Thyroid status regulates particulate but not soluble TRH-degrading pyroglutamate aminopeptidase activity in the rat...

Rat liver contains two topologically different TRH-degrading pyroglutamate aminopeptidases. The particulate pyroglutamate aminopeptidase, unlike the soluble one, was highly specific for TRH and shared many physico-chemical properties with serum thyroliberinase, which is controlled by thyroid hormones. Both enzymes convert pGlu His Pro NH2 into His Pro NH2; the latter may be cyclized to cyclo His Pro known to possess several biological activities and specific binding sites in liver. The aim of the present study was to determine the effects of thyroid status on the particulate and soluble enzymes activity and gain more insight into their biological role. The regulatory pathway for the particulate pyroglutamate aminopeptidase was found similar to that of serum thyroliberinase: their specific activities decrease in hypothyroid rats and increase in hyperthyroid rats, whereas that of soluble enzyme remains unchanged. We postulate that the particulate pyroglutamate aminopeptidase may be a determining factor in the concentrations of TRH and/or cyclo His Pro reaching liver cells and a possible source for serum thyroliberinase. Taken together, these data suggest that this "converting enzyme" acts as a physiological regulator.