Actinomycin D-Insensitive Induction of Rat Liver Glucose-6-phosphate Dehydrogenase by Carbon Tetrachloride Injury

Abstract

The activity of liver glucose-6-phosphate dehydrogenase [D-glucose-6-phosphate NADP⁺ oxidoreductase, EC 1.1.1.49](G6PD) increased in rats following a single dose of CCl₄. From the time course of the increase in enzyme activity, the rate constants for synthesis and degradation of G6PD in vivo were calculated to be increased 4.3- and 1.2-fold, respectively, over the initial rates. A corresponding increase in antigenic activity was found by titration of the induced enzyme with a specific antibody prepared to rat liver G6PD. Pulse labeling of the dehydrogenase protein in vivo with [¹⁴C]-leucine demonstrated that the maximum increase of 2.9-fold in the rate of G6PD synthesis was attained 24 hr after CCl₄ liver injury in spite of inhibited incorporation of the labeled amino acid into soluble hepatic proteins and albumin. Cycloheximide effectively blocked the increased synthesis of the enzyme induced by CCl₄ intoxication, whereas actinomycin D, at a dose sufficient to inhibit most RNA synthesis in the liver, had no effect on either the enzyme activity or the rate of enzyme synthesis in the liver after CCl₄ injury. These results indicate that the increased production of liver G6PD induced by CCl₄ is due to an altered mechanism of posttranscriptional regulation of the enzyme synthesis, probably at the level of translation. The apparent half-life of G6PD during the period of CCl₄-mediated induction of the enzyme was estimated to be reduced to 17 hr from the control value of 20hr, and this agreed well with value calculated from kinetic data. Thus the turnover rate of enzyme protein in the damaged liver seems to be slightly increased. That the enhanced synthesis of liver G6PD induced by CCl4 is caused by hepatic injury and is not the consequence of liver regeneration following hepatic cell necrosis is discussed.