Kinetics of hepatic thyroxine 5'-deiodinase

The enzymatic kinetic constants of 5'-deiodination of thyroxine in the rat liver were determined by studying the enzymatic activity at several sets of concentrations of thyroxine and dithiothreitol. Parallel increases in apparent Vmax and Km for thyroxine were observed by increasing the concentration of dithiothreitol. Double-reciprocal replots of these apparent values vs the dithiothreitol concentration gave straight lines. These results provide further evidence for a ping-pong mechanism of the enzymatic process. The 'true' Km values for thyroxine and dithiothreitol were computed to be 16 +/- 5 microM and 1.2 +/- 0.4 mM, respectively. The Vmax was 126 +/- 23 pmol/mg protein/min.