Summary
Single injections of various doses (0.1, 0.25, 0.5, 5 and 20 μg) of T3 significantly increased the cytosolic malic enzyme activity (ΔOD/min/mg cytosolic
protein) in liver of Singi fish Heteropneustes fossilis Bloch, in a dose-dependent nature, maximum up to 5 μg/g dose on the 3rd day in comparison
to the control. There was no difference in the enzyme activity between 5 and 20 μg/g
of T3 doses. When the enzyme activity was expressed per mg DNA, the dose-dependent increase
in the malic enzyme activity was observed upto 0.5 μg/g of T3 , whereas a fall in the enzyme activity was noticed with 5 and 20 μg/g of T3 doses. Lowering the dose of T3 to 0.05 μg/g was without any effect on the malic enzyme activity (ΔOD/min/mg cytosolic
protein or DNA). Hepatic cytosolic protein content showed a biphasic nature of variation,
significant increase with single injections of 0.05, 0.1, 0.25 and 0.5 μg/g and a
fall with 5 and 20 μg/g of T3 doses in comparison to the untreated control. Cycloheximide treatments of the Singi
fishes counteracted both the T3 -induced rise in the hepatic cytosolic malic enzyme activity (ΔOD/min/mg cytosolic
protein or DNA) and the hepatic cytosolic protein contents.
Thiourea-treated hypothyroid fishes showed significantly decreased level of malic
enzyme activity (ΔOD/min/mg cytosolic protein or DNA) and cytosolic protein content
in liver. A single injection of T3 at 0.25 μg/g to the thiourea-treated fishes not only recovered but also increased
the enzyme activity and cytosolic protein content above the untreated control values.
Total DNA content of Singi fish liver remained unchanged after T3 , cycloheximide and thiourea treatments.
The present experiment indicates the thyroid hormone-sensitive nature of hepatic cytosolic
malic enzyme activity in Singi fish as found in mammalian vertebrates.
Key-Words
Fish Liver Malic Enzyme
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T3 Action in Fish
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T3 Induction of Malic Enzyme
