UNUSUAL KINETIC-PROPERTIES OF ANIONIC TOBACCO PEROXIDASE RELATED TO THE MECHANISM OF OXIDATION OF INDOLE-3-ACETIC-ACID

Anionic tobacco peroxidase (TOP) (mol wt 36 kDa, pI 3.5) was purified from transgenic tobacco plants with the yield of 60 mg/1 kg leaves. Th e enzyme exhibits unusual properties, i.e., Compound I is less reactiv e than Compound II. The enzyme was investigated in oxidation of indole -3-acetic acid (IAA) oxidation by oxygen in the air. The aerobic stead y-state spectral studies reveal that Compound II is the key intermedia te of the reaction mechanism. This was confirmed in the anaerobic stop ped-flow experiments. No reaction between the enzyme and IAA is observ ed under anaerobic conditions. The data obtained are interpreted in te rms of a ternary complex formation (ferric enzyme-IAA-oxygen) at the i nitiation step resulting in production of IAA radicals. The latter int eracts with the ferric enzyme and oxygen producing Compound II. The ox idative cycle involves the ferric enzyme and Compound II, and is indep endent from the peroxidative one.