Induction of NADP-dependent malic dehydrogenase activity in brain of singi fish, heteropneustes fossilis (bloch), by 3,5,3?-triiodothyronine.
For elucidation of thyroid hormone-induced responsiveness of fish brain, various doses (0.012, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2 and 4 ?g/g) of triiodothyronine (T(3)) were injected in Singi fish, Heteropneustes fossilis (Bloch), for 3 consecutive days and the changes in cytosolic NADP-dependent malic enzyme (ME, EC 1.1.1.40) activity in whole brain tissue were determined. Compared to the control, the ME activity increased with lower doses (0.012, 0.025 and 0.05 ?g/g) and decreased with higher doses (1, 2 and 4 ?g/g) of T(3), showing a biphasic nature of thyroid hormone action. The enzyme activity remained unaltered with 0.1, 0.25 and 0.5 ?g of T(3)/g in comparison to the control. Immersion of the fishes in cycloheximide-containing medium (0.5 mg/l) inhibited the T(3) (0.025 ?g/g)-induced rise in ME activity. On the other hand, the NAD-dependent cytosolic malate dehydrogenase (EC 1.1.1.37) activity and the total protein content of brain cytosol remained unaltered with all doses of T(3) used. The thyroid hormone specificity of cytosolic NADP-dependent malic enzyme in fish brain is thus documented.
Srabani De
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AK
Animal Physiology Department, Bose Institute, Kankurgachi, Calcutta-700054, India.
Ray
D A
DA
Medda
A K
AK