IN-VITRO PHOTOINACTIVATION OF CATALASE ISOFORMS FROM COTYLEDONS OF SUNFLOWER (HELIANTHUS-ANNUUS L.)

Catalase (EC 1.11.1.6) isoforms CAT 2 through CAT 8 were purified from peroxisomes of sunflower (Helianthus annuus L.) cotyledons and photoi nactivated in vitro, Action and absorbance spectra between 380 and 727 nm wavelength showed most prominent maxima at 405 nm suggesting an in activation mediated by light absorption of heme groups. First order ki netics of inactivation were observed for CAT 6 through CAT 8 (isoform group B), which are composed of four 55-kDa subunits. Inactivation con stants hi depended on photon fluence rates in the studied range betwee n 8.3 and 660 mu E m(-2) s(-1). The maximal value of k(i) was 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 i rradiation, but both degradation processes occurred at rates lower tha n those of inactivation. Quantitative evaluations contradicted the vie w 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 su bunits, were less photosensitive than the isoforms of group B, In addi tion, irradiated group A isoforms reached a low plateau of residual ac tivity, whereas group B isoforms were inactivated completely, The 59-k Da subunits in group A isoforms were much more resistant to photodegra dation than the 55-kDa subunits of group B isoforms and also much more resistant than their own 55-kDa cosubunits, Results presented here ar e compared with catalase photoinactivation and turnover in vivo and di scussed with respect to a physiological significance of catalase isofo rms in plant peroxisomes. (C) 1997 Academic Press.