Hepatic outer-ring deiodinase in a Mexican endemic lizard (Sceloporus grammicus)

The kinetic characterization of the outer-ring deiodination pathway using r T(3) (rT(3)-ORD) in male, female, and pregnant female livers of an endemic lizard, Sceloporus grammicus, is reported. The ORD pathway does not have th e characteristics of deiodinase type II; it is exclusively carried out by d eiodinase type I (DI). DI enzymatic activity in lizard liver contains one o f the highest activities reported in vertebrates. This activity is sexually dimorphic, with males presenting the highest activity during the reproduct ive season. The properties of this enzyme correspond to those described in mammals, such as specificity for rT(3), susceptibility to inhibition by 6-n -propyl-2-thiouracil and gold-thioglucose, cofactor requirement, and kineti c pattern. Unlike other vertebrates, the lizard DI exhibits conspicuous sta bility in the thermal range of 15 to 42 degrees C and in the pH range of 5. 0 to 9.0. Male true kinetic constants exhibit a direct correlation with tem perature. This is in agreement with short term adaptation to microenvironme ntal changes and the feasible expression of enzymatic forms/variants which, together, endow this lizard species with a greater adaptation to natural d aily ambient thermal fluctuations. (C) 2000 Academic Press.