Mechanism of inhibition of malate dehydrogenase by thyroxine derivatives and reactivation by antibodies: homogeneous enzyme immunoassay for thryroxine.

Pig heart mitochondrial malate dehydrogenase (L-malate:NAD+ oxidoreductase, EC 1.1.1.37) is about 90% inhibited upon labelling an average of two amino groups per subunit with an active ester of thyroxine. Inhibition is probably associated primarily with thyroxine binding to one specific group which is normally unreactive but becomes activated upon noncovalent binding of thyroxine derivatives to the enzyme. Enzyme inhibition is due to a decrease in the rate of association of NAD. Antibodies to thyroxine induce a slow conformational change with partial reversal of inhibition of more heavily labelled conjugates. The antibody-induced activation is not cooperative and does not require bivalent association of the antibody. Activation can be blocked by the presence of free thyroxine and is the basis for a clinically useful assay for serum thyroxine.