Redox reactions of tonoplast and plasma membranes isolated from soybean hypocotyls by free-flow electrophoresis

Redox reactions were studied in more than 90% pure tonoplast and plasma membranes isolated by free-flow electrophoresis from soybean (Glycine max) hypocotyls. Both types of membrane contained a b-type cytochrome (_max = 561 nm) and a noncovalently bound flavin, two possible components of a transmembrane electron-transport chain. Isolated tonoplast and plasma membranes reduced ferricyanide, indophenol and various iron complexes with NADH or NADPH as electron donors. The redox activity was inhibited in tonoplast membranes by about 60% by 10 μM p-chloromercuribenzene sulfonate, 8% by 500 μM lanthanum nitrate and 10% by 100 μM nitrophenyl acetate. In contrast, the redox activity of isolated plasma membranes was inhibited by about 60% by 500 μM lanthanum nitrate or 100 μM nitrophenyl acetate, but only 25% by 10 μM p-chloromercuribenzene sulfonate. The results show that both tonoplast and plasma membranes of soybean contain active electron-transport systems, but that the two systems respond differently to inhibitors.