Nitrogen utilisation and ureogenesis as affected by dietary nucleic acid in rainbow trout ( Oncorhynchus mykiss ) and turbot ( Psetta maxima )

Abstract

We evaluated the influence of dietary nucleic acid (NA) supplied either as an RNA extract (RNA) or as brewer's yeast (BY) on nitrogen (N) utilisation, N excretion and ureogenesis in a freshwater teleost, the rainbow trout (Oncorhynchus mykiss) and a marine teleost, the turbot (Psetta maxima). Five diets containing two levels of NA or BY were formulated for each species, and fed to juvenile rainbow trout (initial body weight (IBW): 21.7 g) and turbot (IBW: 11.8 g) over 8–10 weeks. Besides growth and N utilisation, we monitored total ammonia-N and urea-N excretion rates and measured the activities of selected enzymes (glutamate dehydrogenase, ornithine carbamoyltransferase, arginase and uricase). There was no clear N sparing effect of dietary NA or BY supplementations in either species. Feeding diets containing the RNA extract led to an almost two-fold increase in postprandial plasma urea-N and uric acid concentrations and in urea-N excretion rates. Glutamate dehydrogenase and arginase activities were decreased in fish fed NA supplemented diets. Uricase activities were inversely related to dietary NA levels. Data on N excretion and enzyme activities clearly suggest that ureogenesis is influenced by dietary NA in both species.