Postprandial nitrogen metabolism and excretion in juvenile marble goby, Oxyeleotris marmorata (Bleeker, 1852)

The objective of this study was to examine postprandial nitrogen metabolism and excretion in the juvenile marble goby, Oxyeleotris marmorata (30-80 g), fed a high protein diet (cod fish fillet). Feeding led to only a slight increase in plasma ammonia concentration and had no significant effect on the ammonia content in the brain of juvenile O. marmorata during the subsequent 24 h. Unlike other fishes, juvenile O. marmorata could apparently avoid postprandial ammonia toxicity, and as a result there was no increase in the brain glutamine content, which decreased significantly instead at certain post-feeding time point. Since no prominent increases in tissue glutamine and urea contents were observed after feeding despite its ability to detoxify ammonia to glutamine during emersion and its possession of a full complement of hepatic ornithine-urea cycle enzymes, it can be concluded that only a moderate amount of ammonia was produced after feeding. Traditionally, it has been accepted that excess amino acid would be degraded in the liver through transdeamination which involves the deamination of glutamate catalyzed by glutamate dehydrogenase (GDH). However, we report for the first time that glutamate content in liver and intestine of juvenile O. marmorata increased to 10.8 μmol g⁻¹ and 3.3 μmol g⁻¹, respectively, 6 h post-feeding. These results indicate that the rate of glutamate catabolism in the liver was lower than the total rate of glutamate released directly from protein degradation and produced indirectly from other amino acids through transamination. Indeed, feeding led to significant increases in hepatic GDH amination activity and intestinal GDH activity, in both amination and deamination directions, in juvenile O. marmorata. Thus, there could be an increase in the capacity of the liver, and perhaps the intestine, to produce and retain glutamate after feeding. As a result, only 33% of the ingested nitrogen was excreted during the 24-h post-feeding period, with 67% being retained for somatic growth. Results obtained from this study can be useful for designing re-circulating aquaculture systems for juvenile marble goby, and they offer insights into mechanisms that can be induced by feeding to retain ingested nitrogen.