[Methylviologen photoreduction by chloroplasts].

The condition of methylviologen photoreduction by chloroplasts was investigated. Argon bubbling through the suspension of chloroplasts or degasing in vacuum caused inhibition of methylviologen reduction probably due to the denaturation of chloroplast membranes at the water/air boundary. Adding glycerol or bovine serum albumine or removing oxygen from chloroplast suspension with the aid of the oxygen absorbing-systems preserved the activity of chloroplasts. Methylviologen photoreduction is inhibited by DCMU (10(-7) M) and Tris-buffer treatment and is activated by uncouples. The pH-dependence is similar to that of the Hill reaction. Triton X-100 (0.007%), ethyl ether (2%) and heating up to 42 degrees activated the Hill reaction but inhibited methylviologen reduction. Water molecule probably acts as an initial electron donor in this reaction. It is proposed that the steady level of methylviologen photoreduction is determined by a relationship between the rate of methylviologen electron acceptance and cyclic electron flow short-circuiting photosystem I.