In-vitro conversion of thyroxine to tri-iodothyronine by chicken hepatic 5'-deiodinase: kinetic studies.

In-vitro studies with chicken liver homogenates demonstrate that the conversion of thyroxine (T4) to tri-iodothyronine (T3) is dependent upon tissue concentration, time of incubation, pH, temperature, the presence of dithiothreitol (DTT) and the concentration of substrate (T4), and is heat-labile. The generation of T3 is inhibited by iopanoic acid and 6-n-propyl-2-thiouracil. The kinetics of conversion of T4 to T3, determined by Lineweaver-Burke analysis, indicated an apparent Michaelis-Menten constant (Km) of 1.16 mumol/l with a maximum velocity (Vmax) of 44.57 pmol T3 generated/mg protein per min from T4. Dithiothreitol appears to behave as a co-substrate for this system with an apparent Km of 98.5 mumol/l and a Vmax of 1.41 pmol T3 generated/mg protein per min at a T4 concentration of 5 mumol/l. These data suggest that the conversion of T4 to T3 in fowl proceeds by means of an enzymatic system, probably 5'-monodeiodinase, and is responsible for maintaining T3 levels in vivo.