The synthesis of (3H)-xylamine, an irreversible inhibitor of norepinephrine uptake

Covalent labeling of the cocaine-sensitive catecholamine transporter

Xylamine is an alkylating agent that is a substrate for and specific irreversible inhibitor of the cocaine-sensitive catecholamine transporter that functions in catecholamine reuptake into neurons and PC12 cells. [3H]xylamine prominently labels nine PC12 proteins; the relative xylamine-alkylation of a Mr 54,000 protein was decreased by cocaine and increased in the case of a PC12 variant, B9, which is deficient in catecholamine transport. [3H]xylamine labels no such protein in another transport variant, MPT1. We propose that this Mr 54,000 protein 1) is a component of the catecholamine transporter, 2) is present in B9 cells but in a conformation that reduces transporter activity and makes alkylation by xylamine more likely, and 3) is absent in MPT1 cells. Nerve growth factor treatment restores transporter activity in B9 cells but not in other transporter-deficient variants.

Characterization of xylamine binding to proteins of PC12 pheochromocytoma cells

PC12 pheochromocytoma cells take up 3,4-dihydroxyphenylethylamine (dopamine) and norepinephrine by a Na+-dependent, cocaine-sensitive system. The kinetics suggest that the same transporter functions for both substrates. Xylamine, a nitrogen mustard that blocks catecholamine uptake into neurons, irreversibly inhibited norepinephrine uptake into PC12 (IC50 = 15 microM). Pretreatment with 10 microM xylamine did not inhibit norepinephrine transport if 10 microM cocaine or 100 microM norepinephrine was also present during the pretreatment period or if Na+ was absent. These results indicate that xylamine must interact with the norepinephrine transporter to inhibit norepinephrine uptake. PC12 accumulated [3H]xylamine; this uptake had Na+-dependent and Na+-independent components. The Na+-dependent uptake was saturable (Km = 13 microM), and it was inhibited by cocaine (IC50 = 0.6 microM), desipramine (IC50 less than 1 nM), and norepinephrine (IC50 = 1 microM). Several proteins became prominently labeled when intact PC12 cells were incubated with [3H]xylamine; these proteins were enriched in a plasma membrane fraction and have molecular weights of 17,000, 24,000, 31,000, 33,000, 41,000, 42,000, 52,000, and 80,000. Other proteins were labeled less prominently. The labeling of all proteins was markedly decreased when the incubation with [3H]xylamine occurred in the presence of cocaine, desipramine, gramicidin D, or in a Na+-free buffer. These results indicate that xylamine must be transported into the cells for covalent binding to proteins to occur. [3H]Xylamine labeled essentially the same proteins when incubated with cell homogenates, but competition experiments with bretylium, desipramine, and cocaine failed to reveal which of the [3H]xylamine-labeled proteins is associated with the norepinephrine transporter.

[3H]Xylamine accumulation by two sympathetically innervated tissues

The accumulation of [3H]xylamine [( ring-G-3H]-N-2'-chloroethyl-N-ethyl-2-methylbenzylamine; [3H]XYL) by rabbit thoracic aorta and rat vas deferens was studied in vitro. [3H]XYL uptake in both tissues saturated at the same concentrations and displayed the same time course as that for maximal inhibition of [3H]noradrenaline [( 3H]NA) accumulation by XYL. Hydrolysis of [3H]XYL or coincubation with Na2S2O3 reduced tissue accumulation of tritium by 80%. In aorta and vas deferens, about 45% and 65%, respectively, of the total [3H]XYL accumulation was blocked by 100 mumol/L desmethylimipramine (DMI), l-NA, or bretylium. In the absence of sodium ion, the uptake of [3H]XYL was reduced by approximately these same amounts. In both tissues nearly 70% of the [3H]XYL taken up was protein-bound when measured as trichloroacetic acid-precipitated tritium. Of this radioactivity, the same proportion was sensitive to 100 mumol/L DMI or the absence of sodium ion as found for total tissue accumulation of [3H]XYL. Hydrolysis of [3H]XYL prior to incubation with vas deferens reduced the protein-bound tritium by more than 95%. The studies indicate that [3H]XYL interacts with NA uptake carrier in both rabbit aorta and rat vas deferens and that a substantial portion of the resulting protein-bound radioactivity is carrier-dependent.