Abstract
Partially purified cell wall proteinases of eight strains of Streptococcus cremoris were compared in their action on bovine αs1-, β-, and κ-casein, as visualized by starch gel electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and thin-layer chromatography. Characteristic degradation profiles could be distinguished, from which the occurrence of two proteinases, represented by strain HP and strain AM1, was concluded. The action of the HP-type proteinase P1 (also detectable in strains Wg2, C13, and TR) was established by electrophoretic methods to be directed preferentially towards β-casein. The AM1-type proteinase PIII (also detectable in strain SK11) was also able to degrade β-casein, but at the same time split αs1- and κ-casein more extensively than did PI. Strain FD27 exhibited mainly PI activity but also detectable PIII degradation characteristics. The cell wall proteinase preparation of strain E8 showed low PI as well as low PIII activity. All proteinase preparations produced from κ-casein positively charged degradation products with electrophoretic mobilities similar to those of degradation products released by the action of the milk-clotting enzyme chymosin. The differences between PI and PIII in mode of action, as detected by gel electrophoresis and thin-layer chromatography, were reflected by the courses of the initial degradation of methyl-14C-labeled β-casein and by the effect of αs1- plus κ-casein on these degradations. The results are discussed in the light of previous comparative studies of cell wall proteinases in strains of S. cremoris and with respect to the growth of this organism in milk.
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