CONTROL OF THE ACTIVITY OF PLANT PLASMA-MEMBRANE MGATPASE BY THE VISCOSITY OF THE AQUEOUS-PHASE

The activity of plant plasma membrane (PM) MgATPase (EC 3.6.1.35) was studied in PM vesicles purified from spring wheat (Triticum aestivum L. cv. Drabant) roots, winter wheat (Triticum aestivum L. cv. Martonvasari-8) roots, and soybean (Glycine max L. cv. Williams) hypocotyls by aqueous polymer two-phase partitioning. MgATPase from spring wheat roots was assayed at 23-degrees-C (a) in the absence and presence of Triton X- 100, (b) in the presence of either 1 mM or 3 mM MgATp2-, and in the presence of increasing concentration of sucrose from 10 mM upto 1.2 M. Activity of MgATPase in PM vesicles from winter wheat roots was measured at 21-degrees-C at sucrose concentrations of 15 mM-1.1 M in the presence of 3 mM MgATP2- and absence of Triton X- 100 only. MgATPase activity from soybean hypocotyls was studied (a) in the absence and presence of Triton X-100, (b) both at 21 and 31-degrees-C, in the presence of 3 mM MgATP2- and varying concentrations of sucrose between 10 mM and 1.1 M. In all cases, independently of the assay conditions and the source of PM, the MgATPase activity decreased with increasing sucrose concentration. Latency of the MgATPase activity depended only slightly on the concentration of sucrose. The Q10 value for the MgATPase activity from soybean hypocotyls (and thereby the Arrhenius activation energy of the enzymatic reaction) was independent of the sucrose concentration and of the presence of Triton X-100. At optimal assay conditions, the ATP-hydrolyzing activity of plant PM MgATPase (v) was inversely proportional to the m-th power of the viscosity of aqueous phase (eta) as predicted by the modified Kramers' theory of enzymatic catalysis: v is-proportional-to (1/eta)m, where m is an empirical parameter between 0 and 1. For the activity of MgATPase in the three species studied, m varied between 0.5 and 1.1 in good agreement with the theory. We thus conclude that (a) the activity of integral membrane enzyme-proteins may be controlled not only by the property(ies) of the membrane lipid phase but also by the viscosity of the aqueous phase in the vicinity of such enzymes, and (b) the determination of vesicle sidedness based on enzyme latency may need a minor revision. Our interpretation is in agreement with the molecular dynamics approach of enzymatic catalysis worked out for soluble enzymes.