Partial amino acid catabolism leading to the formation of alanine in Periophthalmadon schlosseri (mudskipper): A strategy that facilitates the use ofamino acids as an energy source during locomotory activity on land

When the mudskipper Periophthalmodon schlosseri was exposed to terrestrial conditions under a 12h:12h dark:light regime the fish could be very active, and levels of total free amino acids increased significantly in the muscle and plasma. Alanine levels increased threefold in the muscle, fourfold in the liver and twofold in the plasma. Similar phenomena were not observed in the more aquatic mudskipper, Boleophthalmus boddaerti. From these results, we concluded that P. schlosseri was capable of partial catabolism of certa in amino acids to support activity on land. The amino groups of these amino acids were transferred directly or indirectly to pyruvate to form alanine. The resulting carbon chain was fed into the Krebs cycle and partially oxid ized to malate, which could replenish pyruvate through the function of mali c enzyme. This favourable ATP yield from partial amino acid catabolism was not accompanied by a net release of ammonia. Such an adaptation would be ad vantageous to P. schlosseri confronted with the problem of ammonia excretio n during aerial exposure. Indeed, when P. schlosseri were forced to exercis e on land after 24 h of aerial exposure, the alanine level in the muscles i ncreased significantly, with no apparent change in glycogen content. In add ition, there was no significant change in the ATP level and energy charge o f the muscle. In contrast, when B. boddaerti were exercised on land, glycog en levels in the muscles decreased significantly and lactate levels increas ed. In addition, muscle energy charge was not maintained and the ATP level decreased significantly. Hence, it was concluded that when P. schlosseri we re active on land, they were capable of using certain amino acids as a meta bolic fuel, and avoided ammonia toxicity through partial amino acid catabol ism. Such a strategy is the most cost-effective way of slowing down interna l ammonia build-up without involving energy-expensive ammonia detoxificatio n pathways. Furthermore, an examination of the balance between nitrogenous excretion and accumulation in a 70 g P. schlosseri revealed that degradatio n of amino acids in general was likely to be suppressed to slow down the bu ild-up of ammonia internally. It is possible that such a strategy may be wi dely adopted, especially by obligatory air-breathing fishes, to avoid ammon ia intoxication during aerial exposure.