The modification of nisin with homocysteine thiolactone and its effect on antimicrobial activity

The aim of the present study is to make an important contribution to the literature by focusing on the preparation of the N-homocysteine conjugate of nisin and evaluating the effect of the N-homocysteinylation reaction on its antimicriobial activity. The modification process was monitored using both acetic acid urea polyacrylamide gel electrophoresis (AAU-PAGE) and tricine sodium dodecyl sulphate polyacrylamide gel electrophoresis (tricine SDS-PAGE). The antibacterial effectiveness of modified nisin was assessed against Staphylococcus aureus ATCC 6538, Enterococcus faecium ATCC 9097, Bacillus subtilis ATCC 6633, Lactococcus lactis ssp. cremoris AÜ, Listeria monocytogenes NCTC 5348, and Escherichia coli RSKK. Optimal conditions for achieving the highest N-homocysteinylation degree (6.30%) were determined as 6 mg/mL nisin, 150 mM homocysteine thiolactone, 150 rpm shaking rate, pH of 3.0, and a reaction time of 6 h. The modified nisin obtained did not have a significant inhibitory effect on the strains tested except E. faecium. E. faecium was inhibited by the modified nisin and its antibacterial activity was determined as approximately 10% of the antibacterial activity of unmodified nisin. On the other hand, hydrolysis of nisin by trypsin and thermolysin resulted in significant specific side chain modifications induced by the homocysteine-thiolactone reaction, especially at Lys12 and Lys22. The results provide valuable insights into the potential of N-homocysteinylation to improve the antibacterial properties of nisin and also suggest that the effects of specific modifications identified during the modification process should be investigated.