ラット肝におけるThyroxineから3, 3′, 5-triiodothyronineへの転換に関する研究還元型グルタチオン, その他のSH化合物のT45′-Deiodinase促進作用の機序について

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In this study, it was attempted to look for an endogenous stimulatory factor for T4 5′-deiodinase activity in rat liver and to investigate the mechanism of this stimulation by thiol. The TCA extract of rat liver homogenate was subjected to gel chromatography on a Sephadex G-15 column using 0.4M sodium acetate buffer (pH 4.0) as eluate. The collected fractions were examined for stimulatory factors for 3, 3′, 5-triiodothyronine (T3) production from thyroxine (T4). It was found that a stimulatory factor was eluted at the same position as that of a thiol substance. Furthermore, the eluted position corresponded to that of authentic reduced glutathione (GSH). These results indicated that endogenous GSH is responsible for the stimulation of the conversion of T4 to T3. The stimulatory effects of GSH and cysteine on the T4 to T3 conversion in rat liver microsomes were maximum above 8 mM, and maximal stimulation attained was about 3 and 2.5-fold respectively, over basal activity observed in the absence of thiols. Considering the high concentration of GSH in the liver, GSH possibly functions as an endogenous activator in the conversion of T4 to T3. Among the thiol substances examined, dithiothreitol (DTT) showed the most potential activity for enhancing T4 5′-deiodinase. Maximal activity was approximately 9 times that of the basal activity. The mechanism of the activation by thiol was studied using DTT. Rat liver microsomes, pre-treated with 2 mM DTT for 30 min at 0°C, were subjected to Sepharose CL-6B gel chromatography at 4°C, using 20 mM Tris HC1/1 mM EDTA (pH 7.4) as eluate, and the microsomes were separated from DTT in 30 min. When these microsomes were incubated with T4 alone, T3 production was 2.5-fold compared with the original activity of the microsomes eluted from the chromatography. This rate of T3 production was only 10% of that obtained by incubating with T4 in the presence of 2 mM DTT. It was clearly shown from this result that 90% of the enhanced activity was sustained by coexisting with DTT. Furthermore, the same type of experiment was carried out using solubilized rat liver microsomes. The enzyme solubilized with 0.1% deoxycholate was incubated with 1 mM DTT for 30 min at 0°C and then subjected to a Sepharose CL-6B column (1.4 × 65 cm) at 4°C. All of the collected fractions gave only marginal activity of T3 formation when they were incubated with T4 alone. However, when activity was assayed in the presence of 1 mM DTT, there appeared substantial activity in the fractions containing high molecular weight components. In conclusion, one of the effects of thiol for T4 5′-deiodinase is to protect sulfhydryl groups (s) in this enzyme. On the other hand, the present finding, that the separation of the enzyme from thiol resulted in substantial diminution in activity, suggests that thiol compounds may act as cofactors in the T4 to T3 conversion.
日本内分泌学会の論文

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今井幸宏
名古屋大学医学部第一内科

ラット肝におけるThyroxineから3, 3′, 5-triiodothyronineへの転換に関する研究還元型グルタチオン, その他のSH化合物のT<SUB>4</SUB>5′-Deiodinase促進作用の機序について

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