Hepatic 5′-monodeiodinase activity in teleosts in vitro: A survey of thirty-three species

Abstract

The in vitro hepatic 5'-monodeiodination of thyroxine (T4) to triiodothyronine (T3) in Oreochromis mossambicus, Channa striata, Clarias batrachus, Cyprinus carpio and Oxyeleotris marmorata was found to be time, pH and temperature dependent, and related to the amount of substrate (T4) and homogenate introduced into the reaction vessel, in a manner which was consistent with Menton-Michaelis kinetics, and thus indicative of an enzyme-regulated process. Dithiothreitol introduced into the reaction vessel stimulated T3 production in a dose-related manner.Hepatic 5'-monodeiodinase activity was also detected in a further 28 species of teleosts suggesting that the peripheral monodeiodination of T4, which is well-documented in salmonids, is also widespread amongst other teleost fishes. All species examined exhibited evidence of enzymatic deiodination, but there were marked differences in Km and Vmax values between the species. There was no apparent phylogenetic or environmental relationships to explain the widely divergent Km and/or Vmax values, nor was there a correlation between Km and Vmax when the species were considered together.

Different thyroid hormone‐deiodinating enzymes in tilapia (Oreochromis niloticus) liver and kidney

“…Both enzymes show (a) location in the microsomal fraction of the tissues, (b) preference for T4 over rT3 as the substrate, (c) K,,, values for the substrates in the nM range, (d) a relatively high cofactor requirement (-25 mM DTT), and (e) lack of sensitivity to inhibition by PTU, IAc and ATG. These findings extend recent studies of Leatherland [18] and Eales [19] and coworkers, who also reported on low-K,, T4 ORD activities in liver of different teleost species with a low sensitivity for PTU.…”

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): II. Changes in the hormones and 5′-monodeiodinase activity during development

“…It is dependent on the presence of DTT in the incubation medium and the T 3 production increases with increase in the concentration of DTT, as also reported by several workers (Eales et al 1984;McNabb et al 1986;Vijayan et al 1988;Flett and Leatherland 1989;Leatherland et al 1990). The production of T 3 was found to be dependent on the volume of tissue homogenate introduced into the medium.…”

“…Leatherland et al (1990) have standardized the assay for tilapia (O. mossambicus) liver homogenates and reported the kinetics of this enzyme. Adopting the same method, the appropriate incubation conditions were determined using the whole body homogenates of 15-day-old tilapia post-larvae.…”

“…The liver in fish, as in mammals, is the principal organ of T 4 deiodination (Eales 1979(Eales , 1982Leatherland 1981). Recently, Leatherland et al (1990) surveyed 33 species of freshwater and marine fishes from around South-East Asia and Western Australia and detected 5'-monodeiodinase (5'-D) activity in the livers of all the species studies although there were marked differences in K m and Vma x values among the species. The kinetics of this enzyme were studied in detail in five of the species, including tilapia, O. mossambicus.…”

Endocrine, Physiological and Histopathological Responses of Fish and their Larvae to Stress with Emphasis on Exposure to Crude Oil and Various Petroleum Hydrocarbons

“…Free T 4 enters cells of peripheral tissue (brain, liver, gill, kidney, heart and muscle) and is then deiodinated to form triidothyronine, (T 3 ) by L-thyroxine 5'monodeiodinase (MDI), the enzyme responsible for peripheral conversion of T 4 to T 3 Leatherland et al 1990;Byamungu et al 1992;Eales, 1990, 1992;Eales et al 1993a,b). Eales and Brown (1993) suggest that in teleosts the 'peripheral model' of plasma T 3 regulation by MDI activity predominates over the 'central model' via the hypothalamic-hypophyseal-thyroidal axis.…”