Studies on peroxidase-catalysed formation of thyroid hormones on a protein isolated from submaxillary gland

A protein has been solubilized and purified to homogeneity from the microsomal fraction of goat submaxillary gland. This protein can preferentially be iodinated to form triidothyronine and thyroxine with the help of submaxillary peroxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7) solubilized and purified from the same microsomal fraction. The protein can also be isolated from soluble supernatant and was found to be identical to the microsomal protein as judged by their moelcular properties as well as the formation of triiodothyronine and thyroxine. The protein has the molecular weight of 120 000 and contains two unequal subunits of molecular weight of 80 000 and 44 000. The molecular weight of the peroxidase is 72 000 and consists of a single polypeptide chain. The enzyme has the Rz value of 0.4 and is inhibited by azide and cyanide. Mersalyl, a mercurial, strongly inhibits the enzyme activity while N-ethylmaleimide cannot. The enzyme can catalyze the formation of 62 mumol of I3-/min per mg of protein at its optimun pH of 5.2. The apparent Km for H2O2 and KI is 0.16 . 10(-3) M and 1 . 10(-3) M, respectively.