Thyroxine dynamically regulates levels of type II iodothyronine 5'-deiodinase 
(5'D-II) by modulating enzyme inactivation and targeting the enzyme to different 
pathways of internalization. 5'D-II is an approximately 200-kDa multimeric 
protein containing a 29-kDa substrate-binding subunit (p29) and an unknown 
number of other subunits. In the absence of thyroxine (T4), p29 is slowly 
endocytosed and transported to the lysosomes. T4 treatment rapidly activates an 
actin-mediated endocytotic pathway and targets the enzyme to the endosomes. In 
this study, we have characterized the influence of T4 on the intracellular 
trafficking of 5'D-II. We show that T4 accelerates the rate of 5'D-II 
inactivation by translocating the enzyme to the interior of the cell and by 
sequestering p29 in the endosomal pool without accelerating the rate of 
degradation of p29. This dichotomy between the rapid inactivation of catalytic 
activity and the much slower degradation of p29 is consistent with the reuse of 
p29 in the production of 5'D-II activity. Immunocytochemical analysis with a 
specific anti-p29 IgG shows that pulse affinity-labeled p29 reappears on the 
plasma membrane approximately 2 h after enzyme internalization in the presence 
of T4, indicating that p29 is recycled. Despite the ability of p29 to be 
recycled in the T4-treated cell, 5'D-II catalytic activity requires ongoing 
protein synthesis, presumably of another enzyme component(s) or an accessory 
enzyme-related protein. In the absence of T4, enzyme inactivation and p29 
degradation are temporally linked, and pulse affinity-labeled p29 is 
internalized and sequestered in discrete intracellular pools. These data suggest 
that T4 regulates fundamental processes involved with the turnover of integral 
membrane proteins and participates in regulating the inter-relationships between 
the degradation, recycling, and synthetic pathways.