DIFFERENT THYROID HORMONE-DEIODINATING ENZYMES IN TILAPIA (OREOCHROMIS-NILOTICUS) LIVER AND KIDNEY

Enzymes catalyzing the outer ring deiodination (ORD) of iodothyronines are important for the regulation of thyroid hormone bioactivity. We h ave studied ORD of thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) i n liver and kidney microsomes of fish, i.e. tilapia (Oreochromis nilot icus). Tilapia kidney contains an enzyme which resembles the mammalian selenoenzyme type I iodothyronine deiodinase (ID-I) with respect to s ubstrate preference (rT3>T4) and high (almost-equal-to muM) K(m) value s, but is much less sensitive to selenocysteine (Sec)-targeted inhibit ors, including 6-propyl-2-thiouracil (PTU). In contrast, tilapia liver contains an enzyme very similar to mammalian type II deiodinase (ID-I I) with respect to substrate preference (T4>rT3), low (almost-equal-to nM) K(m) values, and lack of sensitivity to Sec inhibitors.