DIFFERENTIAL STIMULATION BY CL- OF H-MEMBRANE VESICLES FROM MAIZE (ZEA-MAYS L) ROOTS( TRANSPORT AND ATPASE ACTIVITY IN PURIFIED PLASMA)

Maize (Zea mays L.) root plasma membranes purified by the aqueous poly mer two-phase technique have previously been shown to be very low in t onoplast H+-ATPase and H+-PPase activities. Western blots of a similar preparation showed that, compared to a microsomal fraction, there was practically no reaction with antibodies to the tonoplast enzymes, but a strong reaction with an antibody to the plasma membrane H+-ATPase. Freeze/thaw treatment of the plasma membrane vesicles increased the pr oportion with an inside-out orientation to about 40%. This preparation was used to demonstrate that substitution of KCl for K2SO4 resulted i n a 14-fold stimulation of H+ transport, but an increase in ATPase act ivity of less than 10%. In contrast to its effect on tonoplast vesicle s, Cl- had only a small effect on the membrane potential of plasma mem brane vesicles, assayed by oxonol V fluorescence quench recovery. To a ccount for the apparent variability in the H+/ATP coupling ratio, it m ay be necessary to devise a model that takes into consideration the po ssibility of non-linear behaviour with respect to the membrane potenti al of the proton leak and/or of slip in the ATPase.