Cysteine protease in bovine milk capable of hydrolyzing casein as the substrate and elevation of the activity during the course of mastitis

Cysteine protease was found to be present in bovine milk that catalyzed casein as the substrate. The protease was activated by reducing agents such as 2-mercaptoethanol and inhibited by monoiodoacetic acid, but not affected by the addition of phenylmethylsulfonyl fluoride, calcium or ethylene glycol bis (beta-aminoethyl)-N,N,N',N'-tetraacetic acid. The protease activity was linear as a function of protein amount and incubation time, and showed maximum at pH 6.0. By Sephacryl S-200 chromatography, at least two types of cysteine proteases having molecular weights of 45 kDa and more than 150 kDa were detected. The activity was increased in mastitic milk, and well correlated with the stages of mastitis, as indicated by the California mastitis test, somatic cell count and protein concentration. These results suggested that cysteine protease(s) is involved in the pathogenesis of mastitis.