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. Martonvas ari-8) roots, and soybean (Glycine max L. cv. Williams) hypocotyls by aqueous polymer two-phase partitioning. MgATPase from spring wheat roo ts was assayed at 23-degrees-C (a) in the absence and presence of Trit on 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 m easured at 21-degrees-C at sucrose concentrations of 15 mM-1.1 M in th e presence of 3 mM MgATP2- and absence of Triton X- 100 only. MgATPase activity from soybean hypocotyls was studied (a) in the absence and p resence of Triton X-100, (b) both at 21 and 31-degrees-C, in the prese nce of 3 mM MgATP2- and varying concentrations of sucrose between 10 m M and 1.1 M. In all cases, independently of the assay conditions and t he source of PM, the MgATPase activity decreased with increasing sucro se concentration. Latency of the MgATPase activity depended only sligh tly on the concentration of sucrose. The Q10 value for the MgATPase ac tivity from soybean hypocotyls (and thereby the Arrhenius activation e nergy of the enzymatic reaction) was independent of the sucrose concen tration and of the presence of Triton X-100. At optimal assay conditio ns, the ATP-hydrolyzing activity of plant PM MgATPase (v) was inversel y proportional to the m-th power of the viscosity of aqueous phase (et a) as predicted by the modified Kramers' theory of enzymatic catalysis : v is-proportional-to (1/eta)m, where m is an empirical parameter bet ween 0 and 1. For the activity of MgATPase in the three species studie d, 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-protei ns may be controlled not only by the property(ies) of the membrane lip id phase but also by the viscosity of the aqueous phase in the vicinit y of such enzymes, and (b) the determination of vesicle sidedness base d on enzyme latency may need a minor revision. Our interpretation is i n agreement with the molecular dynamics approach of enzymatic catalysi s worked out for soluble enzymes.