Mode of action of lactoperoxidase as related to its antimicrobial activity: a review

Abstract

Lactoperoxidase is a member of the family of the mammalian heme peroxidases which have a broad spectrum of activity. Their best known effect is their antimicrobial activity that arouses much interest in in vivo and in vitro applications. In this context, the proper use of lactoperoxidase needs a good understanding of its mode of action, of the factors that favor or limit its activity, and of the features and properties of the active molecules. The first part of this review describes briefly the classification of mammalian peroxidases and their role in the human immune system and in host cell damage. The second part summarizes present knowledge on the mode of action of lactoperoxidase, with special focus on the characteristics to be taken into account for in vitro or in vivo antimicrobial use. The last part looks upon the characteristics of the active molecule produced by lactoperoxidase in the presence of thiocyanate and/or iodide with implication(s) on its antimicrobial activity.

Figure 1

Halogenation or peroxidase cycle of peroxidases Compound I.

Figure 2

Apparent second-order rate constant at pH 7 (×10⁴ M⁻¹ s⁻¹) of the reaction between myeloperoxidase Compound I or lactoperoxidase Compound I with (pseudo)halides [15, 16].

Figure 3

Illustration (according to [17]) of the pH threshold value above which the oxidation of the halogen by mammalian heme peroxidase will be thermodically unfavorable. The reduction potential of the couple Compound I/native enzyme and the couple halogen (X = chloride, bromide) HOX/OX⁻ is expressed with an illustrative function of the pH, at a specific concentration of enzyme and substrates.

Figure 4

Illustration of the interaction between the biodisponibility of a peroxidase, the (pseudo)halogen concentration in plasma, in saliva, and in milk, and the production of oxidant molecules. MPO: myeloperoxidase; SPO: salivary peroxidase; LPO: bovine lactoperoxidase; OCl⁻: hypochlorite; and OSCN⁻: hypothiocyanite. Although chloride is the most available substrate compared to thiocyanate, bromide, and iodide, thiocyanate is the most effective substrate for the Compound I and hypothiocyanite could be produced at equal or superior levels compared to hypohalides.

Figure 5

Target group of hypothiocyanite, hypoiodite, and iodine. Due to its low oxidation power, hypothiocyanite is relatively specific and is not reactive against all thiols. In vivo, hypoiodite seems to be selectively directed against reduced pyridine nucleotide because even the presence of excess glutathione and methionine does not thoroughly inhibit their oxidation. HOSCN/OSCN⁻: acidic or basic form of hypothiocyanite; HOI/OI⁻: acidic or basic form of hypoiodite; and I₂: iodine.

Figure 6

Illustration of the cofactor role of SCN⁻ or I⁻. When the necessary conditions are fulfilled, that is, (i) no substrate competitor for SCN⁻ or I⁻ for binding to lactoperoxidase, (ii) enough peroxidase, H₂O₂ and SCN⁻ or I⁻, (iii) enough R-SH, and (iv) no incorporation of SCN⁻ or I⁻ in stable byproducts, the quantity of OSCN⁻ or OI⁻ produced depends only on the amount of H₂O₂. SCN⁻: thiocyanate; I⁻: iodide; H₂O₂: hydrogen peroxide; LPO: lactoperoxidase; R-SH: peptide or protein with a thiol moiety; R-S-SCN or R-S-I sulfenyl thiocyanate or iodide; R-SOH: sulfenic acid; OSCN⁻: hypothiocyanite; and OI⁻: hypoiodite.

Figure 7

Biological activity of hypothiocyanite on bacteria and possible defense mechanism of the bacteria. Reversible inhibition is observed in that (i) hypothiocyanite is not reactive against all thiols and (ii) if hypothiocyanite is removed or diluted, the pathogen recovers. Irreversible inhibition is linked to (i) long period of incubation, (ii) the bacterial species, and (iii) hypothiocyanite concentration. HOSCN/OSCN⁻: acidic or basic form of hypothiocyanite and GSH: glutathione.

Figure 8

Illustration of the molecules that can be present after oxidation of iodide by lactoperoxidase in presence of H₂O₂. The active species depend mainly on the concentration of iodide (upper part) and the pH (lower part). The species with an oxidant power are represented in bold.

Figure 9

Biological activity of hypoiodite or iodine on bacteria. Irreversible inhibition is observed and could be linked to (i) oxidation of thiol groups, NAD(P)H, and thioether groups, (ii) high reactivity of HOI/I₂ against thiol and reduced nicotinamide nucleotides, and (iii) the incorporation of iodide in tyrosine residue of protein (iodination of protein). HOI/OI⁻: acid or basic form of hypoiodite and I₂: iodine.