Abstract
Publisher Summary The transport of choline (Ch) into synaptosomes has been suggested to be mediated by two mechanisms that differ in their affinity toward the substrate. Initial studies showed only one Km (about 10–5M) for the uptake of choline into synaptosomes. This chapter discusses the uptake and metabolic distribution of Me[14C]choline through a study in clone M1 of mouse neuroblastoma 1300. Total incorporation of 14C and labelling of phosphorylcholine were linear with time, while the free choline (Ch) compartment was rapidly saturated, Incubation of cells at different temperature, and under high affinity uptake conditions resulted in abnormal Arrhenius plots. This anomaly was not shown when cells were incubated with 30μM choline. The results support the previously published data suggesting that, the conditions used to study high affinity Ch uptake produce a non steady-state in the endocellular pool of choline. It is postulated that the uptake of Ch may be regulated by the exo-endocellular concentrations of choline, and that the high and low affinity uptake systems may represent a single mechanism that changes in conformation with different concentrations of substrate.
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