Rate-limiting step in electron transport. Osmotically sensitive diffusion of quinones through voids in the bilayer.

Respiration in mitochondria and photosynthesis in chloroplasts varied with the osmotic stretch of the membrane such that these processes were uniformly inhibited at higher osmolalities. A systematic evaluation of segmental electron transport in these intact particles showed that no individual complex exhibited osmotic sensitivity, whereas osmotic sensitivity appeared wherever the assay involved crossing over the corresponding quinone in the electron transport chain. The evidence was consistent with the rate-limiting step in electron transport being the availability of voids for quinone migration rather than any of the components of electron transport chain per se. Evidence based on quinone reconstitution in mitochondria depleted of quinone by acetone treatment clearly distinguished the kinetic control in the hypotonic domain and diffusive control via availability of voids in the hypertonic domain. Influence as well as the presence of voids was further confirmed in quinone-depleted mitochondria reconstituted with quinone as well as cholesterol. Decrease in lateral diffusion of the fluorescent probe, 12-(9-anthroyl)stearic acid, on osmotic compression of the bilayer is consistent with a change in void size distribution on osmotic compression of the bilayer. A direct correlation between succinate cytochrome c oxidoreductase activity and diffusivity of fluorescent probe 12-(9-anthroyl)stearic acid confirmed the availability of voids as the rate-limiting step in electron transport.