Inhibition of Oxygen Evolution in Chloroplasts by Ferricyanide

Abstract

Brewer and Jagendorf (3) observed that dark preincubation of chloroplasts with 0.5 mm FeCN' caused an inhibition of electron transport which they attributed to a step related to 02 evolution.Ikegami and Katoh (10) have found that preincubation of chloroplasts with FeCN slowed down the rise kinetics of fluorescence in the presence of DCMU. This was interpreted as due to oxidation of an internal acceptor during preincubation. Contrary to the interpretation of Brewer and Jagendorf (3), they suggested that the site inhibited is located between the reaction center of PSII and the DCMU-sensitive site.In the present work, the effects of preincubation with FeCN on electron transport and fluorescence were reinvestigated and compared. We conclude that the interpretation of Brewer and Jagendorf (3) is valid and that the inhibition in the rate of the fluorescence rise is also caused by inhibiting electron flow between H20 and PSII.MATERIALS AND METHODS Isolation of Chloroplasts. Chloroplasts were prepared from 2-week-old pea seedlings (Pisum sativum L. cv. Kelvedon) by a modified method of Nakatani and Barber (11). Ten g of leaves were homogenized for 5 s in a homogenizer with 75 ml of medium containing 0.33 M sorbitol, 0.2 mm MgCl2, and 20 mm Mes, pH 6.5. The homogenate was filtrated through 8 layers of gauze and centrifuged at 2500g for 40 s. The pellet was resuspended in the same medium.Tris Treatment. The chloroplast pellet was resuspended in 16 ml of0.8 M Tris, pH 8, and incubated on ice for 25 min. Thereafter, the suspension was centrifuged at 10,000g for 7 min and the pellet was resuspended in the sorbitol-MgCl2-Mes medium.

On the participation of chloride in bicarbonate‐induced reversal of anion inhibition of photosystem II electron transport in spinach thylakoids

“…Heat-shocked thylakoids were then immediately cooled in an ice bath and were kept in dark until further use (Sabat et al 1986). Inactivation ofthe oxygen evolving complex was performed by other treatments also, such as washing with Tris (Drechsler and Neumann 1982) or hydroxylamine (Ghanotakis et al 1984). No detectable oxygen evolution was observed in inactivated samples when measured by an oxygen electrode.…”