Growth, protein-turnover rates and nucleic-acid concentrations in the white muscle of rainbow trout during development

We have studied the growth rate, nucleic-acid concentration, protein-accumulation rate (K(G)), and several other parameters relating to protein turnover, such as the protein-synthesis (K(S)), and protein-degradation rates (K(D)), protein-synthesis capacity (C(S)), protein-synthesis efficiency (K(RNA)), protein-synthesis rate per DNA unit (K(DNA)) and protein-retention efficiency (PRE), in the white muscle of rainbow trout during development. Both growth rate and relative food intake decreased significantly with age and weight, as did the food-efficiency ratio (FER) and protein-efficiency ratio (PER). Although absolute RNA and DNA contents increased with age, their relative concentrations decreased. The RNA/DNA ratio increased sharply from 14 to 28 weeks but afterwards decreased towards initial values. Hypertrophy increased rapidly to the 28-week stage but henceforth increased much more slowly. Hyperplasia, on the other hand, continued to increase linearly, resulting in a significant four- to fivefold predominance in this type of growth at the end of the 96-week experimental period. K(G) decreased significantly with age, as did K(S), and C(S), whereas at the 14-week stage, K(D) was significantly lower than at other ages. K(RNA) increased until 28 weeks. K(DNA) increased significantly in juvenile fish compared to both fingerlings and adults, where it showed similar lower values. PRE remained high at all ages.

Carbohydrate deprivation reduces NADPH-production in fish liver but not in adipose tissue

Little is known about the way in which carnivorous fish such as salmonids mobilise and metabolise dietary carbohydrates, which are essential to lipid metabolism. Thus we have studied changes caused by the absence of dietary carbohydrates to the kinetics and molecular behaviour of the four cellular NADPH-production systems [glucose 6-phosphate dehydrogenase (G6PDH); 6-phosphogluconate dehydrogenase (6PGDH); malic enzyme (ME); and isocitrate dehydrogenase NADP-dependent (NADP-IDH)] in the liver and adipose tissue of rainbow trout (Oncorhynchus mykiss). We used spectrophotometry to study enzyme kinetics and nucleic acid concentrations, and immunoblot analysis to determine specific protein concentrations. The absence of carbohydrate reduced specific enzyme activity, maximum rate and catalytic efficiency by almost 65% in G6PDH and 6PGDH, by more than 50% in ME, and by almost 25% in NADP-IDH but caused no significant changes in the K(m) values or activity ratios in any of these hepatic enzymes. Molecular analysis clearly showed that this kinetic behaviour reflected concomitant changes in intracellular enzyme concentrations, produced by protein-induction/repression processes rather than changes in the activity of pre-existing enzymes. We conclude that the absence of carbohydrates significantly reduces intracellular concentrations of G6PDH, ME and NADP-IDH in trout liver in percentages similar to those recorded for enzyme activity. We found no such variations in the concentrations of any of these enzymes in adipose tissue and no change in the levels of their activity, suggesting that the liver and adipose tissues are subject to different regulation systems with regard to carbohydrates and play distinct roles in lipid metabolism.

Dietary alterations in protein, carbohydrates and fat increase liver protein-turnover rate and decrease overall growth rate in the rainbow trout (Oncorhynchus mykiss)

We have determined the protein-turnover rates and nucleic-acid concentrations in the liver of trout (Oncorhynchus mykiss) fed on two different isocaloric diets: low-protein/high-fat and non-carbohydrate/high-fat. Compared to controls, the partial replacement of protein with fat significantly decreased the protein accumulation rate and protein-retention efficiency in the liver whilst increasing the fractional protein-synthesis and protein-degradation rates as well as protein-synthesis efficiency. The complete replacement of carbohydrates with fat significantly lowered the protein-accumulation rate and protein-retention efficiency, but enhanced both the protein-synthesis and protein-degradation rates as well as protein-synthesis capacity. The protein:DNA and RNA:DNA ratios decreased considerably on both diets. Total DNA decreased in fish on a low-protein/high-fat diet but did not change in those on a non-carbohydrate/high-fat diet. The absolute protein-synthesis rate registered no significant change under any of the nutritional conditions. Both the experimental diets did however raise the fractional protein-synthesis rate significantly, due to enhanced protein-synthesis efficiency when protein was partially replaced with fat and to enhanced protein-synthesis capacity when carbohydrates were completely replaced with fat. Our results show the capacity of the liver to adapt its turnover rates and conform to different nutritional conditions. They also point to the possibility of controlling fish growth by dietary means.

Selective changes in the protein-turnover rates and nature of growth induced in trout liver by long-term starvation followed by re-feeding

We report upon the effects of a cycle of long-term starvation followed by re-feeding on the liver-protein turnover rates and nature of protein growth in the rainbow trout (Oncorhynchus mykiss). We determined the protein-turnover rate and its relationship with the nucleic-acid concentrations in the livers of juvenile trout starved for 70 days and then re-fed for 9 days. During starvation the total hepatic-protein and RNA contents decreased significantly and the absolute protein-synthesis rate (A(S)) also fell, whilst the fractional protein-synthesis rate (K(S)) remained unchanged and the fractional protein-degradation rate (K(D)) increased significantly. Total DNA content, an indicator of hyperplasia, and the protein:DNA ratio, an indicator of hypertrophy, both fell considerably. After re-feeding for 9 days the protein-accumulation rates (K(G), A(G)) rose sharply, as did K(S), A(S), K(D)), protein-synthesis efficiency (K(RNA)) and the protein-synthesis rate/DNA unit (K(DNA)). The total hepatic protein and RNA contents increased but still remained below the control values. The protein:DNA and RNA:DNA ratios increased significantly compared to starved fish. These changes demonstrate the high response capacity of the protein-turnover rates in trout liver upon re-feeding after long-term starvation. Upon re-feeding hypertrophic growth increased considerably whilst hyperplasia remained at starvation levels.

Stimulation of rat-liver branched-chain alpha-keto acid dehydrogenase activity by chronic metabolic acidosis

During chronic metabolic acidosis, the degradation of protein and amino acids reportedly increases. Branched-chain alpha-keto acid dehydrogenase complex (BCKDH) relates amino-acid catabolism and mitochondrial-energy metabolism. This study was designed to evaluate the effect of acidosis on the activity of liver BCKDH, the key regulatory enzyme in the catabolism of branched-chain amino acids. Experimental acidosis was induced in rats by ingestion of 0.28 M ammonium chloride solution for 10 days. We made two different liver-mitochondrial extracts to assay independently the active form of BCKDH and the total BCKDH activity. Acidosis significantly increased both active and total BCKDH specific activities (P < 0.05). The mean value for the active form of the BCKDH complex was 9.27 +/- 1.10 (S.E.M., n = 7) mU/mg of mitochondrial protein in acidotic rats and 5.18 +/- 0.84 (n = 7) for the control rats. The value of the total complex was 16.10 +/- 1.22 (n = 7) for the acidosis and 11.51 +/- 0.58 (n = 7) for the control. No significant changes were found in the activity state of the complex. Citrate synthase activity did not show significant variations between treatments. The stimulation of liver BCKDH activities by the acidosis may contribute to maintaining the level of intermediates of the tricarboxylic-acid cycle in this metabolic situation in which the net release of glutamine are produced.

Deprivation of dietary nucleotides decreases protein synthesis in the liver and small intestine in rats

BACKGROUND & AIMS

Dietary nucleotides are reported to influence the growth and functioning of the liver and small intestine. The aim of this study was to examine the mechanism by which nucleotides exert their effects in these tissues by assessing protein synthesis activity and related parameters in the presence or absence of dietary nucleotides.

METHODS

Rats were fed a purified diet with or without nucleotides for 10 days. Fractional protein synthesis rate, RNA and DNA concentrations, polysome size distribution, and number of ribosomes were assessed.

RESULTS

Fractional protein synthesis rates of the liver and small intestine were lower in the nucleotide-deprived group than in the control group. In the liver, RNA concentration was also lower in the nucleotide-deprived group, but values in the small intestine were similar in the two groups. In the liver, deprivation of nucleotides resulted in a reduction in the number of ribosomes and in polysome breakdown. Protein and DNA concentrations did not vary in the liver; however, the concentration of DNA was lower in the small intestine of the nucleotide-deprived group than in the control group.

CONCLUSIONS

Dietary nucleotides can modulate protein synthesis in the liver and small intestine as a result of tissue-specific nucleic acid changes.

Kinetic properties of hexose-monophosphate dehydrogenases. I. Isolation and partial purification of glucose-6-phosphate dehydrogenase from rat liver and kidney cortex

Glucose-6-phosphate dehydrogenase (G6PDH) from rat-liver and kidney-cortex cytosol has been partially purified and almost completely separated from 6-phosphogluconate dehydrogenase activity. The purification and isolation procedures included high-speed centrifugation, 40-55% ammonium sulphate fractionation, by which both enzyme activities were separated, and finally, the application of the protein fraction to a column of Sephadex G-25 equilibrated with 10 mM Tris-EDTA-NADP buffer, pH 7.6, to eliminate any contaminating metabolites. The kinetic properties of isolated liver and renal G6PDH were examined. Both enzymes showed a typical Michaelis-Menten kinetic saturation curve with no evidence of co-operativity. The optimum pH of both liver and kidney cortex G6PDH was 9.4. The Km values for glucose-6-phosphate (G6P) and for NADP were 3.29 x 10(-4) M and 1.00 x 10(-4) M respectively. The specific activity measured at 37 degrees C and optimum pH was 327.1 mU/ mg of protein. NADPH caused a competitive inhibition with a Ki of 10 microM. The Km values for the G6P and NADP of kidney-cortex G6PDH were 2.06 x 10(-4) and 0.25 x 10(-4) M respectively. The specific activity at pH 9.4 and 37 degrees C was 76.55 mU/mg of protein. The Ki value for NADPH inhibition was 4 microM. This work describes an easy, rapid and reliable method for the separation of the two dehydrogenases involved in the hexose-monophosphate shunt in animal tissues.

Long-term adaptive response to dietary protein of hexose monophosphate shunt dehydrogenases in rat kidney tubules

We have studied the effects of several different macronutrients on the kinetic behaviour of rat renal glucose 6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Rats were meal-fed with high-carbohydrate/low-protein, high-protein/low-carbohydrate and high-fat diets. High-protein increased renal G6PDH and 6PDGH activities by 66 per cent and 70 per cent respectively, without significantly changing the Km values of either and each Hexose monophosphate dehydrogenase activity increased steadily, reaching a significant difference on day 4. A rise in carbohydrate or fat in the diets, produced no significant change in either the activity or the kinetic parameters, Vmax and Km of the two dehydrogenases. In addition, the administration of a high-protein diet for 8 days significantly increased both the pentose phosphate pathway flux (92.6 per cent) and the kidney weigth (35 per cent), whereas no significant changes in these parameters were found when the animals were treated with the other diets. Our results suggest that an increase in the levels of dietary protein induces a rise in the intracellular levels of these enzymes. The possible role of this metabolic pathway in the kidneys under these nutritional conditions is also discussed.

Stimulation of rat-kidney hexose monophosphate shunt dehydrogenase activity by chronic metabolic acidosis

The effect of chronic metabolic acidosis on the kinetic behaviour of renal glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase have been investigated. Acidosis induced a significant increase in both enzyme activities at all substrate concentrations used. Saturation curves of both dehydrogenases were hyperbolic with no evidence of sigmoidicity. Maximum activities were found after 7 days of acidosis with no significant change in the Km values. The results suggest that stimulated renal hexose monophosphate dehydrogenases activities are probably due to an increased intracellular concentration of these enzymes. The relationship between these changes and those generated in the metabolic acidosis are also discussed.

Influence of experimental diabetes on the kinetic behaviour of renal cortex hexose monophosphate dehydrogenases

1. Short term (1-2 hr) and long-term (2 days) effects of experimental alloxan induced diabetes on the kinetics of the renal hexose monophosphate shunt dehydrogenases are reported. 2. Alloxan diabetes for 2 days significantly increased kidney weight (16%) adding about 80 mg/day per g of kidney. No significant changes were found in renal growth 1-2 hr after alloxan injection. 3. Under these experimental conditions, the activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase significantly increased (103 and 33% respectively) at all substrate concentrations, without affecting the KmS of either enzyme. 4. There was no effect of alloxan on the activity of these enzymes at 1-2 hr. Saturation curves show that all enzymes exhibited a M-M kinetic without evidence of sigmoidicity. 5. The results suggest that increased renal hexose monophosphate dehydrogenases activities are due to increased concentrations of the rate limiting proteins. 6. The relationship between these changes and renal hypertrophy is also discussed.