CATALYTIC ACTIVITY OF TYPE-II IODOTHYRONINE 5'-DEIODINASE POLYPEPTIDEIS DEPENDENT UPON A CYCLIC-AMP ACTIVATION FACTOR

Type II iodothyronine 5'-deiodinase is an similar to 200-kDa multimeri c enzyme in the brain that catalyzes the deiodination of thyroxine (T- 4) to its active metabolite, 3,5,3'-triiodothyronine. In astrocytes, c AMP stimulation is required to express catalytically active type II io dothyronine 5'-deiodinase. The affinity ligand N-bromoacetyl-L-T-4 spe cifically labels the 29-kDa substrate-binding subunit (p29) of this en zyme in cAMP-stimulated astro cytes, To determine the requirements for cAMP-induced activation of this enzyme, we optimized N-bromoacetyl-L- T-4 labeling of p29 in astrocytes lacking type II iodothyronine 5'-dei odinase activity and examined the effects of cAMP on the hydrodynamic properties and subcellular location of the enzyme, We show that the p2 9 subunit is expressed in unstimulated astrocytes and requires 10-fold higher concentrations of N-bromoacetyl-L-T-4 to achieve incorporation levels equal to those of p29 in cAMP-stimulated cells, Gel filtration showed that p29 was part of a multimeric membrane-associated complex in both cAMP-stimulated and unstimulated astrocytes and that cAMP stim ulation led to an increase of similar to 60 kDa in the mass of the hol oenzyme, In unstimulated astrocytes, p29 resides in the perinuclear sp ace, Cyclic AMP stimulation leads to the translocation of p29 to the p lasma membrane coincident with the appearance of deiodinating activity . These data show that cAMP dependent activation of type II iodothyron ine 5'-deiodinase activity results from the synthesis of additional ac tivating factor(s) that associates with inactive enzyme and leads to t he translocation of enzyme polypeptide(s) from the perinuclear space t o the plasma membrane.