The presence of two proteinases associated with the cell wall ofLactobacillus bulgaricus

Whole cells of Lactobacillus delbrueckii subsp. bulgaricus ACA DC235 are able to hydrolyse casein. This proteolytic activity is greatly enhanced when cells are grown in milk rather than in a peptide-rich synthetic medium such as De Man-Sharp-Rogosa. A significant part of the caseinolytic activity can be extracted by treating the intact cells with lysozyme, which suggests that the enzyme(s) involved are associated with the cell wall. The soluble lysozyme extract has been partially fractionated by ultrafiltration using different membranes. Biphasic kinetics of irreversible thermal denaturation, partial inactivation by various agents, and selective reactivation by zinc ions indicated that the overall caseinolytic activity was due to two distinct enzymes. The first one was rapidly inactivated upon heating, inhibited by EDTA, reactivated by Zn2+ ions, and was probably a zinc-dependent metalloprotease. The other one was more stable towards thermal inactivation, inhibited by phenylmethanesulfonyl fluoride, insensitive to N-ethylmaleimide, activated by Ca2+ ions, and was probably a serine protease.