Comparison of bovine beta-casein hydrolysis by PI and PIII-type proteinases from Lactococcus lactis subsp. cremoris [corrected].

The action of the cell-wall-associated proteinases from Lactococcus lactis subsp. cremoris strains H2 and SK112 on bovine beta-casein was compared. The proteinase from the H2 strain was characterised as a PI-type proteinase since it did not hydrolyse alpha s1-casein and the initial trifluoroacetic acid-soluble products of beta-casein hydrolysis were identical to those previously identified as hydrolysis products of PI-type lactococcal proteinase action. The time-course of product formation by the proteinase from the H2 strain indicated that the bonds Tyr193-Gln194 and Gln182-Arg183 were the first to be hydrolysed. Cleavage of the bonds Gln175-Lys176, Ser168-Lys169, Ser166-Gln167 and Leu163-Ser164 was also very rapid. Four of the five bonds in beta-casein most susceptible to hydrolysis by the PIII-type proteinase from strain SK112 were different from those cleaved by the PI-type proteinase, initial hydrolysis being at the sites Tyr193-Gln194, Leu192-Tyr193, Asp43-Glu44, Gln46-Asp47 and Phe52-Ala53. Early hydrolysis at the three sites in the N-terminal region of beta-casein, leading to cleavage of the N-terminal phosphopeptide and rapid precipitation of the residual fragment, represents a marked contrast to the action of PI-type proteinases where cleavage at sites in the N-terminal region occurs only very slowly.