A complementary DNA clone encoding the type I iodothyronine 5′- deiodinase (5’DI), which converts T4 to T3, has been isolated recently. The 5′DI messenger RNA (mRNA) is most abundant in kidney and liver. To gain insight into the function of 5’DI in the kidney, Northern blot analysis was used to localize the expression of this mRNA. Our results show that 5′DI mRNA was expressed in the cortex and outer medulla, but not inner medulla and papilla, indicating that there are regional differences in its expression. Using in situ hybridization, we demonstrate that the 5′DI hybridization signal is localized predominantly over tubules in the outer stripe of the outer medulla and in medullary rays, suggesting localization to proximal tubules. To identify which tubular cells express 5′DI mRNA, we compared the profile of 5′DI message from in situ hybridization with the immunostaining of adjacent sections with proximal tubular Si, S2, and S3 segment-specific antibodies. Most of the 5′DI antisense complementary RNA hybridized to the same proximal tubular cells with which the S3-specific antibody, anti-ecto-ATPase, reacted. Cells staining with an Si or S2 segment antibody showed little, if any, 5′DI mRNA. We conclude that the expression of 5′DI mRNA is restricted to the tubular cells of the proximal S3 segment. The S3 segment is also known to express high levels of proteins required for glutathione synthesis consistent with the requirement for a reduced thiol cofactor for iodothyronine deiodination by the 5´DI pathway.
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