[Determination of dipeptidyl-peptidase IV in biological materials]

Dipeptidyl-peptidase IV (EC 3.4.14.5) can be assayed relatively specifically in crude biological material by determination of the initial rate of formation of 4-nitroaniline from aminoacyl-proline-4-nitroanilides. For clinical purposes tricine buffer, pH 7.4, is employed, because unspecific cleavage of the substrate is minimal under these conditions. Various types of chromogenic and fluorogenic leaving groups can be used to detect the enzyme on solid supports. During the enzymatic hydrolysis of the tripeptides Xaa-Pro-Yaa, there is an increase of absorbance at 228 nm, which is useful for monitoring the enzyme activity. Enzymic activity can also be measured titrimetrically in unbuffered solutions, whereby corrections must be made for the protonation of both substrate and products.