Adjustments of Protein Metabolism in Fasting Arctic Charr, Salvelinus alpinus.

Protein metabolism, including the interrelated processes of synthesis and degradation, mediates the growth of an animal. In ectothermic animals, protein metabolism is responsive to changes in both biotic and abiotic conditions. This study aimed to characterise responses of protein metabolism to food deprivation that occur in the coldwater salmonid, Arctic charr, Salvelinus alpinus. We compared two groups of Arctic charr: one fed continuously and the other deprived of food for 36 days. We measured the fractional rate of protein synthesis (KS) in individuals from the fed and fasted groups using a flooding dose technique modified for the use of deuterium-labelled phenylalanine. The enzyme activities of the three major protein degradation pathways (ubiquitin proteasome, lysosomal cathepsins and the calpain systems) were measured in the same fish. This study is the first to measure both KS and the enzymatic activity of protein degradation in the same fish, allowing us to examine the apparent contribution of different protein degradation pathways to protein turnover in various tissues (red and white muscle, liver, heart and gills). KS was lower in the white muscle and in liver of the fasted fish compared to the fed fish. There were no observable effects of food deprivation on the protease activities in any of the tissues with the exception of liver, where the ubiquitin proteasome pathway seemed to be activated during fasting conditions. Lysosomal proteolysis appears to be the primary degradation pathway for muscle protein, while the ubiquitin proteasome pathway seems to predominate in the liver. We speculate that Arctic charr regulate protein metabolism during food deprivation to conserve proteins

Levels of polyubiquitinated proteins in tissues of fed and fasted Arctic charr (white and black, respectively).

<p>* Indicates a significant difference between the fed and fasted groups. Values are expressed as means ± s.e.m. (n = 7).</p

20S proteasome, cathepsin pH 2.5, pH 5.5 and calpain activity in white muscle (WM), red muscle (RM), liver, heart and gill of fed and fasted Arctic charr.

<p>Activities are expressed in fluorescent units per mg protein per minute (means ± s.e.m., n = 7). *Indicates a significant difference between fed and fasted groups.</p

Average protein concentration (mg · g of tissue^-1) in various tissues of fed and fasted Arctic charr.

<p>Values are expressed as means ± s.e.m. (n = 7).</p

Initial mass (g), final mass (g) and specific growth rate (%·day^-1) of Arctic charr following a 36-day fasting period.

<p> Values are expressed as means ± s.e.m. (n = 10).</p

Ratios of protease activity relative to fractional rates of protein synthesis (<i>K</i>_<i>s</i>) in tissues of fed Arctic charr.

<p>(A) maximal 20S proteasome activity, (B) cathepsin pH 2.5 activity and (C) cathepsin pH 5.5 activity. Note that these ratios do not have units and are for comparison purposes only. Different letters indicate significant difference (p < 0.05). Values are expressed as means ± s.e.m. (n = 7).</p