5'- and 5-deiodinase activities in adult rat cecum and large bowel contents inhibited by intestinal microflora.

Enzymatic mechanisms for deiodination of 3,5,3'-triiodothyronine or thyroxine in the phenolic ring (5'-deiodinase) or tyrosyl ring (5-deiodinase) are found in cells of many organs, including the intestinal wall. Deiodinases are highly active in intestinal tissue of developing rat fetuses and relatively inactive in adult intestinal cells, but little is known about these systems in the luminal contents of intestines. We have found both 5- and 5'-deiodinase activities in adult rat intestinal contents and have shown that their expression is inhibited by resident intestinal microflora, which are normally present in the adult but not in the fetus, possibly because they are bound by intestinal bacteria in the adult.