis and bovine liver. The enzyme from S. faecalis, homogeneous on electrophoresis in starch gel and in the ultracentrifuge (s20,w = 9.06 S), has a specific activity of 3,200 at pH 8.5, 37°, and an extinction coefficient (A1%280 nm) of 8.32. Its molecular weight is 223,000 in dilute buffer and 38,000 in 6 m guanidine HCl, both determined by measurement at sedimentation equilibrium, and 39,600 by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. Quantitative determination of the NH2-terminal residues gave one methionine per 40,000. Therefore, the enzyme is a hexamer. In 0.1 m NH4Cl, pH 9.5, the enzyme dissociates to the dimer, which is inactive but reassociates rapidly at pH 6.0 to form the active enzyme. The activity was also recovered nearly quantitatively on reassociation of a sample of the enzyme that had been dissociated in 6 m guanidine HCl. The preparation of bovine liver ornithine transcarbamylase is homogeneous in the ultracentrifuge (s20,w = 5.8 S), but electrophoretograms in polyacrylamide gel show five to six components evenly spaced and present in successively decreasing amounts. These have been separated by isoelectric focusing. They all have approximately the same specific activity and are immunologically identical. The differences in their pI values are consistent with each differing from its immediate neighbor by a single charge, perhaps because of hydrolysis of amide groups. The enzyme has a specific activity of 780 at pH 8.5, 37°, and an extinction coefficient (A1%280 nm) of 12.3. Its molecular weight is 108,000 in dilute buffer and 36,000 in 6 m guanidine HCl, both determined by measurement at sedimentation equilibrium, and 37,800 by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. Quantitative determination of the NH2-terminal residues gave one aspartic acid per 38,000 g. Therefore, the enzyme is a trimer. The enzyme recovered, with a yield of 50%, after dissociation in 6 m guanidine HCl had the same specific activity and electrophoretic composition as the original.