Kinetic studies on the oxidation of nitrite by horseradish peroxidase and lactoperoxidase

The reaction of nitrite (NO2-) with horseradish peroxidase and lactoperoxid ase was studied. Sequential mixing stopped-flow measurements gave the follo wing values for the rate constants of the reaction of nitrite with compound s II (oxoferryl heme intermediates) of horseradish peroxidase and lactopero xidase at pH 7.0, 13.3 +/- 0.07 mol(-1) dm(3) s(-1) and 3.5 +/- 0.05 . 10(4 ) mol(-1) dm(3) s(-1), respectively. Nitrite, at neutral pH, influenced mea surements of activity of lactoperoxidase with typical substrates like 2,2'- azino-bis[ethyl-benzothiazoline-(6)-sulphonic acid] (ABTS), guaiacol or thi ocyanate (SCN-). The rate of ABTS and guaiacol oxidation increased linearly with nitrite concentration up to 2.5-5 mmol dm(-3). On the other hand, two -electron SCN oxidation was inhibited in the presence of nitrite. Thus, nit rite competed with the investigated substrates of lactoperoxidase. The inte rmediate, most probably nitrogen dioxide ((NO2)-N-.), reacted more rapidly with ABTS or guaiacol than did lactoperoxidase compound II. It did not, how ever, effectively oxidize SCN- to OSCN-. NO2- did not influence the activit y measurements of horseradish peroxidase by ABTS or guaiacol method.