The Mechanism of Biochemical Action of Thyroid Hormones

Inosine mono phosphate (IMP) is a precursor of the two purine nucleotides, adenosine mono phosphate (AMP) and guanosine monophosphate (GMP): IMP -----adenylosuccinate ------AMP and IMP to xanthine monophosphate (XMP)---GMP. The reaction IMP----- adenylosuccinate is catalyzed by the enzyme synthetase which was purified and IMP---XMP is catalyzed by the enzyme IMP dehydrogenase which was purified also . Adenylosuccinate synthetase of rat liver is stimulated by L-T3 but not D-T4 and the effect of other analogues paralled their in vivo effects . The enzyme IMP dehydrogenase is inhibited by thyroid hormones, but not by D-T4, and also, the effects of other analogues paralled their in vivo effects.