The stability of electron transport in in vitro chloroplast membranes

The stability and stabilization of the electron transport system of chloroplast membranes under physiological conditions of temperature and illumination were studied in relation to two separate and often competing mechanisms of decay. Photochemical inactivation (photoinhibition) of the electron transport system of ageing spinach chloroplasts was not normally found to limit stability even at saturating light intensities. Only when the membranes were protected from dark (fatty acid) inhibition did photoinhibition limit stability.Electron transport could be partially protected from dark inhibition by the addition of high concentrations of recrystallized (i.e. fatty acid free) bovine serum albumin, ovalbumin, polyethyleneimine cellulose, Biomesh SM2 beads or with Ficoll 400. Some improvement in stability was achieved with N,N, dimethylphenethylamine but other esterase and phospholipase inhibitors were ineffective in preventing thermal inactivation.Photoinhibition was apparently delayed by phenazine methosulphate under certain conditions but was unaffected either by artificial scavengers of reactive oxygen species (butylated hydroxytoluene), and 1,4-diazobycyclo (2, 2, 2 octane) or by natural scavengers which constitute part of the in vivo protective mechanism (α-tocopherol, β-carotene, SOD, catalase and glutathione) or by anaerobic incubation. Photoinhibition may therefore be by a separate mechanism which does not initially involve free radical damage.