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 wer e 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 s ensitivity appeared wherever the assay involved crossing over the corr esponding quinone in the electron transport chain. The evidence was co nsistent with the rate-limiting step in electron transport being the a vailability of voids for quinone migration rather than any of the comp onents of electron transport chain per se. Evidence based on quinone r econstitution 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 q uinone-depleted mitochondria reconstituted with quinone as well as cho lesterol. Decrease in lateral diffusion of the fluorescent probe, 12-( 9-anthroyl)stearic acid, on osmotic compression of the bilayer is cons istent with a change in void size distribution on osmotic compression of the bilayer. A direct correlation between succinate cytochrome c ox idoreductase activity and diffusivity of fluorescent probe 12-(9-anthr oyl)stearic acid confirmed the availability of voids as the rate-limit ing step in electron transport.