Carboxypeptidase E activity is deficient in mice with the fat mutation: Effect on peptide processing

Carboxypeptidase E (CPE) is involved in the biosynthesis of many peptide hormones and neurotransmitters. Mice with the fat mutation have previously been found to have a point mutation in the cpe gene, and to have greatly reduced levels of CPE-like enzyme activity in the pituitary and pancreatic islets (Naggert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). In the present report, we examined CPE-like activity and peptide processing in several tissues of C57BLKS/LtJ- Cpe(fat)/Cpe(fat) mutant (Cpe(fat)/Cpe(fat)) mice. Whereas CPE-like activity is detected in homogenates of Cpe(fat)/Cpe(fat) mouse tissues, the majority of this activity is not due to CPE based on the sensitivity to p- chloromercuriphenyl sulfonate. In addition, the Cpe(fat)/Cpe(fat) activity does not bind to a substrate affinity column under conditions that bind CPE. Furthermore, the enzyme activity and immunoreactive properties of the activity purified from Cpe(fat)/Cpe(fat) brain are distinct from those of CPE. Taken together, these data suggest that CPE is completely inactive in the Cpe(fat)/Cpe(fat) mice, and that all of the CPE-like activity is due to other carboxypeptidases such as carboxypeptidase D. Levels of Leu-enkephalin in Cpe(fat)/Cpe(fat) mouse brain are approximately 5-fold lower than those in control brain. Treatment of the Cpe(fat)/Cpe(fat) brain extract with carboxypeptidase R restores the level of Leu-enkephalin to the level in control brain. Interestingly, the large molecular weight enkephalin- containing peptides are elevated 2-3-fold in Cpe(fat)/Cpe(fat) mouse brain. These data indicate that CPE plays an important role in the processing of peptide hormones in various tissues, but that other carboxypeptidases also contribute to peptide processing. Furthermore, the increase in levels of high molecular weight enkephalin peptides in the Cpe(fat)/Cpe(fat) mouse suggests that CPE is required for efficient peptide processing by the endopeptidases.