Relationships between the synthesis and breakdown of protein, dietary absorption and turnovers of nitrogen and carbon in the blue mussel, Mytilus edulis L.

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Abstract
Seasonal and nutritionally induced changes of whole body protein metabolism have been studied in 45 to 57 mm shell-length Mytilus edulis from Whitsand Bay, southwest England. The subtraction of measured net protein balances from coincident rates of protein synthesis, determined in vivo by supplying 15N-labelled alga and monitoring the enrichment of excreted ammonia, enabled computation of protein breakdown rates. Over the range of protein absorption from zero to 0.58% of total soft tissue protein 24 h-1, fractional rates of protein breakdown decreased from 0.41 to 0.03%, whereas protein synthesis and net protein balance both increased from 0.25% to 0.39% and from-0.16% to 0.36%, respectively. The progressive reduction in fractional protein degradation with elevated net protein balance represented a “protein sparing” effect, whereby the efficiency of protein synthesis (defined as net synthesis/overall synthesis) confirmed theoretical predictions of as much as 92% during periods of maximal growth. In addition, 38% of breakdown products were recycled directly to synthesis under conditions of zero net balance, with an increasing contribution evident upon further decreases of protein absorption. The overall response was characterized by a consistently conservative elemental turnover of nitrogen relative to carbon, so that as a fraction of each element absorbed, between 1.2 and 1.9 times as much nitrogen was incorporated within structural materials. Such conservation was most pronounced among mussels starved prior to experimentation, indicating nutritionally related efficiencies in the utilization of resources for synthesis. The changing balance between individual processes also effected large alterations in proportional size of the metabolic pool of free amino acids (0.2 to 14.5% of total soft tissue nitrogen). Finally, it is suggested that adjustments of protein synthetic rate may be significant in the regulation of energy expenditure, accounting for at least 16% of basal energy requirements. Results throughout have been compared and contrasted with those for mammals, and whole-body measurements of both protein synthesis and breakdown proposed as a valuable index for environmental effects on instantaneous growth and metabolism.