Influence of physico-chemical factors on the oligomerization and biological activity of bacteriocin AS-48

Bacteriocin AS-48 forms a mixture of monomers and oligomers in aqueous solu tions. Such oligomers can be clearly differentiated by SDS-PAGE after forma ldehyde crosslinking, and we have verified that these associates are stable to acid treatment after fixation. In addition, they show antimicrobial act ivity and are recognized by anti-AS-48 antibodies. AS-48 oligomers can be d issociated by the detergents SDS and Triton X-100. The degree of oligomeriz ation of AS-48 depends on the pH of the solution and the protein concentrat ion. At pH below 5, AS-48 is in the monomeric state at protein concentratio ns below 0.55.mg ml(-1), but it also forms dimers above this protein concen tration. This bacteriocin forms oligomers at pH values above 5, in agreemen t with the observation that it is also more hydrophobic at neutral pH. AS-4 8 is stable to mild heat treatments irrespectively of pH. At 120 degreesC i t is more heat resistant under acidic conditions, but it inactivates at neu tral pH. Activity of AS-48 against E. faecalis is highest at neutral pH, bu t it is highest at pH 4 for E. coli. The influence of pH on bacteriocin act ivity could be owing to changes in the conformation/oligomerization of the bacteriocin peptide as well as to changes in the surface charge of the targ et bacteria.