Dark reoxidation of the plastoquinone-pool is mediated by the low-potential form of cytochrome b-559 in spinach thylakoids

We have found that in isolated spinach thylakoids, plastoquinone-pool (PQ-p ool), after its photoreduction, undergoes dark-reoxidation with the half-ti me of tau(1/2) = 43 +/- 3 s. To explain the observed rates of PQ-pool reoxi dation, a nonenzymatic plastoquinol (PQH(2)) autoxidation under molecular o xygen and an enzymatic oxidation by the low-potential form of cytochrome b- 559 (cyt. b-559LP), as the postulated PQ-oxidase in chlororespiration, were investigated. It was found that the autoxidation rate of PQH(2) in organic solvents and liposomes was too low to account for the observed oxidation r ate of PQH(2) in thylakoids. The rate of cyt. b-559LP autoxidation in isola ted Photosystem II was found to be similar (tau(1/2) = 26 +/- 5 s) to that of the PQ-pool. This suggests that the LP form of cyt. b-559 is probably re sponsible for the PQ-oxidase activity observed during chlororespiration.