Intracellular Concentration of Hepatic Glycerol-3-phosphate Dehydrogenase in Normal, Diabetic, and Hormonally Manipulated Rabbits 1

Abstract A radioimmunoassay (RIA) for cytosolic glycerol-3-phosphate dehydrogenase (GPDH) was developed as a means of determining enzyme concentration independent of assayable activity. The GPDH concentration (nmole/g wet wt), activity (units/g wet wt), specific activity (units/nmole GPDH), total liver concentration (nmole/liver/kg body wt), and total liver activity (units/liver/kg body wt) were determined under conditions favorable and unfavorable to gluconeogenesis. Fasted rabbits showed a decrease in total liver nanomoles of GPDH per killigram of body weight. Cortisone, triamcinolone, and thyroxine administration caused increases in total liver units and total liver nanomoles of GPDH per killigram of body weight in the fed animal. The increases or decrease observed correlated directly with changes in liver size. Diabetic, insulin-controlled diabetic, and glucagon-treated fed animals showed neither GPDH activity nor concentration changes. There was no change in the specific activity of GPDH under any conditions. The constant specific activity observed indicates a single catalytic state for the enzyme. Thus, changes in total liver enzyme rather than in specific activity become critical in the regulation of the GPDH reaction.