Maturational changes in the regulatory mechanisms of hepatic thyroxine 5'-monodeiodination in growing rats.

Abstract

Summary: Changes in the regulatory mechanisms of hepatic thyroxine (T4) 5′-monodeiodination during maturational process were studied in rats aged 1,3, and 6 wk and in adults. Despite low T4 5′-deiodinase activity in the neonates, a similar degree of activation in older rats was obtained with graded doses of dithiothreitol. Lineweaver-Burk plot analysis showed that Vmax increased 1–3 wk of age, decreasing with age thereafter, whereas a high Km value in young rats (7.0–7.8 × 10−4 M) fell to a level of 4.8 × 10−4 ? by 6 wk of age. T4 5′-deiodinase at 3 wk of age was relatively resistant to iodoacetamide, a SH blocking agent (11% inhibition at 10−6 M versus 47% in adult). Furthermore, it was markedly enhanced with 3 mM EDTA (125% versus 10–20% in older rats). Among various bivalent cations tested, Cu++ and Zn++ had a strong inhibitory effect on the reaction, whereas livers from 3-wk-old rats were less sensitive to Zn++ (7% inhibition at 10−6 M versus 40% in adult). Responses to graded doses of reduced glutatbione (GSH) or to its blocker, diamide, suggest that GSH exerts its promoting effect through preservation of protein SH radicals in reduced form. In contrast, NADPH stimulates the reaction directly, and a marked increase in the sensitivity to NADPH was observed 1–3 wk of age. Dose-response relation to methylene blue (MB), inhibitor of NADPH, exhibited a biphasic effect on the reaction: stimulatory at smaller dose and inhibitory at larger dose. The critical dose of MB producing this reversal shifted to a lower level with advancing age, which appears to be due to the content of endogenous NADPH as well as to the reactivity of the enzyme to it. These results indicate that (1) protein SH radicals appear to change from a relatively inactive to an active state with age, in which an interaction with Zn++ might be involved; (2) GSH is probably associated with the conversion of SH groups; (3) NADPH enhances directly the enzyme activity, playing a pivotal role in the regulation of the reaction; and (4) maturation of T4 5′-deiodination includes changes in the protein SH groups and GSH-NADPH generating system.