Inhibition and inactivation of presynaptic cholinergic markers using redox-reactive choline analogs

Inhibition and inactivation of two presynaptic cholinergic "markers", choline acetyltransferase and high affinity choline transporter, has been investigated using inhibitors designed with a redox-reactive catechol tethered to a quaternary ammonium group. Two quaternary ammonium alkyl-substituted catechols, 3[(trimethylammonio)methyl]catechol (TMC, 1) and N,N-dimethylepinephrine (catecholine, 2) were shown to bind weakly and noncompetitively to bovine choline acetyltransferase yet inactivated the enzyme in a time course consistent with the involvement of early intermediates in the spontaneous oxidation of these catechols. Both agents also inhibited high-affinity choline uptake. The time course of TMC and catecholine spontaneous oxidation-dependent inactivation of high affinity choline uptake sites was slower than, if it occurred at all, the spontaneous degradation of measurable choline transport in synaptosomes. When compared with inhibition of uptake of other neurotransmitters, it was shown that catecholine demonstrated more selectivity than TMC toward inhibition of choline transport. Km (microM) and Vmax (pmol/min per mg of protein) were measured for high affinity transport of choline, dopamine, and serotonin and were observed to be Km = 2.04 +/- 0.31, Vmax = 22 +/- 1; Km = 1.4, Vmax = 53; and Km = 0.15, Vmax = 23, respectively, in good agreement with published literature values. Ki's (mM) for catecholine and TMC, calculated from experimentally determined IC50's, were for catecholine 0.13 +/- 0.06, 0.53 +/- 0.09, and 0.39 +/- 0.10, and for TMC 0.06 +/- 0.03, 0.09 +/- 0.03, and 0.09 +/- 0.08, for choline, dopamine, and serotonin transport, respectively. In vivo studies using catecholine suggest that this compound impairs learning ability associated with long-term memory. Thus, catecholine represents a lead compound in a potential series of redox-reactive choline analogs, which may become useful irreversible antagonists of the critical cholinergic macromolecular targets underlying cholinergic hypofunction in disorders such as Alzheimer's disease.