Action of plasmin on equine β-casein

Abstract

Equine β-casein and, in a lesser extent, αs1- and κ-caseins were good substrates for plasmin in solution. The Lys47–Ile48 bond of β-casein was readily hydrolysed leading to the production of γ-like caseins of 23 kDa. These γ-like caseins were degraded by plasmin in solution subsequently to their formation. The presence of PP5-like peptides of 20 kDa associated to caseins seemed to arise from endogenous plasmin activity in equine milk. Their formation was, however, not obvious in solution, as the amino-terminal region of β-casein seemed to be readily hydrolysed by plasmin. γ-caseins were generated when a sodium caseinate solution was incubated at 37°C and pH 8.0 for 24 h or when caseinate was stored in a freeze-dried form at 7°C for 2 years. This might be due to the presence of low amount of endogenous plasmin associated to equine caseinate as evidenced by assays with a specific synthetic substrate.