Purification and characterization of a peroxidase isozyme from Indian turnip roots

A peroxidase isozyme (TP I) from Indian turnip roots ( Brassica rapa ) was purified. TP I had a minimum molecular mass of 45 000 Da as determined from sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). A far-UV circular dichroism (CD) spectroscopy study of TP I revealed the presence of 44% alpha-helix, 16% beta-sheet, and 40% random structure. The N-terminal sequence of TP I was found to be Gln-Phe-Val-Ile-Pro-Thr-Tyr-Ala-Trp-Gln. Pyromellitic dianhydride (PMDA)-modified TP I showed enhanced thermal stability and p-chlorophenol removal efficiency. In the absence of polyethylene glycol (PEG), PMDA-modified TP I (dose of 50 units mL(-1)) converted 100% p-chlorophenol, while at the same time, native TP I could convert only 85%. In the presence of PEG, PMDA-modified TP I (dose of 0.05 units mL(-1)) converted p-chlorophenol completely in 45 min, while native TP I required 60 min for complete conversion. The K(M) value toward the substrates p-chlorophenol and o-cresol decreased after PMDA modification of TP I, which indicated increased affinity for these substrates.