INFLUENCE OF STEROLS ON PLASMA-MEMBRANE PROTON-PUMPING ATPASE ACTIVITY AND MEMBRANE FLUIDITY IN OAT SHOOTS

The aim of this work was to determine the influence of sterols on the activity of proton-pumping Hf ATPase (EC 3.6.1.35) and membrane fluidi ty. Plasma membrane vesicles were isolated and purified from the shoot s of 10-d-old winter oat seedlings (Avena sativa L.) cv. Peniarth, gro wn in a controlled environment. Membranes were incubated separately, i n vitro, with cholesterol, stigmasterol or a mixture of sitosterol and campesterol and the proton-pumping activity of H+-ATPase was measured . In membranes incubated with cholesterol or stigmasterol, enzyme acti vity increased, whereas in those incubated with campesterol plus silos terol these was a reduction. Increases in membrane fluidity, as determ ined by steady-state fluorescence polarisation, were found with choles terol- and stigmasterol-incubated membranes, but no change was observe d with the sitosterol/campesterol treatment. It was concluded that pro ton-pumping H+-ATPase activity was independent of fluidity.