Spectrophotometric determination of proteolytic activity in coloured solutions

A simple procedure for spectrophotometric determination of proteolytic activity in coloured solutions is described. Gelatin cross-linked with glutaric dialdehyde or formaldehyde in the presence of black drawing ink is used as an insoluble chromolytic substrate. The absorbances of reaction mixture filtrates are read in the near infra-red region (at 800-900 nm) where the majority of coloured substances does not absorb; on the contrary black drawing ink released from the substrate during the proteolysis absorbs strongly even at these wavelengths.