Kinetic assay of human pepsin with albumin-bromphenol blue as substrate

A novel substrate, albumin complexed with bromphenol blue, has been developed for the assay of human gastric juice pepsin by a kinetic method in the Cobas centrifugal analyzer. The action of pepsin on the complex degrades the albumin and releases the dye. The change in the color of the substrate is a zero-order reaction. Human and porcine pepsin have different Km's with the new substrate. This kinetic method has a throughput of 28 tests in approximately 10 min and good precision (CV = 2.0%). Other advantages are analysis in homogeneous solution (thereby eliminating the need to separate substrate and products), lack of interference from bilirubin or phenol red, and the expression of pepsin activity in IUB enzyme units.