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Effects of ammonia toxicity on the activities of enzymes involved in nitrogen and carbohydrate metabolism in the muscle and liver of two species of mudskippers
Author
Fong, Siao Wan
Supervisor
Chew, Shit Fun
Abstract
Periophthalmodon schlosseri is active on land during high or low tides and can tolerate high environmental ammonia. In contrast, Boleophthalmus boddarti grazes on the mudflats during low tides, disappears into the burrow during high tides and has a lower tolerance for environmental ammonia compared to P. schlosseri. When exposed to environmental ammonia or terrestrial conditions, they encounter difficulty in excreting ammonia. The accumulation of ammonia in the body is toxic. This study was undertaken to evaluate the involvement of enzymes associated with nitrogen and carbohydrate metabolism in the muscle and liver of both species of mudskippers during environmental ammonia exposure or terrestrial conditions. Since P. schlosseri is very active on land, experiments were also performed to evaluate both nitrogen and carbohydrate metabolism in its muscle and liver after it had been exercised to exhaustion. When P. schlosseri was exposed to 75 mmol l-1 NH4Cl for 6 d or terrestrial conditions for 4 d, no significant changes in the activities of alanine transaminase (Alt), aspartate transaminase (Ast), glutamine synthetase (Gs) and glutamate dehydrogenase (Gdh) were observed in the muscle and liver. Similarly, there were no changes to the activities of Alt, Ast, and Gs when B. boddarti was exposed to 10 mmol l-1 NH4Cl for 6 d or terrestrial conditions for 4 d. However, there was an increased in the activity of Gdh in the liver of B. boddarti when exposed to NH4Cl or terrestrial conditions. Thus, the enzymes associated with nitrogen metabolism were not affected in both species of mudskippers exposed to environmental ammonia or terrestrial conditions. The increase in Gdh activity indicated that B. boddarti might be detoxifying ammonia to glutamate during environmental ammonia or terrestrial exposure. In the muscle of P. schlosseri, the activity of pyruvate kinase (Pk) increased after exposure to terrestrial conditions while that of phoshofructokinase (Pfk) increased after exposure to both environmental ammonia and terrestrial conditions. These results indicated that there could be an increase in the capacity for carbohydrate metabolism when P. schlosseri is on land. However, whether the fish is resorting to carbohydrate metabolism to provide energy for its activity on land remains unclear. On the other hand, no significant changes were observed in the activities of Pk, Pfk and malic enzyme (Me) in the liver of P. schlosseri. Only an increase in the activity of Pk in the liver of B. boddarti was observed after the fish was exposed to terrestrial conditions. As the level of Pk activity in the liver of B. boddarti was at least 10-fold lower than that in the muscle, thus the possibility of B. boddarti relying on carbohydrate metabolism maybe negligible. The increase in the activities of Alt and Me in the liver of P. schlosseri after 3 min of intense exercise corroborates the proposal that it resorts to partial catabolism of free amino acids to alanine without ammonia production. In addition, the increase in Gs activity indicated that it detoxifies ammonia to glutamine. There may also be an increase in capacity for carbohydrate metabolism in the muscle of P. schlosseri after 3 min of exercise since there was an increase in Pfk activity.
Date Issued
2015
Call Number
QL638.G7 Fon
Date Submitted
2015