Diurnal Variation in Liver and Muscle Glutathione Pools of Molting and Nonmolting White-Crowned Sparrows

To test the hypothesis that the tripeptide glutathione (γ-glutamylcysteinylglycine) might function as a free-cysteine buffer in adequately nourished birds, we measured liver and muscle glutathione concentrations early and late in the 8-h overnight fast of molting and nonmolting white-crowned sparrows. During molt-when the overnight requirement for cysteine in keratin synthesis is greatest-liver glutathione concentration early in the overnight fast was greater than that during the pre- or postmolt stages, and the overnight loss of glutathione was greater than that in the postmolt stage. Liver glutathione concentration remained constant overnight or increased slightly during the premolt stage. Variation in muscle glutathione paralleled liver glutathione except that concentrations in muscle were greatest during the postmolt stage. These results suggest that glutathione pools in the liver and muscle of adequately nourished birds (1) could serve as a buffer that accepts or donates cysteine as required by the exigencies of supply (feeding bouts) and demand (intervals between feeding bouts) and (2) may have a homeostatic role in protein synthesis.