Different responses between thyroxine 5'-monodeiodinase and protein disulfide isomerase activities to dietary fat and thyroid status in rat liver

On the hypothesis that rat hepatic microsomal type I iodothyronine 5'-monodeiodinase (MDI) would be identical to hepatic protein disulfide isomerase (PDI) (Boado et al. Biochem. Biophys. Res. Commun. 155 1297-1304 (1988)), we examined the responses of these enzyme activities to dietary fat and thyroid status in rats. The hepatic MDI activity was higher in rats fed high fat diet than in rats fed low fat diet, but the hepatic PDI activity showed the reverse responses to the diets. Propylthiouracil administration (hypothyroidism) lowered the MDI activity, but elevated the PDI activity. Thyroxine administration (hyperthyroidism) elevated the MDI activity but had no effect on the PDI activity. These results indicate that the two enzyme activities are regulated by different mechanisms in vivo, suggesting that MDI and PDI are not identical enzymes.