The Effects of Digitonin on Photochemical Activities of Isolated Chloroplasts

Abstract

The addition of digitonin to chloroplasts stimulated the rate of oxygen evolution followed by a gradual inhibition. The effect of digitonin was dependent on the digitonin to chlorophyll ratio and on temperature and time. The initial stimulation of oxygen evolution appeared to be a result of uncoupling as digitonin did not stimulate oxygen evolution by uncoupled chloroplasts. The stimulatory effect occurred more rapidly at high digitonin to chlorophyll ratios but the extent of stimulation was low and inhibition occurred soon after addition of the detergent. The inhibition of electron flow by digitonin was due to a site of action near photosystem II which resembled the inhibition reported for tris buffer and resulted in photobleaching. However, digitonin inhibition could not be recovered by washing with reducing agents and was only partially recovered by the addition of artificial electron donors to photosystem II. Electron flow mediated by photosystem I was unaffected by the addition of digitonin but was decreased when the chloroplasts were separated by subsequent centrifuging. This suggested that digitonin solubilizes photosystem I components which remain active in the soluble form.

Function of sterols

“…The photochemical activity of chloroplasts is also reduced with filipin treatment, probably through interaction with membrane sterols (Bishop 1973). The steroid digitonin, a sapogenin, forms equimolar insoluble sterol complexes and treatm ent of chloroplasts with digitonin reduces photosynthesis as measured by oxygen evolu tion (Robinson & Wiskich 1975).…”