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 μmol/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 μmol/l and a Vmax of 1·41 pmol T3 generated/mg protein per min at a T4 concentration of 5 μmol/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.
J. Endocr. (1986) 110, 441–446.
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