Diurnal Changes in Tissue Glutathione and Protein Pools of Molting White-crowned Sparrows: The Influence of Photoperiod and Feeding Schedule

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Abstract
In molting birds keratin synthesis is intense and continuous through day and night. Keratins are much richer in cystine than are most tissue proteins. We hypothesized that glutathione (γ-glutamylcysteinylglycine) helps to sustain feather synthesis and spare bodyprotein during overnight fasting by serving as a cysteine reservoir. To test this hypothesis, we measured the responses of liver and muscle glutathione and protein pools to various feeding schedules (cysteine supply) andperiods of overnight fasting (cysteine demand) in molting white-crowned sparrows. Our main purpose was to ascertain whether these pools change passively or are instead regulated in relation to cysteine need in the postabsorptive state. During molt, significantly more liver glutathione and pectoralis protein were stored by day and utilized overnight than in postmolt. Liver glutathione and pectoralis protein were stored and utilized diurnally during molt in proportion to the duration of the overnight fast (12 h > 8 h > 4 h). Diurnal changes in these pools resulted from significantly larger evening pools, not from significantly smaller morning pools. Liver glutathione and pectoralis protein pools appeared to be specifically regulated to support the demands of overnight feather synthesis. In contrast, changes in liver protein pools during molt were smaller than in postmolt and were unrelated to the duration of the overnight fast. The sizes of liver protein pools in evening were nearly the same in molting and nonmolting birds. Similarly, the size of pectoralis glutathione pools in evening was nearly the same in molting and nonmolting birds, butpectoralis glutathione was utilized overnight in molt in relation to the duration of the overnight fast. Liver protein pools and possibly other tissue protein pools appeared to be spared by storage of glutathione in the liver. Pectoralis glutathione pools appeared to be passively emptied according to overnight cysteine demand. Overnight force-feeding of chick starter mash or serial injection of cysteine (im) attenuated overnight utilization of liver and muscle glutathione, indicating a specific role for GSH as a cysteine reservoir. The dynamics of tissue glutathione and protein pools during molt in relation to photoperiod may have important consequences for the energetic and amino acid costs of molt.

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