Labile Protein Reserves and Protein Turnover

Abstract

The concept of a labile protein reserve is based on the relatively slow establishment of a new equilibrium in the rate of nitrogen excretion after an abrupt change in dietary supply. The evidence reviewed shows that a majority of this nitrogen is derived from or deposited in skeletal muscle proteins. The rates of synthesis and degradation of total body protein are rapid in large animals (man and swine) and are correlated with heat production. The rate of protein synthesis in skeletal muscle greatly exceeds the rate of growth and is sensitive to nutritional status. The rate exceeds the rate of degradation in response to the ingestion of an adequate diet so that tissue proteins are accumulated, but it decreases below the rate of degradation under conditions of deprivation. In this latter state, proteins of skeletal muscle supply amino acids for energy or for the synthesis of other more essential proteins, e.g., milk proteins during lactation. Thus, we conclude that the labile protein reserve is a product of the normal, dynamic metabolism of protein.

Feeding lactating primiparous sows to establish three divergent metabolic states: II. Effect on nitrogen partitioning and skeletal muscle composition.

“…Assuming that skeletal muscle accounts for 45% of maternal protein (Young, 1970) and that muscle is the main contributor to mobilizable protein (Swick and Benevenga, 1977), the reduction in pig growth rate seen in restricted-fed sows by wk 3 of lactation suggests that these sows had mobilized 10 to 15% of their maternal protein, or 25 to 30% of their skeletal muscle. Furthermore, assuming that excretion of urinary creatinine is proportional to an animal's muscle mass (Tietz, 1986), the 24% reduction in urinary creatinine observed in restricted-fed sows in late lactation gives an indication of the amount of skeletal muscle protein mobilized during lactation.…”

“…Skeletal muscle is the main source of mobilizable protein (Swick and Benevenga, 1977). Therefore, changes in skeletal muscle composition and N balance reflect changes in whole-body protein mobilization, and in turn changes in urinary creatinine excretion reflect changes in maternal muscle mass.…”

“…Thus, sows mobilize protein reserves, primarily composed of skeletal muscle (Swick and Benevenga, 1977), to make up this deficit during lactation (King et al, 1993;Everts and Dekker, 1994). Dietary N is partitioned in lactating sows toward losses in feces and urine and retention in milk and maternal body mass.…”

Composition of two-week change in body weight under unrestricted free-living conditions

“…1977; Fernandez‐Elias et al. 2015), labile protein stores (Swick and Benevenga 1977), and the accompanying body water (Oian et al. 1987; Ew 1996).…”

Exposure to cold but not exercise increases carbon turnover rates in specific tissues of a passerine

“…We can directly demonstrate phenotypic mass increase within this study. Increased protein synthesis rather than decreased protein breakdown serves as the most likely explanation for phenotypic mass increase (Swick and Benevenga, 1976), which can explain increased carbon turnover. Higher tissue-specific activity is a well documented and common phenomenon among animals that require increased heat production due to cold exposure.…”