Glutamate dehydrogenase biosynthesis in amphibian liver preparations.

Glutamate dehydrogenase (EC 1.4.1.3) (GDH) was studied with a system in vifro of tadpole and frog liver cube suspensions. A rise in GDH activity was observed in liver from tadpoles immersed in thyroxine (2.6 X lo-* M) for 8 days. A small rise in enzyme activity was observed in vitro in liver cubes from premetamorphic tadpoles. In the presence of thyroxine (2.6 X lo-* M to 2.6 X lo-lo M) or triiodothyropropionate (2.6 X 10vg M or 2.6 X lo-l1 M), the mean increase in the specific activity in uifro was twice that of the control over a 4%hour incubation period. Thyroxine stimulated synthesis de novo of immunoprecipitable glutamate dehydrogenase as well as soluble mitochondrial proteins. A specific stimulation of GDH synthesis relative to soluble mitochondrial protein synthesis in vitro was observed at a thyroxine concentration of 2.6 X 10m8 M. Higher concentrations of thyroxine resulted in a specific effect only in liver cubes prepared from tadpoles previously treated with thiourea. Synthesis de novo of GDH was inhibited by puromycin and actinomycin but the total enzyme activity in vifro was increased in the presence of these antibiotics. A continuous conversion of labeled nonimmunoprecipitable precursors into immunoprecipitable GDH was observed in vitro in the absence of any additives to the incubation system. In a single experiment, thyroxine itself appeared to stimulate precursor conversion. The half -disappearance time of labeled immunoprecipitable GDH was 21.2 f 1.7 hours in vifro and 22.8 hours in vivo. Actinomycin and puromycin inhibited breakdown of immunoprecipitable GDH. Synthesis de nouo, precursor conversion, and a rapid turnover of GDH were observed in uifro in liver cubes from premetamorphic and metamorphosing tadpoles as well as in the adult frog. The implications of these basic mechanisms of enzyme regulation are discussed.