Bipyridylium herbicide toxicity in an isolated lung cell culture system

Abstract

An investigation has been made into the effect of low concentrations of 2,6-dichlorophenolindophenol (DCIP) and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), in the presence of ascorbate, on light-induced absorbance changes in the α-band region of cytochrome f and b-type cytochromes. Strong photo-oxidation of cytochrome f occurred with DCIP, in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (2 μM), but not with TMPD. However, with TMPD a large light-induced absorbance increase occurred due to a broad band centred at 566 nm. With DCIP, the uncoupler, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP), in conjunction with the electron acceptor 1,1′-ethylene-2,2′-bipyridylium dibromide (diquat), inhibited the cytochrome f response and an absorbance increase in the 560–575 nm region occurred similar to that with TMPD. Neither FCCP nor diquat had any great effect on the TMPD system. The results support a pathway of electron transport between the two photosystems in which (a) DCIP-ascorbate interacts with an intermediate on the Photosystem II side of a coupling site; (b) TMPD-ascorbate interacts after this site, and (c) cytochrome f is located on the Photosystem I side of the site.