Purification and properties of a neutral peroxidase isozyme from turnip (Brassica napus L. Var. Purple Top White Globe) roots

A neutral peroxidase isozyme (pI 7.2) from turnip roots (TNP) was purified to homogeneity and partially characterized. TNP is a monomeric glycoprotein with 9.1% carbohydrate content and a molecular weight of 36 kDa. Optimum pH values for activity using 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid (ABTS) and guaiacol as H donors were 4.5 and 5.5, whereas the K(m) values were 0.7 and 3.7 mM, respectively. The ABTS K(m) was approximately 7 times higher than that reported for basic commercial horseradish peroxidase (HRP-C). TNP retained approximately 70% activity after 11 min of heating at 65 degrees C, whereas the activation energy for inactivation (132 kJ/mol) was higher than or comparable to that of other peroxidases. The low ABTS K(m) and high specific activity (1930 units/mg) gave a high catalytic efficiency (500 M(-1) s(-1)). These properties make TNP an enzyme with a high potential as an alternative to HRP in various applications.