Abstract
Kinetic studies were performed on the partially purified preparation of pig thyroid peroxidase using guaiacol as a hydrogen donor. The peroxidase was inhibited by ex-cess of hydrogen peroxide, but, below its optimal concentration, the linearity of Line-weaver-Burk plots was confirmed. The en-zyme was inhibited by potassium cyanide, sodium azide, sodium fluoride, potassium thiocyanate, and potassium perchlorate. From the mode of this inhibition, it seemed likely that the peroxidase is a hematin enzyme as are other peroxidases. Propylthiouracil, thiouracil, thiourea, aminotriazole, and p-aminobenzoate also showed inhibitory effect on the enzyme, their mode of inhibition be-ing somewhat different from that of the for-mer group.
Thyrotropic hormone did not cause any appreciable effect on the peroxidase activity. Most of metal ions except Hg2+ also did not affect the enzyme activity.
Peroxidation of iodide was studied by photometrically measuring the formation of triiodide. Excess of iodide as well as hydro-gen peroxide inhibited the enzyme activity.
I wish to express my sincere thanks to Prof. N. Ui for his valuable advice. This work was aided in part by a Grant-in-Aid for Fundamental Scientific Research from the Ministry of Education.
