The prolyl oligopeptidase family of enzymes cleave the post proline bond and include the cell surface and soluble glycoproteins fibroblast activation protein (FAP) and dipeptidyl peptidase 4 (DPP4) which have roles in diabetes and liver injury.  FAP has both dipeptidyl and endopeptidase activity and a FAP-specific substrate (ARI-3144) has recently been developed, which we have used to quantify FAP enzyme activity in organs and fluids from three mammalian species.  DPP4 activity was also measured as a comparator.

This study showed the in vivo specificity of the novel FAP-specific substrate ARI-3144 and quantified FAP enzyme activity in tissue and blood samples from human, baboon and mouse.  FAP was detected in baboon plasma/serum at levels approx ~20-fold and 1.3-fold lower than mouse and human plasma, respectively.  Baboon urine and bile had no detectable FAP enzyme activity.  FAP was detected in all wild type mouse organs at varying levels, however all organs and blood from FAP gene knockout mice were negative, thus confirming the in vivo specificity of this substrate.  The highest levels of FAP in mice were found in pancreas, uterus, submaxillary gland, lymph node and ovary.  The large vital organs such as heart, liver, lung and kidney had little FAP.  The organ distribution in baboon was more variable than mouse.  High-FAP organs in baboon included skin, epididymis, bladder, adipose tissue, nerve and colon.

FAP was also examined in some disease states.  In baboons, we observed increased FAP in tumours and in diseased skin and lymph nodes.  Additionally, FAP enzyme activity was ~15-fold greater in cirrhotic compared to non-diseased human liver.

This new reagent is a useful tool for specific quantitation of both soluble and cell-surface FAP enzyme activity and paves the way for research into the expression patterns of this unique enzyme.