P-28 Searching for the Function of an Intestinal Peptidase

Carboxypeptidase O (CPO) is a metallocarboxypeptidase that is expressed in the small intestine. CPO is able to cleave C-terminal polar and, more readily, acidic amino acids. It is not strongly regulated by pH, does not contain a prodomain, and has been observed both on the plasma membrane and intracellularly. Hence, CPO appears to exist as a constitutively active enzyme with broad subcellular localization. The physiological function of CPO remains somewhat of a mystery, and has been the focus of our research for a number of years. Currently, four working hypotheses are being considered: 1) CPO has no important function, but is a remnant of genomic change. Analysis of the gene suggests that it has been subject to many changes, including duplication and pseudogenization events. 2) CPO functions in dietary protein digestion. This was initially proposed based on its location and fundamental role as a peptidase. 3) CPO is necessary for the processing and absorption of folate produced by the intestinal microbiome. Folate, a vitamin necessary for many cellular biosynthetic reactions, is naturally produced with a polyglutamate extension, which must be proteolytically removed for absorption. 4) CPO is involved in lipid absorption and processing in the intestine. We have shown that CPO associates with intracellular lipid droplets and the presence of CPO increases lipid droplet formation. Due to similarities in the processes of lipid droplet and chylomicron formation, we hypothesize that CPO may play a role in chylomicron formation in the small intestine. Evidence exists for each one of these hypotheses, opening many doors for future research into the pleiotropic functions of this enzyme.