Solubilization of thyroxine-5'-deiodinase activity from rat liver microsome fraction

The greater part of T3 is converted from T4 in liver or kidney. The majority of this activity exists in microsomal fraction. In the present study, we investigated whether this activity can be solubilized from rat liver microsomal pellet with various concentrations of deoxycholate (DOC). The extent of solubilization was compared with that of protein, rotenone insensitive NADH cytochrome c reductase or NADH cytochrome b5 reductase, which have been shown to associate with microsomal membrane rather than luminar contents. When 0.05% of DOC which was capable of releasing luminar contents of microsomal vesicles was applied to microsomal suspension, only a limited part of NADH cytochrome b5 reductase, rotenone insensitive NADH cytochrome c reductase or T4-5'-deiodinase activity was solubilized. When the concentration of DOC was increased to 0.125%, 41% of T4-5'-deiodinase activity was solubilized. Solubilization of protein, NADH cytochrome b5 reductase or rotenone insensitive NADH cytochrome c reductase was increased abruptly to 66%, 58% or 63%, respectively. The highest specific activity was obtained at 0.125% DOC. These results suggest that the T4-5'-deiodinase is associated with microsomal membrane instead of luminar contents.