Use of dye-labeled protein as spectrophotometric assay for protein precipitants such as tannin.

Bovine serum albumin has been covalently labeled with Remazol brilliant blue R to provide a substrate for a convenient spectrophotometric assay for protein precipitants. The blue protein is especially useful for measuring protein precipitation by vegetable tannins because its absorption maximum is at a wavelength where plant pigments exhibit minimum absorption. Blue BSA has been used to determine, by competition experiments, the relative affinity of various proteins for tannins. A procedure for purifying condensed tannin from commercially available quebracho extract is described.