Analysis of the Key Metabolites and Genes Involving in Ornithine-Urea Cycle in Mytilus coruscus / 中国生物化学与分子生物学报

The ornithine-urea cycle (OUC) plays important roles in metabolism. However, there is a lack of study of OUC in shellfish. For filling this gap, the mussel Mytilus coruscus was selected, and the key genes together with the metabolites of OUC pathway were analyzed in the mantle and adductor muscle, respectively, using a real-time fluorescent quantitative PCR and an amino-acid analyzer. Moreover, the changes of the metabolite concentrations and the gene relative expression level of OUC were analyzed after arginine injection. The δ

Influence of environmental hypertonicity on the induction of ureogenesis and amino acid metabolism in air-breathing walking catfish (Clarias batrachus, Bloch).

Effect of environmental hypertonicity, due to exposure to 300 mM mannitol solution for 7 days, on the induction of ureogenesis and also on amino acid metabolism was studied in the air-breathing walking catfish, C. batrachus, which is already known to have the capacity to face the problem of osmolarity stress in addition to other environmental stresses in its natural habitats. Exposure to hypertonic mannitol solution led to reduction of ammonia excretion rate by about 2-fold with a concomitant increase of urea-N excretion rate by about 2-fold. This was accompanied by significant increase in the levels of both ammonia and urea in different tissues and also in plasma. Further, the environmental hypertonicity also led to significant accumulation of different non-essential free amino acids (FAAs) and to some extent the essential FAAs, thereby causing a total increase of non-essential FAA pool by 2-3-fold and essential FAA pool by 1.5-2.0-fold in most of the tissues studied including the plasma. The activities of three ornithine-urea cycle (OUC) enzymes such as carbamoyl phosphate synthetase, argininosuccinate synthetase and argininosuccinate lyase in liver and kidney tissues, and four key amino acid metabolism-related enzymes such as glutamine synthetase, glutamate dehydrogenase (reductive amination), alanine aminotransaminase and aspartate aminotransaminase were also significantly up-regulated in different tissues of the fish while exposing to hypertonic environment. Thus, more accumulation and excretion of urea-N observed during hypertonic exposure were probably associated with the induction of ureogenesis through the induced OUC, and the increase of amino acid pool was probably mainly associated with the up-regulation of amino acid synthesizing machineries in this catfish in hypertonic environment. These might have helped the walking catfish in defending the osmotic stress and to acclimatize better under hypertonic environment, which is very much uncommon among freshwater teleosts.

Functional ureogenesis and changes of amino acid metabolism in amphihaline shad hilsa (Tenualosa ilisha, Hamilton-Buchanan) while inhabiting in estuary and freshwater habitats.

The possible occurrence of a functional ornithine-urea cycle (OUC) and changes of activity of key amino acid metabolism-related enzymes were studied in the amphihaline shad hilsa (Tenualosa ilisha) that were collected from estuarine water of Kakdwip and from freshwater river basin of Bhrahmaputra during the breeding season. Very high concentration of urea was detected in different tissues and plasma of shad hilsa collected from estuarine water compared to the one collected from freshwater river basin. This observation clearly suggests that the shad hilsa has the potential of synthesizing and retaining urea inside the body for the purpose of osmoregulation while living in hypertonic saline environment of estuary. This was accompanied by the presence of high activity of all the five OUC enzymes in hepatic and in certain non-hepatic tissues such as the kidney and muscle of shad hilsa in support of its potential ureogenic capacity while inhabiting in estuarine water. The activities of different key amino acid metabolism-related enzymes such as glutamine synthetase, glutamate dehydrogenase, alanine aminotransaminase and aspartate aminotransaminase were also found to be significantly higher in shad hilsa of estuarine water compared to the one collected from freshwater habitat. Thus the adjustment to amino acid metabolism in shad hilsa in different environmental salinities appears to play significant roles for osmotic balance and also for proper energy supply in addition to the presence of a functional OUC while migrating between marine and freshwater habitats throughout their life cycle.