The intracellular distribution of glutathione into kinetically distinct pools and the determinants of glutathione turnover were examined in vivo. Glutathione turnover was measured in individual, restrained rats with a biliary fistula by administration of acetaminophen to trap the previously labeled hepatic glutathione as an excretable acetaminophen adduct. Fasting for 48 h resulted in a decrease of hepatic glutathione from 4.7±0.9 to 3.6±0.8 μmol/g liver and a marked increase in the fractional rate of glutathione turnover from 0.19±0.04 to 0.43±0.07/h. Within 6 h following refeeding, the rate of glutathione turnover and the hepatic glutathione concentration returned to normal. The simultaneously determined specific activities of free intrahepatic glutathione and the acetaminophen-glutathione adduct in bile were identical, indicating that the hepatic glutathione pool is kinetically homogeneous. The synthesis of glutathione could, therefore, be estimated from the rate constant and the intrahepatic glutathione concentration. During fasting hepatic synthesis of glutathione increased from 0.86±0.17 to 1.50±0.23 μmol/g per h. In fed animals the administration of dibutyryl cyclic adenosine monophosphate and theophylline stimulated the rate of hepatic glutathione turnover similar to fasting. In contrast, glucose given intraduodenally to fasted animals decreased the rate of glutathione turnover. These data are consistent with the view that the increased glutathione turnover that occurs during fasting results from two mechanisms. Because of a decrease in the intrahepatic free glutathione/mixed disulfide ratio, which is apparently mediated by cyclic adenosine monophosphate, the free glutathione pool contracts and turns over more rapidly in order to maintain glutathione synthesis. In addition, glutathione consumption via the gamma-glutamyl cycle apparently is increased, which may be related to the increased uptake of amino acids for gluconeogenesis during fasting.
Bernhard H. Lauterburg, Jerry R. Mitchell