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This paper presents the first purification of the mitochondrial branched‐chain amino acid aminotransferase (BCATm) from sheep placenta. It is a homodimer with an apparent subunit molecular mass of 41 kDa. The enzyme differs from those of the rat and human as it appears to form at least one intermolecular disulfide bond. The sheep BCATm cDNA (1.4 kb) encodes a mature polypeptide of 366 amino acids with a calculated molecular mass of 41 329 Da and a partial mitochondrial targeting sequence of seven amino acids. The sheep BCATm sequence shares higher identity with other mammalian BCATm isoenzymes (82–85%) than with the cytosolic isoenzymes (60%). By Northern blot analysis, a message of 1.7 kb was detected in sheep placenta and skeletal muscle. Measurements of BCAT activity, mRNA and BCATm protein in sheep placenta and skeletal muscle revealed that BCATm is the sole BCAT isoenzyme expressed in placenta, whereas it contributes 57 and 71% of the BCAT activity in tensor fascia latae and masseter muscles from weaned lambs respectively. Skeletal muscle, the main site of branched‐chain amino acid transamination, exhibits significantly lower BCAT activity in sheep than in rat. Our results suggest that the low BCATm mRNA level probably accounts for the low BCAT activity in sheep skeletal muscle, and that the metabolic scheme for branched‐chain amino acid catabolism is specific to each species. [ABSTRACT FROM AUTHOR]

A simple and convenient procedure is described for the isolation in good yield of two aminotransferases from various strains of Escherichia coli. On the basis of their substrate specificities one of the enzymes has been classified as an aromatic amino acid aminotransferase and the other as an aspartate aminotransferase, but both act on a wide range of substrates. Pyridoxal phosphate is bound more strongly to the aspartate aminotransferase than to the aromatic amino transferase which cannot be fully re-activated after removal of the prosthetic group. Both enzymes are composed of two subunits which appear to be identical. [ABSTRACT FROM AUTHOR]

Both holo- and apo-aspartate-aminotransferase are sensitive to tetranitromethane; however, the inactivation which follows nitration is partial for the holoenzyme, and complete for the apoenzyme. Cysteinyl and tyrosyl residues are modified in both forms of the enzyme; in the apoenzyme a greater number of tyrosines are available to the reagent. The complete inactivation of the apoenzyme is correlated with the loss of only one tyrosyl residue, which is selectively modified under particular conditions. The loss of activity is caused by tile failure of the nitrated apoenzyme to recombine with the coenzyme. [ABSTRACT FROM AUTHOR]

The amino acid and protein metabolism of roots of maize has been studied. The important role of the free amino acids and proteins of the roots as active agents in nitrogen assimilation is pointed out. Nitrogen supplied as nitrate is preferably incorporated into α-ketoglutaric acid, and then by transaminases transferred to other ketoacids. In the case of ammonia supply the function of a nitrogen-accumulating assimilation system leading to the formation of Arg, Glu-NH2 and Asp-NH2 is shown.