On thyrotropin-releasing hormone (TRH) metabolism, pyroglutamyl aminopeptidase II (PAP-II), a zinc-dependent ectoenzyme primarily located in the central nervous system, is believed to play a predominant role. Recently we cloned pyroglutamyl aminopeptidase I (PAP-I) which is known for specifically removing a L-pyroglutamate (L-pGlu) residue from the amino terminus of proteins and peptides including TRH. To investigate possible contribution of PAP-I toward TRH metabolism, we conducted biochemical and immunohistochemical characterization using recombinant rat, mouse and human PAP-Is and an antibody raised against rat PAP-I. The Km values toward TRH by the recombinant PAP-Is were about 0.05 mM, being similar value to the reported value of recombinant PAP-II. The L-pGlu-cleaving activities toward TRH in rat brain homogenate were inhibited by a PAP-II specific inhibitor 1,10-phenanthroline, but not inhibited by the antibody against rat PAP-I. Immunohistochemical study in rats revealed heterogeneous distribution of PAP-I in the pituitary, the target tissue of TRH, but the distribution was cytosolic. Taken together, these results suggested that PAP-I might not be dominantly involved in the degradation of TRH in rats. Additionally, we found that PAP-I was localized in the renal proximal tubules. Further investigations are needed for elucidating the function of PAP-I in these restricted sites.