Factors affecting the caseinolytic activity of Lactobacillus casei and Lactobacillus plantarum

Whole cell suspensions of some strains of each Lactobacillus casei and Lactobacillus plantarum were assayed for their caseinolytic activity in 0.1 M NaH₂PO₄ buffer, pH 6.5, at 30 °C, using different assay methods. Azocasein was not as sensitive as casein (Hammarsten) as a substrate. Inclusion of glucose in the assay mixture reduced the released α-amino groups as evidenced by fluorescent labelling, but generally increased the amounts of excreted amino acids. Divalent cations, including calcium ions, played only a minor role in the activation of the cell-bound proteinase, whereas NaCl inhibited it markedly. Inhibitor studies suggest that the enzyme is a serine proteinase. The different assay methods used did not give identical results. Fluorescent labelling of the free α-amino groups at pH 6.0 appears, on the contrary, to be a more reliable method.