Thyroid hormone deiodination in tissues of American plaice, Hippoglossoides platessoides: characterization and short-term responses to polychlorinated biphenyls (PCBs) 77 and 126

We have described the tissue distribution and properties of thyroid hormone (TH) deiodination activities of the marine American plaice, Hippoglossoide s platessoides. We then studied the 1- or 4-week responses of the plaice li ver and brain deiodination activities and the plasma thyroxine (T4) and 3,5 ,3'-triiodothyronine (T3) levels to an intraperitoneal injection (5-500 ng/ g) of the polychlorinated biphenyl (PCB) congeners 77 (3,3'-4,4'-tetrachlor obiphenyl) or 126 (3,3',4,4',5 -pentachlorobiphenyl). T4 and 3,3'5'-triiodo thyronine (rT3) outer-ring deiodination (ORD) activities were greater in li ver than in kidney, gill, heart, brain, intestine or muscle; inner-ring dei odination (IRD) activity occurred in all tissues but was consistently highe r in brain. Deiodination characteristics (optimal pH: optimal dithiothreito l concentration, responses to inhibitors and apparent K-m values of 0.6-4 n M) fell in the same rage as those of low-K-m deiodinases in other teleosts. Deiodination activities were maximal when assayed at 25 degreesC but unifo rmly low over the natural range of 0-9 degreesC. Neither PCB 77 nor PCB 126 altered brain T4ORD activity or plasma T4 levels (P < 0.05). However, at 1 week post injection hepatic T4ORD activity was increased and plasma T3 lev els lowered by PCB 77 (5 and 25 ng/g), while hepatic IRD activity was incre ased by PCB 126 (50 and 500 ng/g). Neither PCB 77, PCB 126 nor selected hyd roxylated. PCBs given in vitro compared with T4 for binding sites on plasma proteins or altered hepatic deiodination activity, indicating no direct ac tion on plasma proteins or deiodinases We conclude that plaice TH deiodinat ion tissue distribution and characteristics resemble those of other teleost s. Deiodination activities are low at natural assay temperatures but at I w eek show some responses to PCBs 77 and 126. (C) 2000 Elsevier Science Inc. All rights reserved.