The thiol-activated enzymatic outer-ring monodeiodination of iodothyronines by rat kidney microsomes at low (nanomolar) substrate concentrations shows an apparently sequential reaction mechanism and is further characterized by insensitivity to inhibition by dicoumarol, a moderate sensitivity to inhibition by propylthiouracil (Ki = 100 microM) and iopanoic acid (Ki = 0.9 mM), responsiveness to 5 mM glutathione (GSH), and a thermal activation profile that is concave downward with a Td of approximately 20 degrees C. In contrast, the activity at high (micromolar) substrate concentrations shows a ping-pong reaction mechanism, is inhibited by micromolar concentrations of propylthiouracil, iopanoic acid and dicoumarol, is unresponsive to 5 mM GSH, and shows a concave upward thermal activation profile. Analysis of the microsomal deiodinase reaction over a wide range of 3,3',5'-triiodothyronine (rT3) concentrations (0.1 nM to 10 microM) suggested the presence of two enzymatic activities, with apparent Michaelis constants (Km) of 0.5 microM and 2.5 nM. Lineweaver-Burk plots of reaction velocities at nanomolar substrate concentrations in presence of 100 microM propylthiouracil also revealed an operationally distinct enzymatic activity with Km's of 2.5 and 0.63 nM and maximum velocities (Vmax's) of 16 and 0.58 pmol/mg protein per h for rT3 and thyroxine (T4), respectively. These findings are consistent with the presence of a low Km iodothyronine 5'-deiodinase in rat kidney microsomes distinct from the well characterized high Km enzyme and suggest that at circulating levels of free T4 the postulated low Km enzyme could be physiologically important.