Energy-driven uptake of the neurotoxin 1-methyl-4-phenylpyridinium into chromaffin granules via the catecholamine transporter

Chromaffin granules take up and concentrate 1-methyl-4-phenylpyridinium (MPP+) through a temperature-sensitive and saturable mechanism. The uptake displays an apparent Km of 51.2 microM and a Vmax of 7.1 nmol/min/mg of protein. MPP+ uptake is markedly depressed in the absence of ATP or by inhibition of the membrane Mg2+-dependent ATPase, and it is completely blocked by reserpine. Reversal of the membrane potential by carbonyl cyanide m-chlorophenylhydrazone or dissipation of the pH gradient in the presence of nigericin plus potassium ions produces a marked inhibition of MPP+ uptake indicating that the process is dependent upon the integrity of the transmembrane proton electrochemical gradient generated and maintained by the membrane Mg2+-dependent ATPase. Furthermore, the data shows that a permanently charged compound is capable of entering the granule through the catecholamine carrier.