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Spontaneous enzymic release of renal dipeptidase (RDPase; EC 3.4.13.19), a glycosylphosphatidylinositol (GPI)-linked ectoenzyme, was observed in vitro during incubation of porcine proximal tubules at 37°C. Triton X-114 phase separation of the released RDPase showed that the majority of the enzyme activity partitioned into the aqueous phase, indicating its hydrophilic nature. Immunoblot analyses using an antibody against the cross-reacting determinant (CRD) inositol 1,2-cyclic monophosphate, the epitope formed by phospholipase C (PLC) cleavage of the GPI anchor on a protein, detected the released RDPase. Reprobing the immunoblot with an anti-RDPase serum showed the RDPase band co-migrating with the CRD band. The release of RDPase from the proximal tubules was a Ca2+-dependent process and had a pH optimum of 9.0. These results indicate that RDPase is released from the proximal tubules by the action of a distinct endogenous GPI-specific PLC.

The differential diagnosis of acute renal failure (ARF) and chronic renal failure (CRF) may be possible by measuring urinary dipeptidase (Udpase) activity and serum creatinine (Scr) concentration. When the mass test of 246 individuals was examined on a 2-dimensional plot of Udpase (y-axis) versus Scr (x-axis) with the data obtained from healthy volunteers (n = 189), ARF (n = 19) and CRF (n = 38) patients, the characteristic distribution of each group was obvious. It is summarized by the mean values of healthy volunteers (1.44 +/- 0.39 mg/dL, 1.19 (0.59 mU/mL), ARF (6.04 +/- 5.04 mg/dL, 0.12 +/- 0.08 mU/mL), and CRF patients (8.72 +/- 2.93 mg/dL, 0.81 +/- 0.44 mU/mL). The healthy volunteers are distributed along the y-axis and the ARF patients the x-axis, thus separating the two groups 90 degrees apart. The CRF patients are scattered away from both x-, and y-axis. This 2-dimensional approach is thought to be very useful for the differential diagnosis of ARF suggesting Udpase as a new member of the marker enzymes of renal disease.