Essential amino acid metabolism in molting and non-molting sparrows in different nutritional states

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We investigated the abilities of sparrows, Zonotrichia leucophrys, to adjust their rates of oxidation of endogenously derived essential amino acids and to retain these indicator amino acids (IAA) in tissue proteins in response to either a dietary deficiency of total protein, threonine or sulfur amino acids (SAA). Tissue proteins of birds were uniformly labeled with either 1- 14C-phe or 1- 14C-val by injecting birds IM with a tracer dose of labeled amino acid 21 days before dietary treatments began. Oxidation rates and retention of IAA in tissue proteins were compared between well-nourished birds and those fed deficient diets during two phases of their annual cycle, molt and winter maintenance. Non-molting birds fed diets low in protein or in threonine and molting birds fed diets low in protein reduced oxidation rates and increased retention of IAA after 9 days of dietary treatment as compared with well-nourished birds. In contrast, non-molting birds fed diets low in SAA showed no adjustments in oxidation or retention of IAA and molting birds fed diets low in SAA showed increased rates of oxidation and lower retention of IAA as compared with well-nourished control birds. In those birds able to adjust to dietary deficiencies, decreased oxidation and increased retention rates closely approximated earlier reported decreases in muscle protein degradation rates in response to malnutrition. Reduced protein degradation rates may be the preeminent compensatory mechanism in protein sparing during malnutrition, whereas adjustment in oxidation and reutilization of endogenously derived amino acids seems to play only a small role. Differing responses of birds to SAA deficiency as compared with protein or threonine deficiency probably result from depletion of glutathione pools and a resulting disturbance to the normal economy of protein metabolism.