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Other Factors Affecting Metabolism for fish

Apart from genetic adaptation, climatic factors also play an important role in carbohydrate metabolism in fish. Acclimation in fish, in essence, reflects enzyme acclimation, since the animal's ability to survive depends largely upon its ability to carry out normal metabolic functions. Some enzymes for metabolic acclimation show good compensation while others do not. The enzymes associated with energy liberation (enzymes of glycolysis, pentose shunt, tricarboxylic acid cycle, electron transport and fatty acid oxidation) exhibit temperature compensation whereas, those enzymes dealing largely with the degradation of metabolic products show poor or reverse compensation (see Table 1).

Table 1 Enzymes Subject to Metabolic Acclimation

Enzymes exhibiting compensation Enzymes exhibiting reverse or no compensation
phosphofructokinase catalase
aldolase peroxidase
lactic dehydrogenase acid phosphatase
6-phosphogluconate dehydrogenase D-amino acid oxidase
succinic dehydrogenase Mg-ATP ase
malic dehydrogenase choline acetyl transferase
cytochrome oxidase acetylcholine esterase
succinate-cytochrome C reductase alkaline phosphatase
NAD-cytochrome C reductase allantoinase
aminoacyl transferase uricase
Na-K-ATPase amylase
protease lipase
malic enzyme
glucose-6-phosphate dehydrogenase

It is interesting to note that two key enzymes involved in carbohydrate metabolism, amylase and glucose-6-phosphate dehydrogenase, together with an enzyme involved in fat digestion, lipase, show no temperature compensation. It is not certain if this is in any way connected with the cessation of feeding by fish at low temperatures. The molecular mechanism of thermal acclimation are not well understood and may consist of changes in synthesis or amounts of a given enzyme. Differences in kinetics, changes in the proportion of isoenzymes suitable for particular temperatures, and changes in co-factors such as lipids, co-enzymes, or other factors such as pH and ions may be important in the animal's adjustment to temperature changes.

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