Application of high-resolution ultrasonic spectroscopy for detection of the plasmin activity toward Î2-casein

High-resolution ultrasonic spectroscopy (HR-US) was applied for precise detection of plasmin activity towards β-casein in buffer at pH 7.8 and 37 C. The evolution of ultrasonic velocity and ultrasonic attenuation measured at 15.5 MHz is related to the concentration of peptide bonds hydrolyzed and loss of β-casein aggregates, respectively. The ultrasonic assay presents sensitive and direct activity-based quantification of plasmin levels in milk. The variation in plasmin concentration between HR-US and ELISA method owed to the differing detection principles. The real-time ultrasonic profiles of hydrolysis were utilized to describe the kinetic aspect of plasmin activity. The non-linear activity curve was fitted with classic and inverse Michaelis–Menten type models. Within 1–8.6 mg⋅mL 1 β-casein, the Vmax and KM obtained were (6.30 ± 2.21) × 10 5 mol.kg 1⋅min− 1 and 10.33 ± 3.50 mg⋅mL 1, respectively. The maximum peptide bond cleaved was 5–6 (2.7% degree of hydrolysis) achieved at 1 mg⋅mL 1 β-casein.