Effect of cortisol on the in vitro hepatic conversion of thyroxine to triiodothyronine in brook charr (Salvelinus fontinalis Mitchill).

In vitro studies with liver homogenates of brook charr (Salvelinus fontinalis) demonstrate that the conversion of thyroxine (T4) to triiodothyronine (T3) is dependent upon temperature, pH, substrate, and homogenate concentration, supporting the hypothesis of an enzymatic hepatic T4 to T3 conversion. Dithiothreitol has a marked stimulatory effect on the rate of conversion. The kinetics of the conversion, determined by Lineweaver-Burke analysis suggest a Michaelis-Menten (Km) constant of 1.3 x 10(-8) M, and a maximum velocity (Vmax) of 0.42 pmol/mg protein/h. In brook charr given intraperitoneal implants of hydrogenated coconut oil containing cortisol at levels of 0, 5, and 50 mg/kg body weight, and sampled after 56 days, T4 to T3 conversion by homogenates of hepatic tissue in vitro was significantly (P less than 0.01) higher in the fish implanted with 50 mg/kg of cortisol than in either of the other two groups. However, plasma cortisol, L-thyroxine (T4) and triiodo-L-thyronine (T3) concentrations, plasma T3:T4 ratios and hepatosomatic index were not significantly different in the three treatment groups.