Studies on cation-induced membrane vesicle aggregation of porcine intestinal brush borders

Cation specificity and effects of pH and temperature on membrane aggregation of porcine intestinal brush borders were studied in terms of the turbidity change of the membrane suspensions due to the addition of polyvalent cations. Monovalent cations (K+ and Na+) did not induce the membrane aggregation at 5 mM or below. Divalent and trivalent cations induced membrane aggregation even below 0.5 mM in the following orders of effectiveness: Hg2+ much greater than Cd2+ greater than Zn2+ greater than Mn2+ greater than Ca2+ greater than or equal to Ba2+ greater than Sr2+ greater than or equal to Mg2+ and Lu3+ greater than Yb3+ greater than Nd3+ = Er3+ = Tb3+ greater than Ce3+ greater than La3+, respectively. The aggregation of membranes induced by these polyvalent cations was reversible. Membrane aggregation induced by either divalent or trivalent cations followed saturation kinetics with increasing cation concentration and the apparent dissociation constants of these cations for the membranes were estimated to be at 4-6 mM for divalent cations (Ca2+, Mn2+, and Sr2+) and 1-2 mM for trivalent cations (Lu3+, Tb3+, and La3+). Cd2+ and Hg2+ had peculiarly much lower dissociation constants than any other divalent cations tested, being rather comparable to trivalent ones. The membrane aggregation induced by Ca2+, Mn2+, or Sr2+ was independent of pH in the range of 6.6 to 8.0, whereas that induced by Tb3+ or Cd2+ was linearly enhanced with increasing pH, very steeply above pH 7.0. The membrane aggregation induced by Cd2+, Hg2+, or Tb3+ was enhanced steadily with increasing temperature between 15 and 45 degrees C. On the other hand, Ca2+-induced aggregation showed a peak at around 30 degrees C in the temperature dependence profile, still being enhanced above 40 degrees C.