Proteolysis in samples of quarter milk with varying somatic cell counts. 2. Component PP3 and beta-casein-1P (f29-105 and f29-107) of the proteose-peptone fraction.

The proteose-peptone fraction was studied to measure proteolysis in 86 samples of quarter milk from 31 cows with subclinical mastitis. The relative content of component PP3 decreased significantly in the total of proteose-peptone as plasmin activity increased, but the content of this component in milk was not correctly correlated with the plasmin activity (r = .52). Electrophoresis, amino acid composition, and sequence analysis showed that the component beta-CN-1P (f29-105/7) of the proteose-peptone fraction was a terminal product of plasmin-like activity and was located correctly electrophoretically. Correlation factors between the plasmin activity and content in milk of component beta-CN-1P (f29-105/7) was very high (r = .87). This component could be used as indicator of the endogenous proteolysis in milk from cows with subclinical mastitis.