Biochemical studies on the peroxidase activity in the normal and hyperplastic thyroids of rats

The total and specific activity of thyroid homogenate of normal rat was 0.95 times 10- minus 3 GU (guaiacol unit) per rat and 0.61 times 10- minus 3 GU per mg protein. These were increased 3.4 fold and 1.6 fold by chronic administration of thyroid stimulating hormone (TSH) and 5.1 fold and 2.5 fold by treatment with methylthiouracil (MTU), respectively. However, the soluble fractions, which were obtained by centrifugation at 105,000 xg for 120 min from the homogenates, manifested only 2-3% of the activity of the original homogenates, and the specific activity was scarcely affected by the administration of TSH and MTU. Moreover, even the low activity of the soluble fraction might be due to contaminated microsomes. None of the results obtained favors the view that the peroxidase is released from the cell to the colloid lumen. The number of peroxidase molecule per cell of thyroid was roughly estimated to be 1.5 times 10- minus 5 for normal and 3.9 times 10- minus 5 for MTU-treated rat.