Molecular and Biochemical Studies of Rat Vesicular Monoamine Transporter

Abstract

Transport of the monoamines serotonin, doparnine, norepinephrine, epinephrine, and histamine into storage organelles in a variety of cells is catalyzed by vesicular monoamine transporters (VMATs). Accumulation of the neurotransmitter depends on the proton electrochemical gradient generated by the vesicular H + -adenosine triphosphatase and involves the VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. A model of the mechanism of action of VMAT has been proposed based on a large body of biochemical data. A clue for the molecular basis of some of these processes has been obtained using diethyl pyrocarbonate (DEPC), a reagent relatively specific for His residues. The inhibition by DEPC is specific for His groups because transport could be restored by hydroxylamine. Cloning of VMAT has made it feasible to try to identify the residue(s) modified by DEPC. DEPC inhibited transport but had no effect on binding of reserpine, indicating that the inhibition of transport was not because of a direct interaction with either of the known binding sites. The result suggests that either proton transport or a conformational change induced by proton transport is inhibited by DEPC. Practically identical results have been obtained with phenylglyoxal, a reagent specific for Arg residues.