ATPase activity in plasmalemma‐rich vesicles isolated by aqueous two‐phase partitioning from Vicia faba mesophyll and epidermis: Characterization and influence of abscisic acid and fusicoccin

Abstract

Plasmalemma‐rich microsomal vesicles were prepared from whole leaf and acid‐washed epidermal tissue of Vicia faba L. cv. Osnabrücker Markt by aqueous two‐phase partitioning in dextran T‐500 and polyethylenglycol 1350 aqueous phases. These vesicles were tightly sealed and predominantly right‐side out, and contained a K+ ‐stimulated, mg2+‐dependent and vanadate‐sensitive ATPase. The enzyme from both tissues exhibited nearly identical properties: pH optimum 6.4, Km for ATP 0.60 mM(whole leaf) and 0.67 mM (epidermis). Vmax ‐480 nmol (mg protein)1 min1 (whole leaf) and 510 nmol (mg protein)1 min1 (epidermis), I50 (Na3,VO4) 7.5 μM (whole leaf) and 15 μM (epidermis). The enzyme was not inhibited by NO3(50 mM)or sodium azide (I mM). DCCD (20 μM) reduced enzyme activity to 50% (whole leaf) and 58% (epidermis), gramicidin S (20 μM) to 36% (whole leaf) and 41%(epidermis). Ca2+ inhibited the ATPase [I50, C2+: 0.5 mM(whole leaf) and 0.8 mM(epidermis)]. Ca2+ inhibited the ATPase [I50, C2+ 0.5 mM(whole leaf) und 0.8 (epidermis)]. The vanadate‐sensitive ATPase from whole leaf and epidermal tissue was slightly but significantly stimulated by fusicoccin (FC) at a concentration (0.13 μM) promoting stomatal opening. The stimulation was not seen in the solubilized ATPase. Stomata of the cultivar used here were insensitive lo (±)ABA up to 2 μM level which is effective in most other cultivars and species. Likewise, at this concentration no effect of ABA on the activity of the epidermal ATPase was observed. The data are discussed with respect to the interaction of FC and ABA with the ATPase.