In VitroPhotoinactivation of Catalase Isoforms from Cotyledons of Sunflower (Helianthus annuusL.)

Catalase (EC 1.11.1.6) isoforms CAT 2 through CAT 8 were purified from peroxisomes of sunflower (Helianthus annuusL.) cotyledons and photoinactivatedin vitro.Action and absorbance spectra between 380 and 727 nm wavelength showed most prominent maxima at 405 nm suggesting an inactivation mediated by light absorption of heme groups. First order kinetics of inactivation were observed for CAT 6 through CAT 8 (isoform group B), which are composed of four 55-kDa subunits. Inactivation constantskidepended on photon fluence rates in the studied range between 8.3 and 660 μE m−2s−1. The maximal value ofkiwas about 4.0 h−1, corresponding to a half-life of about 10 min. Heme groups and 55-kDa apoprotein moieties of group B isoforms were degraded during irradiation, but both degradation processes occurred at rates lower than those of inactivation. Quantitative evaluations contradicted the view that photoinactivation was caused by destruction or dissociation of heme but suggested apoprotein damage leading to the loss of activity. Group A isoforms CAT 2 through CAT 5, containing both 55- and 59-kDa subunits, were less photosensitive than the isoforms of group B. In addition, irradiated group A isoforms reached a low plateau of residual activity, whereas group B isoforms were inactivated completely. The 59-kDa subunits in group A isoforms were much more resistant to photodegradation than the 55-kDa subunits of group B isoforms and also much more resistant than their own 55-kDa cosubunits. Results presented here are compared with catalase photoinactivation and turnoverin vivoand discussed with respect to a physiological significance of catalase isoforms in plant peroxisomes.