Characteristics of a Low Molecular Weight Minor Anionic Isoperoxidase $A_{3n}$ from Radish

A minor anionic isoperoxidase named $A_{3n}$, was isolated from Korean radish (Raphanus sativus L.) root. Purification of the enzyme was accomplished by CMcellulose chromatography, DEAE-Sephacel chromatography, and Sephadex G-75 gel filtration. The enzyme was a glycoprotein with molecular weight of approximately 31,000 as determined by SDS-PAGE and 33,000 by Sepadex G-150 gel filtration, which is by far the smallest among the reported isoperoxidases. The pI value was 3.5. The optimum pH of the enzyme was 6.5 for guaiacol and $H_2O_2$, and the $K_m$ values for guaiacol and $H_2O_2$ were 13.3 mM and 1.5 mM, respectively. Kinetic studies with various substrates revealed that only A3n, unlike other isoperoxidases from radish, did not use scopoletin as a substrate and had very low $K_m$ value of 0.25 mM for ferolic acid among naturally occurring phenolic substrates.