Synthesis and properties of N-bromoacetyl-L-thyroxine

Substitution of cysteine for selenocysteine in the catalytic center of type III iodothyronine deiodinase reduces catalytic efficiency and alters substrate preference

Human type III iodothyronine deiodinase (D3) catalyzes the conversion of T(4) to rT(3) and of T(3) to 3, 3'-diiodothyronine (T2) by inner-ring deiodination. Like types I and II iodothyronine deiodinases, D3 protein contains selenocysteine (SeC) in the highly conserved core catalytic center at amino acid position 144. To evaluate the contribution of SeC144 to the catalytic properties of D3 enzyme, we generated mutants in which cysteine (D3Cys) or alanine (D3Ala) replaces SeC144 (D3wt). COS cells were transfected with expression vectors encoding D3wt, D3Cys, or D3Ala protein. Kinetic analysis was performed on homogenates with dithiothreitol as reducing cofactor. The Michaelis constant of T(3) was 5-fold higher for D3Cys than for D3wt protein. In contrast, the Michaelis constant of T(4) increased 100-fold. The D3Ala protein was enzymatically inactive. Semiquantitative immunoblotting of homogenates with a D3 antiserum revealed that about 50-fold higher amounts of D3Cys and D3Ala protein are expressed relative to D3wt protein. The relative substrate turnover number of D3Cys is 2-fold reduced for T(3) and 6-fold reduced for T(4) deiodination, compared with D3wt enzyme. Studies in intact COS cells expressing D3wt or D3Cys showed that the D3Cys enzyme is also active under in situ conditions. In conclusion, the SeC residue in the catalytic center of D3 is essential for efficient inner-ring deiodination of T(3) and in particular T(4) at physiological substrate concentrations.

Selective labelling and inactivation of creatine kinase isoenzymes by the thyroid hormone derivative N-bromoacetyl-3,3',5-tri-iodo-L-thyronine

Besides their well-known regulation of transcription by binding to nuclear receptors, thyroid hormones have been suggested to have direct effects on mitochondria. In a previous study, incubation of rat heart mitochondria with 125I-labelled N-bromoacetyl-3,3',5-tri-iodo-L-thyronine (BrAcT3), a thyroid hormone derivative with an alkylating side chain, resulted in the selective labelling of a protein doublet around M(r) 45,000 on SDS/polyacrylamide gels [Rasmussen, Köhrle, Rokos and Hesch (1989) FEBS Lett. 255, 385-390]. Now, this protein doublet has been identified as mitochondrial creatine kinase (Mi-CK). Immunoblotting experiments with the cytoplasmic and mitochondrial fractions of rat heart, brain and liver, as well as inactivation studies with the purified chicken CK isoenzymes have further demonstrated that all four CK isoenzymes (Mia-, Mib-, M- and B-CK) are indeed selectively labelled by BrAcT3. However, in contrast with their bromoalkyl derivatives, thyroid hormones themselves did not compete for CK labelling, suggesting that not the thyroid hormone moiety but rather the bromoacetyl-driven alkylation of the highly reactive 'essential' thiol group of CK accounts for this selective labelling. Therefore the assumption that CK isoenzymes are thyroid-hormone-binding proteins has to be dismissed. Instead, bromoacetyl-based reagents may allow a very specific covalent modification and inactivation of CK isoenzymes in vitro and in vivo.