PROPERTIES OF T4 5'-DEIODINATING SYSTEMS IN VARIOUS TISSUES OF THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

l-Thyroxine (T4) 5′-monodeiodinase (5′D) activity was examined in the microsomal fractions of liver, kidney, gill, white skeletal muscle, and red blood cells (RBC) of fed rainbow trout held in freshwater at 12°. Two distinct 5′D systems were established and were examined at low (0.08-1.3 nM) or high (1.6-25 nM) T4 substrate ranges. The low substrate 5′D occurred in liver, gill, and muscle, but not in kidney or RBC. The pH optimum was 7.0 and the optimum dithiothreitol (DTT) level ranged from 7 to 10 mM. The Km values (nM) were liver, 0.098; muscle, 0.198; and gill, 0.168. The Vmax values (pmol · hr-1 · mg protein-1) were liver, 3.74; muscle, 0.79; and gill, 0.62. DTT affected both the Vmax and the Km, and propylthiouracil (PTU) inhibited the Vmax. These data suggest a ping-pong type mechanism. In contrast, the high substrate 5′D occurred only in liver (pH 7 optimum, DTT optimum 15 mM) and in kidney (pH optima 6 and 8, DTT optimum 15 mM). The Km values (nM) were liver, 10.0; and kidney, 14.7; the Vmax values (pmol · hr-1 · mg protein-1) were liver, 8.21; and kidney, 5.76. DTT affected the Vmax but not the Km and PTU did not inhibit, indicating a sequential type mechanism. In conclusion, in rainbow trout there are at least two types of 5′D which differ in their tissue distribution, T4 substrate affinity, and enzyme mechanism, and which do not resemble in their combined properties the 5′D forms established in higher vertebrate taxa. © 1992.