Participation of ?-carotene in reactivation of PSI of heptane-extracted spinach chloroplasts

Abstract

A carotenoid requirement for photosystem I activity in spinach chloroplasts using extraction-reconstitution technique has been investigated. The transfer of electron from N,N,N',N'-tetramethyl-p-phenylene diamine through the chloroplast photosystem to methyl viologen dye or to NADP(+) was used as an assay of photosystem I activity. Extraction of lyophilized spinach chloroplasts with heptane at near 0°C removed almost all β-carotene and reduced photochemical activities associated with photosystem I to a low level (about 15% of the original activity). Reconstitution of the extracted chloroplasts with β-carotene completely restored photosystem I activity. The maximum rate of methyl viologen photoreduction in reconstituted chloroplasts occurred at an β-carotene/chlorophyll molar ratio of 0.5. Cyclic phosphorylation mediated by phenazine methosulphate was partially restored. Xanthophylls (lutein, neoxanthin, violaxanthin), as components of chloroplast membranes, were not able to replace β-carotene in reconstitution of chloroplasts and had essentially no effect on restoring photoreactions. On the basis of the P700/total chlorophyll ratio it can be assumed that extraction of lyophilized chloroplasts with heptane do not affect photosystem I reaction centre. Therefore it is possible that β-carotene, removed during heptane extraction and belonging mainly to the antenna pigment pool of photosystem I, is effective in the restoration of photosystem I activity.