Method comparison of dipeptidyl peptidase IV activity assays and their application in biological samples containing reversible inhibitors.

Abstract Background Dipeptidyl peptidase IV (DPPIV, DPP4) is a serine protease that releases N-terminal dipeptides. It is a validated drug target for type 2 diabetes and DPPIV inhibitors are currently evaluated for other therapeutic applications. Various assays are used for DPPIV activity measurements in biological samples. Highly sensitive methods are needed to measure also very low activities in inhibited samples. Methods Here, the three most extensively used substrates to quantify DPPIV activity are compared using in-house methods. A luminescent kit was also included. In addition, one of the in-house fluorometric assays was elaborated for use in biological samples containing reversible DPPIV inhibitors to estimate residual DPPIV activity which is usually underestimated due to sample dilution. Results The in-house methods showed a good precision, linearity and specificity. Both fluorometric substrates had a 10-fold higher sensitivity compared to the colorimetric assay. The luminescent kit was found to be the most sensitive. Conclusions All three in-house methods can be used to measure DPPIV activity in non-inhibited biological samples. The more sensitive fluorometric assays are recommended when sample volumes are limited or when using inhibited samples. The elaborated fluorometric method can be used to estimate the residual in vivo DPPIV activity in inhibitor treated subjects.