PRORENIN MEASUREMENT WITH A NOVEL PRORENIN KIT MAKING USE OF A MONOCLONAL ANTIBODY DIRECTED AGAINST RESIDUES 30–43 OF THE PROSEGMENT: EFFECT OF TRYPSIN AND ALISKIREN: PP.24.459

Abstract

Objective and Methods: Prorenin measurement requires prorenin-renin conversion by trypsin, thus allowing its detection in an active-site-specific immunoradiometric assay (IRMA). Alternatively, pretreatment with the renin inhibitor aliskiren induces a stable conformational change in the prorenin molecule, making it recognizable for an antibody directed against the active site, although the prosegment is still present (Batenburg et al., 2008). Recently, a prorenin ELISA has been developed by Molecular Innovations, which applies a prosegment specific monoclonal antibody (4B5-E3, against residues 30–43). This assay measures prorenin directly, without requiring trypsin or aliskiren pretreatment, and yields results within 3 hours. Here we applied this kit to human plasma samples (n = 17). A comparison was made with the prorenin measurements obtained indirectly with the above-mentioned renin IRMA (Cisbio, France). We also verified whether the antibody interferes with prorenin-renin conversion by trypsin. Prorenin generated in CHO cells was used as a standard. Results: Trypsin and aliskiren increased the renin levels detected with the IRMA from 33 ± 10 to 163 ± 43 and 197 ± 34 pg/mL, resp. (P = NS, r = 0.93). The prorenin levels calculated from these data (129 ± 35 and 164 ± 45 pg/mL, resp.) were not different from the prorenin levels measured with the new prorenin ELISA (99 ± 33 pg/mL; r = 0.82 and 0.88, resp.). Trypsin reduced the amount of prorenin detected with the ELISA by maximally 80%. Thus, trypsin cleavage occurs, at least partly, at a site that still allows detection by 4B5-E3. Aliskiren reduced the prorenin levels measured with the ELISA by 42 ± 9%, indicating that the aliskiren-induced conformational change hampers prosegment recognition by 4B5-E3. Finally, 4B5-E3, at concentrations >10 nM, reduced the increase in plasma renin observed with the IRMA after trypsin treatment by 50%. However, since it also reduced the renin measurements at baseline by 50%, it appears that 4B5-E3 binding interferes with the binding of the active site-directed antibody in the IRMA. Conclusion: The new kit allows rapid detection of prorenin, but should be used with care when measuring samples of aliskiren-treated patients.