Kinetic characterization of outer-ring deiodinase activity (ORD) in the liver, gill and retina of the killifish Fundulus heteroclitus

Abstract

Conversion of T 4 to T 3 is the first step in TH action and deiodinases are the major determinants of TH tissue availability and disposal. We here report the kinetic characterization of the outer-ring deiodinating (ORD) enzymes in the liver, gill and retina of sea water-adapted killifish, by using both rT 3 and T 4 as substrates. In liver, by using rT 3, we detected a high K m (84 nM) and a low K m (1.3 nM) component with kinetic characteristics similar to mammalian deiodinases DI and DII. In contrast, T 4-ORD only generated a low K m (0.5 nM) component. As judged by its V max (920 fmol 125I/mg per h) this DII enzyme is very abundant, approximately five and 20 times higher than that found in trout liver and hypothyroid rat, respectively. Kinetic analysis in killifish gill showed only one enzymatic component, with a high rT 3 K m (430 nM) and a relatively low V max (4.3 pmol 125I/mg per h). Our results in killifish retina show the expression of a T 4-low K m (0.6 nM) deiodinase with high cofactor requirements akin to the mammalian DII. The V max value for this enzyme is 182 fmol 125I/mg per h, five times lower than the one found in killifish liver, but comparable to that in hypothyroid rat pituitary. The biochemical similarities between fish and mammalian deiodinases could reflect their high conservation during vertebrate evolution and thus their importance in the regulation of thyroid hormone action.