jb.oxfordjournals.org/cgi/conten … /128/1/121
The NADP(H)-linked oxidoreductase activity of a major isozyme of human liver 3{alpha}-hydroxysteroid dehydrogenase was activated 5-, 4-, and 2-fold by D-thyroxine (T4), L-T4 and DL-3,3’,5’-triiodothyroiiine (reverse T3), respectively. Kinetic analysis of the activation indicated that D-T4, L-T4 and reverse T3, are non-essential activators, showing binding constants of 1.5, 1.1, and 3.6 µM, respectively. Comparison of the effects of the T4 analogs on the activities of the mutant enzymes suggests that the binding site is composed of at least Lye-270, Arg-276, and the C-terminal loop of the enzyme. L-T3, DL-thyro-nine, and D-tyrosine had no effect on the enzyme, but 3,5,3’,5’-tetra- and 3,5,3’-tri-iodo-thyropropionic acids were potent competitive inhibitors with K1, values of 42 and 60 nM, respectively, with respect to the substrate. The inhibition constant was lowered upon the activation of the enzyme by D-T4, and the inhibition by the deamino derivatives of T4 and T3 disappeared upon modification of the C-terminal loop of the enzyme, but not upon replacement of Lys-270 or Arg-276 with Met. These results indicate that, depending on their structures, the T4 analogs bind differently to two distinct sites at the active center of the enzyme to produce stimulatory and inhibitory effects.