Amine transport in chromaffin granule ghosts. pH dependence implies cationic form is translocated

Chromaffin granules have a translocator-mediated uptake system for the monoamines dopamine, serotonin, norepinephrine, and epinephrine. These substrates are predominantly cationic at physiological pH but they also exist in neutral, zwitterionic, and anionic forms. The cationic fraction is nearly pH-independent between pH 6.9 and pH 7.6. Over the same pH range, the neutral and zwitterionic fractions increase by a factor of 6.3 and the anionic fraction increases by a factor of 40. In chromaffin granule ghosts, the apparent Km values for dopamine and serotonin transport are independent of pH between 6.8 and 7.6. Consequently, the translocator probably binds the cationic form of the substrate. Vmax values for dopamine and serotonin uptake increase by a factor of 2 between pH 6.8 and pH 7.6.