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 diet s differing in essential amino acid balance or physical and chemical s tate of the protein source (Hydrolysate vs. intact protein) to induce different growth rates. The reference diet was based on fish meal, whe reas 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. Digestibi lity as well as fractional rate of whole-body protein synthesis was as sessed. Whole-body protein synthesis was determined for each group of fish using a single injection of flooding dose of tritiated phenylalan ine. Protein digestibility of the diets varied only by 5.5%. Specific growth rate and fractional protein specific growth rate, i.e., fractio nal 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 rep laced 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% gre aves meal protein. The fractional protein accretion to fractional prot ein synthesis ratio, i.e., the efficiency of protein deposition, was l ower in fish fed poorer dietary amino acid balance than in the referen ce group. The substitution of fish protein hydrolysate for intact fish protein led to a similar, though less pronounced phenomenon: nonsigni ficant increase in protein synthesis accompanied by significant increa se in protein degradation. The increase in whole-body protein synthesi s 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 pr otein degradation.