Effect of l-triiodothyronine on the mitochondrial ?-glycerophosphate dehydrogenase activity, mitochondrial and total protein contents of brain of Singi fish (Heteropneustes fossilis bloch).

A single injection of various doses (0.25, 0.5, 5, 20 and 50 ?g/g) of l-triiodothyronine increased the mitochondrial cytochrome-linked ?-glycerophosphate dehydrogenase (EC 1.1.99.5) activity and mitochondrial protein content of brain of Singi fish on the 3rd day. l-Triiodothyronine at the dose of 0.1 ?g/g did not alter the ?-glycerophosphate dehydrogenase activity and mitochondrial protein content of brain. The total protein content of the brain also increased on the 3rd day with 0.5 ?g of l-triiodothyronine per g. Increased enzyme activity followed a dose-response relationship of a non-linear fashion. The enhancement of the ?-glycerophosphate dehydrogenase activity of fish brain with a dose of 0.5 ?g/g was found from the 1st day and it reached to a maximum level from the 3rd to the 5th day. The enzyme activity then sharply declined on the 6th or 7th day. Cycloheximide inhibited the l-triiodothyronine-induced increase in the ?-glycerophosphate dehydrogenase activity, mitochondrial and total protein content of fish brain. The present study thus reveals the responsiveness of fish brain to thyroid hormone, and ?-glycerophosphate dehydrogenase activity can be taken as a biochemical indices for the expression of thyroid hormone action in fish brain.