Characterization of isoperoxidase-B2 inactivation in etiolated Lupinus albus hypocotyls

One basic peroxidase isoenzyme, with a pI of 8.8, is present in the interce llular washing fluid in the aerial part of 6-day-old Lupinus albus hypocoty l seedlings. This isoenzyme, called LuP-B2, is the principal soluble compon ent secreted into the apoplastic space and it is a constitutive enzyme alon g the whole length of etiolated hypocotyl. The enzymatic inactivation proce ss which this apoplastic peroxidase undergoes is described for the first ti me. The kinetic constants which describe its inactivation by H2O2 in the ab sence of reductant substrates are determined. LuP-B2 is inactivated in situ and in vitro in a time- and concentration-dependent manner. H2O2 acts as a suicide substrate according to a model previously proposed by us. The cons tant values calculated are similar to those calculated for the basic isoenz yme of horseradish roots, HRP-C. LuP-B2 presents a k(inact) value of 7.5x10 (-3) s(-1) and a k(cat) of 6.7 s(-1). This isoenzyme makes 889 catalytic cy cles for each inactivation event. The similarity in behavior and the consta nt values, together with other situations (both are excreted, soluble and c onstitutive isoenzymes) suggest that the inactivation process could play an important role in plant development and stress situations. (C) 3000 Publis hed by Elsevier Science B.V. All rights reserved.