Effects of the sesquiterpene lactone tetraesters thapsigargicin and thapsigargin, from roots of Thapsia garganica L., on isolated spinach chloroplasts.

The effect of thapsigargicin and thapsigargin, extracted from the roots of Thapsia garganica L., on isolated photosynthetic membranes (thylakoids) and intact chloroplasts from spinach leaves (Spinacia oleracea L.) was investigated. Both sesquiterpene lactone tetraesters impair membranes and organelles in an identical, chlorophyll-dependent manner. In thylakoids these compounds primarily act as inhibitors of photophosphorylation. At lower sesquiterpene lactone tetraester/chlorophyll ratios, cyclic and non-cyclic photophosphorylation, ADP-stimulated electron transport and the photosynthetic control ratio progressively decreased with increasing concentrations of thapsigargicin and thapsigargin, whereas the state 4 electron flow, the coupling efficiency of photophosphorylation, the light-induced proton gradient, and the H+ flux across the membranes remained nearly unaffected. Half-maximal inhibition of photophosphorylation was obtained with 4-5 X 10(-7) moles sesquiterpene lactone tetraesters per mg chlorophyll. At higher sesquiterpene lactone tetraester/chlorophyll ratios, uncoupling of photophosphorylation from electron transport occurred. This was evident from stimulation of the state 4 electron flow, decline in the ADP/2e- ratio, increase in proton permeability and decrease in delta pH, whereas the uncoupled electron transport was only little impaired. In intact chloroplasts inhibition of HCO-3, 3-phosphoglycerate and oxaloacetate reduction by thapsigargicin and thapsigargin was not caused by inactivation of the photochemical reactions of the thylakoid membranes but were rather due to alterations in the permeability properties of the chloroplast envelope. This was concluded from similarities in the kinetics of these reactions. It is suggested that the highly lipid soluble sesquiterpene lactone tetraesters effectively disrupt the lipid-protein associations of biomembranes.