Fish Physiology and Biochemistry vol:18 issue:3 pages:253-266
The presence of outer ring deiodinating (ORD) and inner ring deiodinating (IRD) activities was investigated in different tissues of Oreochromis niloticus (Nile tilapia), Clarias gariepinus (African catfish), Oncorhynchus mykiss (rainbow trout) and Scophthalmus maximus (turbot). High-K-m rT(3) ORD is present in the kidney of most of the fishes studied, except in catfish. In turbot, besides the kidney, rT(3) ORD is also present in liver, heart and ovary. Low-K-m T-4 ORD is found in the liver and low-K-m T-3 IRD in the brain of all the fishes studied. In addition, low levels of low-K-m T-3 IRD were demonstrated in gill and skin of Nile tilapia, liver of rainbow trout and gill and kidney of turbot. For the different teleosts, the biochemical properties of the different rT(3)-deiodinating enzymes mentioned, T-4 ORD in liver and T-3 IRD in brain and tilapia gill were compared to those of the deiodinases formerly characterized in Oreochromis aureus (blue tilapia). In general, the different deiodinases demonstrate analogous sensitivities to iodothyronines and inhibitors, although minor differences occur. The various deiodinating enzymes all depend on addition of dithiothreitol and demonstrate maximal activity at a pH between 6.5 and 7. The optimal incubation temperature of rT(3) ORD and T-4 ORD in tilapia and catfish is 37 degrees C, in trout and turbot it varies, depending on the tissue, between 25 degrees and 37 degrees C. For the different T-3 IRD activities the optimal temperature is 37 degrees C in warmwater as well as in coldwater species. The apparent K-m values for rT(3) ORD lay in the mu M range, for T-4 ORD and T-3 IRD they lay in the nM range. V-max values are usually higher in tilapia as compared to the other teleosts studied. Based on the similarities in susceptibility to inhibition by different iodothyronines and inhibitors and the agreement of the apparent K-m values, we conclude that the deiodinating enzymes in teleosts are more similar to mammalian deiodinases than is generally accepted.