AUGMENTATION OF PROTEIN-SYNTHESIS AND DEGRADATION BY POOR DIETARY AMINO-ACID BALANCE IN EUROPEAN SEA BASS (DICENTRARCHUS-LABRAX)

Sea bass fry were fed a fixed ration of one of six isonitrogenous diets differing in essential amino acid balance or physical and chemical state of the protein source (Hydrolysate vs. intact protein) to induce different growth rates. The reference diet was based on fish meal, whereas the other diets contained fish protein hydrolysate, greaves meal (i.e., defatted collagen meal) or hydrolyzed feather meal added at 30 or 50% of crude protein at the expense of Fish meal protein. Digestibility as well as fractional rate of whole-body protein synthesis was assessed. Whole-body protein synthesis was determined for each group of fish using a single injection of flooding dose of tritiated phenylalanine. Protein digestibility of the diets varied only by 5.5%. Specific growth rate and fractional protein specific growth rate, i.e., fractional protein accretion, were higher in fish fed the reference diet than in those fed the diets in which 50% of fish meal protein had been replaced by greaves or hydrolyzed feather meal protein. Compared with the reference group, whole-body protein synthesis was higher in fish fed these latter diets as well as in those fed the diet containing 30% greaves meal protein. The fractional protein accretion to fractional protein synthesis ratio, i.e., the efficiency of protein deposition, was lower in fish fed poorer dietary amino acid balance than in the reference group. The substitution of fish protein hydrolysate for intact fish protein led to a similar, though less pronounced phenomenon: nonsignificant increase in protein synthesis accompanied by significant increase in protein degradation. The increase in whole-body protein synthesis rate was linked to an increase in the amount of whole-body RNA, but the efficiency of ribosomal activity was not significantly different. Thus, decreased growth rate seemed to be a result of an increase in protein degradation.