One basic peroxidase isoenzyme, with a pI of 8.8, is present in the intercellular washing fluid in the aerial part of 6-day-old Lupinus albus hypocotyl seedlings. This isoenzyme, called LuP-B2, is the principal soluble component secreted into the apoplastic space and it is a constitutive enzyme along the whole length of etiolated hypocotyl. The enzymatic inactivation process which this apoplastic peroxidase undergoes is described for the first time. The kinetic constants which describe its inactivation by H(2)O(2) in the absence of reductant substrates are determined. LuP-B2 is inactivated in situ and in vitro in a time- and concentration-dependent manner. H(2)O(2) acts as a suicide substrate according to a model previously proposed by us. The constant values calculated are similar to those calculated for the basic isoenzyme of horseradish roots, HRP-C. LuP-B2 presents a k(inact) value of 7.5 x 10(-3) s(-1) and a k(cat) of 6.7 s(-1). This isoenzyme makes 889 catalytic cycles for each inactivation event. The similarity in behavior and the constant values, together with other situations (both are excreted, soluble and constitutive isoenzymes) suggest that the inactivation process could play an important role in plant development and stress situations.