Protein synthesis fractional rate

The starving salmon is able to control the rates at which different tissues lose mass and this is in turn a reflection of tissue-specific differences in protein turnover. Fractional rates of protein synthesis are elevated after 4 months of starvation in the liver, ovaries, stomach and ventricle (Fig. 17). White muscle and gill showed no change and only the red muscle showed a decrease. Protein degradation rates are in excess of synthesis rates (leading to protein loss) in the gill, ventricle, red muscle, white muscle and stomach whereas in the ovary protein synthesis exceeded degradation. 

Answer In (a), respiratory control by ADP, the increase in respiratory rate is limited by the rate of diffusion of ADP, and the response would be expected to occur in fractions of a millisecond. The adjustment to (b), hypoxia mediated by HIF-1, requires a change in concentration of several proteins, the result of increased synthesis or degradation. The time scale for protein synthesis or degradation is typically many seconds to hours—much longer than the time required for changes in substrate concentration. 

It was just stated that protein turnover has anabolic and catabolic arms. In a subject in a steady metabolic state, these are exactly equal. It may then be of interest to determine the absolute rates of protein synthesis/degradation. Individual protein turnover rates may be expressed in terms of half-lives (tiy) or fractional catabolic rates or simply in terms of grams protein synthesized and degraded per unit time. The same parameters can be derived for whole-body protein turnover. 

The possible relationship between tryptophan and sodium ions has been considered in a number of studies. Herken and Weber165 reported that under certain conditions tryptophan injections intraperitoneally to rats led to a reduction of elimination of Na++. Subsequently, Reuter et al.166 analyzed the alterations of water and electrolyte balances by the use of clearance experiments. The fractional Na++ reabsorption increases, with no increase in the absolute tubular sodium transport rate since the significant reduced plasma-sodium concentration led to a decreased sodium load. The most probable cause of the decreased plasma-sodium concentrations seemed to be retention of sodium-free water under the conditions of infusion. The water retention is compatible with the antidiuretic effect of serotonin. Another relationship between tryptophan and sodium has been reported on the effects of each agent alone or together in vivo or in vitro, on in vitro hepatic nuclear tryptophan receptor binding and hepatic protein synthesis.167 168 This has been considered in detail in Chapter 4. 

Ponter et al.135 reported that in piglets the fractional protein synthesis rates were generally not increased in duodenal or jejunal mucosa by adequate or excess tryptophan in high carbohydrate or high fat diets, although in stomach small increases occurred. Also, the fractional protein synthesis rate was increased with adequate or excess tryptophan diets compared to inadequate tryptophan diets (controls). 

Ponter et al.135 reported an increase in fractional protein synthesis rate in the skin of piglets tube-fed adequate or excess tryptophan diets compared to controls (tryptophan-inadequate diet). 

Lin et al.137 reported that there was a depressive effect of tryptophan deficiency on the protein synthesis rate in pig muscle. Also, Cortamira et al.138 reported that the fractional protein synthesis rates in piglet muscle (longis-simus dorsi and semitendinosus) were increased in animals fed tryptophan-adequate diets compared to controls (tryptophan-inadequate diets). This was confirmed in a later study by Ponter et al.135... 

Different tissues clearly have different fractional rates of protein synthesis, a point that has been universally found from studies of tissue protein synthetic rates (e.g. Fauconneau 1985) possible explanations for these tissue differences are... 

Fig. 1. Histogram of the fractional rates of protein synthesis and degradation of trout tissues in relation to body size. (Data from Houlihan et al. 1986)...

The fractional rates of protein synthesis can be converted into the amount of protein synthesised in each tissue (Fig. 2) by multiplying it by the protein content of the tissues (data from Houlihan et al. 1986). This calculation reveals that although the white muscle has the lowest fractional rate of protein synthesis, this large mass of protein makes this tissue the major site of protein synthesis and... 

It has been suggested that the postulated high protein synthesis rates of larval fish may be due to the relatively high proportion of the body occupied by the intestine which will have a high rate of synthesis relative to the white muscle (Dabrowski 1986 Weatherly and Gill 1987). Thus as body size increases not only may fractional rates of protein synthesis of individual tissues decline but also the relative proportions of the body may change with a relative increase in slowly synthesising white muscle. 

The decline in fractional rates of protein synthesis and degradation could be the molecular basis for many of the physiological manifestations of ageing (Makrides 1983 Richardson and Cheung 1982 Holehan and Merry 1986) but the relationships between age in ectotherms, protein turnover and the ability to repair damage remain to be investigated. 

Fig. 3. a Whole body fractional rates of protein synthesis and oxygen consumption of cod after a single meal of sand eels, b Absolute rates of protein synthesis of cod liver (O) and stomach ( ) from the same animals as in a. (Data from Lyndon et al. in prep.)... 

Fig. 4. Mean ( S.E.) fractional rates of protein synthesis of the tissues of fasted Carcinus, at various times after feeding and of continuously fed animals. The animals were fed at time zero with Mytilus flesh at a ration level of 2.75% of the crab s live weight. The levels of significance from comparison with the fasted animals using Student s t-test are P < 0.05, P < 0.005, p < 0.001. (Data from Houlihan etal. 1990b)...

Fractionation of the trout liver has told us something about the nature of the proteins that are synthesised (McMillan and Houlihan 1990a). The fractional rates of protein synthesis in the subcellular components were markedly different in fasted fish the highest rates were found in the mitochondrial fraction (5.3 0.43% day ) and the lowest in the nuclear fraction (3.10 0.29% day ). A stimulation in... 

When protein synthesis is expressed in fractional terms there is no indication of the contribution that the different subcellular components make to the total liver synthesis. However, the quantities of protein in each fraction did not differ significantly at any time before or after feeding. Expressing the results in absolute terms reveals that soluble proteins in the post-mitochondrial fraction have the highest rates of synthesis and all the fractions increase with feeding (Fig. 7). It should be noted that all these experiments were carried out with labelled phenylalanine incorporation over 40-min periods and our results indicate that newly synthesised export proteins, e.g. albumin, do not appear in the fishes plasma until at least 1 h after radiolabelled amino acid injection. 

Fig. 7. Absolute rates of protein synthesis (mg protein liver day ) in the liver and liver fractions of rainbow trout. The rates of protein synthesis of fasted animals (6 days without food) and at various times after feeding are shown. (McMillan and Houlihan 1990a)...

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