Abstract
The uptake and metabolism of [14C]choline in dissociated rat brain embryo cell cultures was examined as a function of the extracellular choline concentration. Choline uptake did not follow normal Michaelis-Menten kinetics, but rather exhibited two components with apparent Km of 0.016 mM and 0.96 mM. At low choline concentrations (high affinity uptake) most of the [14C]choline label was present in the phosphocholine fraction prior to the appearance of label in phospholipids. At high choline concentrations (low affinity uptake) a large proportion of the radioactivity was converted into acetylcholine. The dissimilarities between the formation of phosphocholine and acetylcholine as a function of choline concentration might be explained by the existence of two mutually independent enzymatic activities with different Km affinities for choline. Kinetic data augmented by double label studies, suggested that formation of choline phosphoglyceride proceeds entirely via a phosphocholine intermediate. Nearly all radioactivity in the lipid fraction is incorporated into choline phosphoglycerides. A higher turnover rate of choline incorporation into choline phosphoglycerides, accompanied by an increase in the levels of glycerophosphocholine, was observed in older cultures as compared to younger cultures. The metabolic implications of these findings in cultured brain cells in comparison with other in vitro systems are discussed.
Highlights
The uptake and metabolism of [‘,‘C]choline in dissociated rat brain embryo cell cultures was examined as a function of the extracellular choline concentration
At low choline concentrations most of the [“Clcholine label was present in the phosphocholine fraction prior to the appearance of label in phospholipids
At high choline concentrations a large proportion of the radioactivity was converted into acetylcholine
Summary
The uptake and metabolism of [‘,‘C]choline in dissociated rat brain embryo cell cultures was examined as a function of the extracellular choline concentration. At high choline concentrations (low affinity uptake) a large proportion of the radioactivity was converted into acetylcholine. It has been proposed that the major source of choline for brain metabolism is derived from the blood circulation either as free choline [3] or in lipid-bound form [4, 5]. Studies in this direction have shown that choline is incorporated into brain acetylcholine [6] and choline phospholipids [7, 8]. Very low quantities of phosphocholine were formed at relatively high choline concentrations [10, 13]
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