Re-examination of the subcellular localization of thyroxine 5′-deiodination in rat liver

Abstract

We describe the existence of at least two thyroxine 5′-deiodinases in rat liver. They co-fractionate with NADPH-cytochrome c reductase, the marker enzyme for membranes of the endoplasmic reticulum. Subcellular-localization studies of the most active microsomal thyroxine 5′-deiodinase were performed under substrate saturation and at optimal pH 6.8. This enzyme was a Km(app.) of about 3 microM-thyroxine and a Vmax. of about 8 ng of tri-iodothyronine/min per mg of protein. Our study confirms in part the earlier reports of microsomal localization of thyroxine 5′-deiodination. However, this process is not mediated by only a single enzyme.