On the Mechanism of Glucose‐6‐Phosphate Dehydrogenase Regulation in Mouse Liver: 3. The Rate of Enzyme Synthesis and Degradation

The rates of synthesis and degradation of glucose‐6‐phosphate dehydrogenase in the liver of male C57BL mice are followed by a modified immunochemical method. Mice are given labeled amino acids for a predetermined period, and the labeled enzyme is isolated from liver homogenates with goat anti‐enzyme, followed by rabbit anti‐goat IgG serum. The precipitates are either counted directly or analyzed on acrylamide gels. The rate of glucose‐6‐phosphate dehydrogenase synthesis is found to be independent of the state of induction or repression of the animal. The synthesis of the enzyme constitutes 0.20–0.25% of the synthesis of all soluble liver proteins. This rate is maintained even during periods of most rapid formation or disappearance of enzyme activity. Quantitative immunoprecipitation shows that similar amounts of precipitable antigen are present in induced and non‐induced liver homogenates. The results indicate that the large reversible increase in activity of glucose‐6‐phosphate dehydrogenase, observed when animals are transferred from a high‐fat to a fatless diet, does not involve the synthesis of a new enzyme protein, and that mechanisms involving modulation of existing enzyme molecules ought to be considered for the adaptation of this enzyme in mouse liver.