Pyroglutamyl peptidase II inhibition specifically increases recovery of TRH released from rat brain slices

Pyroglutamyl peptidase II (EC 3.4.19-) is a highly specific membrane-bound thyrotropin releasing hormone (TRH) degrading enzyme. To study the functional significance of pyroglutamyl peptidase II in TRH degradation, we synthesized the reversible inhibitor N-1-carboxy-2-phenylethyl (N imbenzyl)-histidyl-β-naphthylamide (CPHNA). CPHNA inhibited the enzyme with a Ki of 8 μM, but had no effect no TRH receptors or no prolyl endopeptidase (EC 3.4.21.26). It weakly inhibited cytosolic pyroglutamyl peptidase I (EC 3.4.19.3). CPHNA at a concentration of 10 −4 M increased both the basal and potassium stimulated recovery of TRH released from hypothalamic slices by approximately two-fold. An even higher recovery was observed in slices from brain regions with relatively high levels of pyroglutamyl peptidase II. CPHNA had no effect on the basal recovery of γ-aminobutyric acid or Met-enkephalin released from brain slices but decreased the potassium stimulated recovery of both Met-enkephalin and γ-aminobutyric acid. These data further support the involvement of pyroglutamyl peptidase II in the extracellular inactivation of brain TRH.