An endopeptidase has been purified from subsp. SK11. The enzyme is a 70 kDa monomer, strongly inhibited by the metalloproteinase inhibitors 1,10-phenanthroline and phosphoramidon but relatively insensitive to EDTA. It is not significantly inhibited by the thiol enzyme inhibitor -chloromercuribenzoate nor by the serine protease inhibitor phenylmethylsulphonyl fluoride. The action of the endopeptidase in catalysing the hydrolysis of several peptide hormones has been studied and the hydrolysis products identified by sequence analysis. The enzyme catalyses hydrolysis of peptide bonds in which a hydrophobic amino acid (most commonly a Phe or Leu) residue occupies the position immediately C-terminal to the hydrolysed bond. It thus has a specificity very similar to that of thermolysin. Two of the oligopeptides produced during the early stages of β-casein digestion by the lactococcal cell-wall proteinases were hydrolysed by the endopeptidase, the others were resistant to hydrolysis. Cell fractionation studies have shown that the distribution of endopeptidase activity between the different cell fractions is the same as that of the intracellular marker enzyme fructose bisphosphate aldolase, and thus indicate a cytoplasmic location for the enzyme. These observations argue against a role for this enzyme in the early stages of casein breakdown by the lactococcal proteolytic system.


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