EFFECTS OF ACCLIMATION AND ASSAY TEMPERATURE ON OUTER-RING AND INNER-RING THYROXINE AND 3,5,3'-TRIIODO-L-THYRONINE DEIODINATION BY LIVER-MICROSOMES OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

We studied effects of temperature on the deiodinating enzymatic pathways governing hepatic production of 3,5,3'-triiodo-L-thyronine (T-3) from L-thyroxine (T-4) in rainbow trout. Kinetics of outer-ring deiodination (ORD) and inner-ring deiodination (IRD) of T-4 and T-3 were measured in vitro at 4 degrees, 12 degrees, and 18 degrees C using liver microsomes of trout that were acclimated for 21 days at 4 degrees, 13 degrees, and 18 degrees C and fed on temperature-compensated food rations. For all temperatures the V-max (level of functional enzyme) and V-max/K-m (index of enzyme efficiency) values for the T-3-generating T(4)ORD pathway greatly exceeded values for the T-3-degrading T(3)IRD pathway; T(4)IRD and T(3)ORD) pathways were negligible. The main effects of temperature occurred between 4 degrees C and 12 degrees/13 degrees C; T(4)ORD V-max/K-m rose from 0.48 at 4 degrees C to 1.62 at 12 degrees/13 degrees C and to 1.83 at 18 degrees C. This response correlated with in vivo deiodination rates measured previously. T(4)ORD apparent K-m did not change over the temperature acclimation range, suggesting an absence of temperature-induced isozymes. However, K-m was depressed at the assay temperature of 4 degrees C, indicating a direct effect of temperature to enhance enzyme-T-4 substrate affinity. T(4)ORD V-max was insensitive to assay temperature but was depressed in trout acclimated at 4 degrees C. We conclude that from 4 degrees to 18 degrees C T(4)ORD is the main thyroid hormone deiodinating pathway in trout liver. T(4)ORD activity increases more than threefold from 4 degrees to 13 degrees C. This is due to a direct effect of temperature to decrease K-m and to an acclimatory increase in V-max. (C) 1995 Wiley-Liss, Inc.