Detection of multiple forms of rat liver ornithine decarboxylase.

Rat liver ornithine decarboxylase induced by injection of thioacetamide has been separated into at least two fractions by covalent chromatography on an activated thiol-Sepharose 4B column. The two major fractions could be distinguished by ion exchange chromatography and electrophoresis on acrylamide gels. In addition, the two forms displayed different Km values for ornithine. Although the two forms are separable, they display identical antigenic properties, pH optima, and they appear to be the same molecular size. The biological significance or the relationship between multiple forms of ornithine decarboxylase is not understood.

Production of monospecific antibodies to rat liver ornithine decarboxylase and their use in turnover studies.

Two forms of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) were purified from the livers of rats which had been treated with thioacetamide for 16 h (for details, see miniprint to Obenrader, M.F., and Prouty, W. F. (1977) J. Biol. Chem. 252, 2860-2865). The enzyme was purified over 7,000-fold from liver cytosol with an overall yield of 8%. Enzyme activity was eluted finally in two distinct fractions by chromatography on activated thiol-Sepharose 4B. Both forms appear to be dimeric proteins having molecular weights of approximately 100,000 by equilibrium sedimentation and analysis on a calibrated Sephadex G-200 column. The apparent subunits are approximately 50,000 daltons as determined by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. Since electrophoresis in the presence of detergent is the only method used here to indicate subunits, the possibility that conditions of sample preparation resulted in splitting of a labile protein cannot be excluded from consideration. Ornithine decarboxylase has a very broad pH-activity curve with an optimum that shifts from pH 7.0 to pH 7.8 as the enzyme is purified. The apparent Km values for a highly purified mixture of the two forms of enzyme for L-ornithine and pyridoxal 5'-phosphate were determined to be 0.13 mM and 0.25 micronM, respectively. Both sodium and potassium chloride were shown to inhibit enzymatic activity; 50% inhibition occurred at 270 mM for each when Km amounts or ornithine were used. Rat liver ornithine decarboxylase antiserum was prepared in rabbits using Form I of the enzyme as the antigen. The antibody was shown to precipitate quantitatively the ornithine decarboxylase activity isolated from induced rat liver and rat ventral prostate. The specificity of the antiserum was demonstrated by rocket immunoelectrophoresis and by gel electrophoresis in the presence of sodium dodecyl sulfate using immunoprecipitates obtained from enzyme preparations labeled either in vivo, with [3H]leucine, or in vitro, by reductive methylation using formaldehyde and sodium [3H]borohydride. The antibody preparation has been used in a titration method to assess the half-life of antigen in livers of rats induced for ornithine decarboxylase by injection of thioacetamide. In two experiments, the t1/2 of activity at the height of induction, following injection of cycloheximide, was 19 and 24 min, while the t1/2 of disappearance of antigen was 28 and 33 min, respectively. In each experiment the t1/2 for antigen was significantly longer than the t1/2 for loss of enzyme activity. Enzyme levels appear to be modulated primarily by synthesis and degradation of antigen. Furthermore, the observation that enzyme activity is lost with a shorter t1/2 than antigen is consistent with the idea that denaturation is an initial step in the degradation of this enzyme...