LIGNIN PEROXIDASE-TYPE ACTIVITY OF SOYBEAN PEROXIDASE

Soybean peroxidase (SBP), an acidic peroxidase isolated from the hulls of the bean, catalyzes the efficient oxidation of veratryl alcohol to veratraldehyde in the presence of H2O2. The reaction is optimal at pH 2.4 in the presence of 0.2 M CaCl2. Soybean peroxidase is highly ther mostable at pH 2.4, with half-lives of 210 and 2.5 h at 30 and 50 degr ees C, respectively. This compares favorably to the thermostability of lignin peroxidase (LiP) from Phanerochaete chrysosporium under equall y acidic conditions. In fact, SBP is at least 150-fold more stable tha n LiP at 30 degrees C and the latter is completely labile at 50 degree s C. Soybean peroxidase follows a ping-pong, bi-bi catalytic reaction mechanism with a k(cat)/K-m (veratryl alcohol) of 2.47 . 10(2) M(-1)s( -1), ca. 1500-fold lower than a similar value for lignin peroxidase. T his lower value of catalytic efficiency is due both to a higher K-m (v eratryl alcohol) and lower k(cat) for SBP as compared to LiP. Oxidatio n of methoxybenzenes suggests that the approximate oxidation potential of SBP is 1.42 V, yet this is high enough to effect the oxidation (an d eventual beta-ether cleavage) of -(3,4-dimethoxyphenyl)-2-(phenoxy)p ropan-1,3-diol, a lignin model dimer. In addition to SBP, horseradish peroxidase (HRP) is also capable of oxidizing veratryl alcohol as well as methoxybenzenes (the latter up to an oxidation potential of 1.34 V ). Horseradish peroxidase, however, is extremely labile at pH 2.4 and is inactivated within minutes under such acidic conditions. The oxidiz ation by SBP, and to a lesser extent by HRP, of veratryl alcohol direc tly in the presence of H2O2 is the first reported care of plant peroxi dases catalyzing the efficient oxidation of high oxidation potential n onphenolics and demonstrates that SBP may be an effective alternative to lignin peroxidase.