Reaction of lactoperoxidase compound I with halides and thiocyanate.
Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions, including milk, tears, and saliva, and has physiological significance in antimicrobial defense which involves (pseudo-) halide oxidation. This study for the first time presents transient kinetic measurements of the reactivity of its competent redox intermediate compound I with halides and thiocyanate, using the sequential stopped-flow technique. Compound I was produced with either H(2)O(2) [(1.1 +/- 0.1) x 10(7) M(-1) s(-1)] or hypochlorous acid [(3.2 +/- 0.1) x 10(7) M(-1) (s-1)]. At pH 7 and 15 degrees C, the two-electron reduction of compound I to native LPO by bromide and iodide has a second-order rate constant of (4.1 +/- 0.1) x 10(4) M(-1) s(-1) and (1.2 +/- 0.04) x 10(8) M(-1) s(-1), respectively. With thiocyanate the reaction is extremely fast (2.0 x 10(8) M(-1) s(-1)), whereas chloride cannot function as electron donor. The results are discussed with respect to known kinetic data of homologous mammalian peroxidases and to the physiological role of LPO in antimicrobial defense.
Furtmüller
Paul Georg
PG
Institute of Chemistry, University of Agricultural Sciences, Muthgasse 18, A-1190 Vienna, Austria.
Jantschko
Walter
W
Regelsberger
Günther
G
Jakopitsch
Christa
C
Arnhold
Jürgen
J
Obinger
Christian
C
D000818:Animals; D001965:Bromides / Q000737:chemistry*; D002417:Cattle; D002712:Chlorides / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry; D006997:Hypochlorous Acid / Q000737:chemistry; D007454:Iodides / Q000737:chemistry*; D007784:Lactoperoxidase / Q000737:chemistry*; D010084:Oxidation-Reduction; D011048:Polarography; D013861:Thiocyanates / Q000737:chemistry*
D001965:Bromides; D002712:Chlorides; D007454:Iodides; D013861:Thiocyanates; D006997:Hypochlorous Acid; D006861:Hydrogen Peroxide; D007784:Lactoperoxidase; C031760:thiocyanate