The effect of NaCl substitution with KCl on proteinase activities of cell-free extract and cell-free supernatant at different pH levels and salt concentrations: Lactobacillus acidophilus and Lactobacillus casei

The aim of this study was to investigate the effect of substitution of NaCl with KCl at different pH levels and salt concentrations on proteinase activity of cell-free extract and cell-free supernatant of the probiotics Lactobacillus acidophilus and Lactobacillus casei. de Man, Rogosa, and Sharpe broth aliquots were mixed with 2 pure salts (NaCl and KCl) and 2 salt concentrations at 2 concentration levels (5 and 10%), inoculated with Lactobacillus acidophilus or Lactobacillus casei, and incubated aerobically at 37°C for 22 h. The cultures were then centrifuged at 4,000 × g for 30 min, and the collected cell pellets were used to prepare cell-wall proteinases and the supernatants used as a source of supernatant (extracellular) proteinases. The proteolytic activity and protein content of both portions were determined. After incubation of both portions with 3 milk caseins (α-, β-, κ-casein), the supernatants were individually subjected to analysis of angiotensin-converting enzyme (ACE)-inhibitory activity and proteolytic activity using the o-phthalaldehyde method. Significant differences were observed in ACE-inhibitory and proteolytic activities between salt substitution treatments of cell-free extract and cell-free supernatant from both probiotic strains at the same salt concentration and pH level.