3-Methylhistidine Excretion as an Index of Muscle Protein Breakdown in Birds in Different States of Malnutrition

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Abstract Using 3-methylhistidine (3-MeHis) excretion as an index, we evaluated the abilities of molting and non-molting sparrows to adjust muscle protein degradation rates in response to three types of nutritional limitations: total food shortage, protein deficiency, or sulfur amino acid (SAA) deficiency. Regardless of nutritional status, molting birds excreted daily at least two-fold as much 3-MeHis as their non-molting counterparts. Both molting and non-molting birds significantly reduced daily excretion of 3-MeHis in response to a 1-day fast. Likewise, both molting and non-molting birds promptly reduced daily excretion of 3-MeHis in response to protein deficiency (within 1 day) and significantly so by day 5 and through day 9 of dietary treatment. Reductions in excretion of 3-MeHis by fasted or protein-limited birds could not be explained solely on the basis of reduced body protein mass and provided evidence of adjustments in muscle protein degradation rates. Responses of birds to SAA deficiency differed between molting and non-molting birds and from responses to the other nutritional limitations. In these birds, 3-MeHis excretion increased significantly on day 1 of dietary treatment. By day 5 and through day 9 of SAA deficiency, non-molting birds excreted 3-MeHis at rates similar to those of their well-nourished counterparts. In contrast, excretion of 3-MeHis by molting birds experiencing SAA deficiency remained significantly higher than by their well-nourished counterparts. Neither molting nor non-molting birds readily reduced muscle protein degradation rates in response to SAA deficiency and it is suggested that their failure to do so may provide evidence of a direct and (or) indirect role for glutathione in the regulation of muscle protein degradation.