The reaction of human erythrocyte catalase with hydroperoxides to form compound I

A kinetic study of the reaction of human erythrocyte catalase with peracetic acid, methyl hydroperoxide, and ethyl hydroperoxide to form the primary oxidized compound (Compound I) has been carried out at 25 degrees C by means of a stopped flow technique. The pH dependence of the apparent second order rate constants indicates that the process occurs by reaction of catalase with un-ionized hydroperoxide molecules, The reaction of catalase with methyl hydroperoxide is pH-independent in the range pH 5.8 to 9.4, and lows as methyl hydroperoxide (pKa, 11.5) is deprotonated. Similarly, the reaction with peracetic acid is independent of pH from pH 5.8 to 6.5 and slows as peracetic acid (pKa 8.2) is deprotonated. The pH-independent rate constants for the formation of Compound I are 1.4 x 10(6), 2.7 x 10(4), and 3.8 x 10(4) M-1s-1 for methyl hydroperoxide, peracetic acid, and ethyl hydroperoxide, respectively, following a trend of decreasing rate as the size of the substrate increases. The optical absorption spectrum of Compound I from 360 to 680 nm at pH 7.1 and 25 degrees C obtained by a rapid scanning stopped flow technique is also reported.