Ontogeny of two topologically distinct TRH-degrading pyroglutamate aminopeptidase activities in the rat liver

We recently separated and characterized two topologically distinct pyroglutamate aminopeptidase (PAP) activities in adult rat liver, which convert TRH to cyclo His-Pro (cHP). The liver possesses high-affinity binding sites to the biologically active dipeptide cHP and is thus a potential target tissue for pancreatic TRH and/or its conversion product cHP, and may be a site of TRH conversion and/or inactivation. This report describes the ontogenic development of two liver PAP activities and compares them with that of plasma thyroliberinase. The particulate high-molecular-weight PAP was absent at birth and during the neonatal period, while the soluble, low-molecular-weight PAP was present at all the developmental stages tested. The changes in particulate PAP activity are similar to those in the plasma of age-matched rats. The peculiar age-dependent changes in particulate PAP activity, plus its cellular location, suggest that it has a regulatory role.