Chemical, physical, and morphological properties of ornithine Aminotransferase from rat liver.

Ornithine aminotransferase was crystallized from rat liver and several properties of the enzyme were studied, including amino acid composition, thiol content, absorbance spectrum, isoelectric point, molecular weight, and appearance under the electron microscope. The half-cystine and thiol contents of the enzyme were equal (0.12 pmole per mg of enzyme), indicating the absence of disulfide bonds in the molecule. One-fourth of the thiol groups in the native enzyme was reactive. The addition of denaturing agents activated the remaining thiol groups, which presumably had been protected in the interior of the molecule. These results indicate that the enzyme must contain at least four thiol groups (cysteine residues). The empirical weight of the enzyme, therefore, was estimated to be 33,000. A value of 33,300 was obtained when the empirical weight was calculated from the total amino acid composition of the enzyme. The minimal-molecular weight of the enzyme as determined by equilibrium ultracentrifugation was 132,000, indicating that the enzyme may be composed of four 33,000 molecular weight subunits. The isoelectric point of the enzyme was 5.38, indicating that the enzyme is an acidic protein. Crystals of the enzyme were examined by light and electron microscopy. Individual protein molecules were visualized in crystals negatively stained with phosphotungstate. The approximate dimensions of these molecules indicate that they are probably the subunits of the enzyme.