Mechanism of Linolenic Acid-induced Inhibition of Photosynthetic Electron Transport.

The effect of linolenic acid on photosynthetic electron transport reactions in chloroplasts has been localized at a site on the donor side of photosystem I and at two functionally distinct sites in photosystem II.In photosystem I, an increase in the electron transport rate occurs in the presence of 10 to 100 micromolar linolenic acid, followed by a decline in rate from 100 to 200 micromolar linolenic acid. The increase may result from an alteration of membrane structure that allows greater reactivity of the artificial donors 2,6-dichlorophenolindophenol (DPIP) and N,N,N',N'-tetramethyl-p-phenylenediamine with plastocyanin. The decrease is due to loss of plastocyanin from the membrane since addition of purified plastocyanin to treated and washed chloroplasts leads to the reestablishment of photosystem I rates.In photosystem II, a reversible site and an irreversible site of inhibition have been located. At the irreversible site, there is a time-dependent loss of the loosely bound pool of Mn implicated in the water-splitting mechanism. At the reversible site, the photochemical charge separation is rapidly inhibited as evidenced by the high initial fluorescence yield upon illumination and the inhibition of artificial donor reactions in NH(2)OH-washed chloroplasts. When chloroplasts are washed after treatment with linolenic acid, the fluorescence returns to its original low value and there is a resumption of artificial donor activity from diphenylcarbazide --> DPIP. This reversible inhibition of the photoact is a unique characteristic of linolenic acid and suggests evidence for a new mode of inhibition of photosystem II.