Peptide-degrading enzymatic activities in GH3 cells and rat anterior pituitary homogenates

The activities of a number of peptide-degrading enzymes were compared in homogenates of GH3 cells and rat anterior pituitaries. The enzymes studied were prolyl endopeptidase (EC 3.4.21.26), a soluble metalloendopeptidase, pyroglutamyl peptide hydrolase (EC 3.4.11.8), a multicatalytic protease complex, cathepsin B (EC 3.4.22.1), cathepsin D (EC 3.4.23.5), aminopeptidase (EC 3.4.11.2), and a membrane-bound neutral metalloendopeptidase (EC 3.4.24.11). Specific substrates were used to measure the activities, and active-site-directed inhibitors were used to verify the identities of the enzymes studied. Of the two lysosomal enzymes studied, cathepsin B, the enzyme with the highest activity in both preparations, had 5 times the activity in GH3 cell homogenates as in anterior pituitary homogenates. Cathespin D had a somewhat higher activity in the anterior pituitary homogenates than in the GH3 cell homogenates. Soluble metalloendopeptidase and prolyl endopeptidase, both cytoplasmic enzymes, had about twice the activity in GH3 cell homogenates as in anterior pituitary homogenates. Membrane-bound neutral metalloendopeptidase in the GH3 cell homogenates had 25% of the activity of the anterior pituitary homogenates. Of the two TRH-degrading enzymes, the activity of prolyl endopeptidase in GH3 cell homogenates was about 25 times higher than that of pyroglutamyl peptide hydrolase. Since the secretory function of the pituitary is in part controlled by neuropeptides, the knowledge of the enzyme profiles of the GH3 cells and the anterior pituitary should be of value in studying the metabolism of neuropeptides and peptide hormones in these systems.