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, Hippoglossoides platessoides. We then studied the 1- or 4-week responses of the plaice liver 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′-tetrachlorobiphenyl) or 126 (3,3′,4,4′,5-pentachlorobiphenyl). T4 and 3,3′5′-triiodothyronine (rT3) outer-ring deiodination (ORD) activities were greater in liver than in kidney, gill, heart, brain, intestine or muscle; inner-ring deiodination (IRD) activity occurred in all tissues but was consistently higher in brain. Deiodination characteristics (optimal pH, optimal dithiothreitol concentration, responses to inhibitors and apparent K_m values of 0.6–4 nM) fell in the same rage as those of low-K_m deiodinases in other teleosts. Deiodination activities were maximal when assayed at 25°C but uniformly low over the natural range of 0–9°C. 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 levels lowered by PCB 77 (5 and 25 ng/g), while hepatic IRD activity was increased by PCB 126 (50 and 500 ng/g). Neither PCB 77, PCB 126 nor selected hydroxylated. PCBs given in vitro compared with T4 for binding sites on plasma proteins or altered hepatic deiodination activity, indicating no direct action on plasma proteins or deiodinases We conclude that plaice TH deiodination tissue distribution and characteristics resemble those of other teleosts. Deiodination activities are low at natural assay temperatures but at 1 week show some responses to PCBs 77 and 126.