Demonstration of Hepatic Cytosolic Malic Enzyme Activity as a Thyroid Hormone Sensitive Physiologic Parameter in a Teleost, Heteropneustes Fossilis Bloch

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 - T₃ Action in Fish - T₃ Induction of Malic Enzyme