A selenium-containing catalytic antibody with Type I deiodinase activity.

Acting as a mimic of type I deiodinase (DI), a selenium-containing catalytic antibody (Se-4C5) prepared by converting the serine residues of monoclonal antibody 4C5 raised against thyroxine (T4) into selenocysteines, can catalyze the deiodination of T(4) to 3,5,3'-triiodothyronine (T(3)) with dithiothreitol (DTT) as cosubstrate. Investigations into the deiodinative reaction by Se-4C5 revealed the relationship between the initial velocity and substrate concentration was subjected to Michaelis-Menten equation and the reaction mechanism was ping-pong one. The kinetic properties of the catalytic antibody were a little similar to those of DI, with Km values for T(4) and DTT of approximately 0.8 microM and 1.8 mM, respectively, and V(m) value of 270 pmol per mg protein per min. The activity could be sensitively inhibited by PTU with a Ki value of approximately 120 microM at 2.0 microM of T(4) concentration, revealing that PTU was a competitive inhibitor for DTT.