Surface charge densities and ionic substrate concentrations at the membrane surface in smooth microsomes isolated...

Salt-induced pH changes of smooth microsomes from rat liver were examined. At pHs higher than 4.9 addition of salt to a microsomal suspension induced a decrease in pH, while at pHs lower than 4.9 it induced a pH increase. The salt-induced pH changes were explained by the change in the degree of dissociation of ionizable groups of membranes due to the change of surface potential and surface pH. On comparison of the experimental data with those of calculations with the Gouy-Chapman equation, a value of 3.1+/-0.1 X 10(-3) carboxyl groups/A2 was obtained, which gives a maximal surface charge density of -1.08+/-0.04 X 10(-3) elementary charge/A2 at neutral pH and -19.2 mV surface potential at 0.15 M monovalent salt. Due to the surface charges of smooth microsomal membranes the surface pH and surface concentrations of ionic substrates become different from those in the bulk aqueous phase depending on the salt concentration. This explains part of the salt-concentration dependence of the activities of membrane enzymes in vitro. The importance of the surface concentrations of ionic substrates of enzymes of smooth microsomal membranes in vivo is also suggested.