Studies on the lactoperoxidase system: reaction kinetics and antibacterial activity using two methods for hydrogen peroxide generation

D.A. DIONYSIUS, P.A. GRIEVE AND A.C. VOS. 1992. Components of the lactoperoxidase system were measured during incubation in Isosensitest broth, with enzymatic (glucose oxidase, GO) or chemical (sodium carbonate peroxyhydrate, SCP) means to generate H₂O₂. When low levels of thiocyanate (SCN^-) were used in the GO system, H₂O₂ was detected and lactoperoxidase (LP) was inactivated when SCN^- was depleted. With 10-fold higher SCN^-, LP remained active and H₂O₂ was not detectable. The oxidation product of the LP reaction, most likely hypothiocyanite, was present in low concentrations. When SCP was used for the immediate generation of H₂O₂ in a system employing low SCN^-, half the LP activity was lost within minutes but thereafter it remained stable. Low concentrations of oxidation product were measured and H₂O₂ was not detected during the course of the experiment. At high SCN^- levels, relatively high concentrations of oxidation product were produced immediately, with H₂O₂ undetectable. The results suggest that the final product of the LP reaction depends on the method of H₂O₂ generation and the relative proportions of the substrates. Antibacterial activity of the two LPS was tested against an enterotoxigenic strain of Escherichia coli. Both systems showed bactericidal activity within 4 h incubation at 37°C.