An experimental study and computer simulation of the turnover of choline in erythrocytes of patients treated with lithium carbonate.

The mechanism by which choline accumulates in erythrocytes during treatment with lithium salts has been elucidated. A component of the study was a kinetic description of erythrocyte phospholipase-D, which catalyses the release of intracellular choline from phospholipids. Apparent steady-state kinetic parameters for calcium ions were determined: Km (+/- SD) = 0.6 +/- 0.3 mmol/l aqueous cell volume and Vmax (+/- SD) = 12 +/- 4 mumol/l packed red blood cells (RBC) min-1. Competitive inhibition of the phospholipase-D by barium ions was also observed. Other information concerning choline and lithium levels and red cell life-time was obtained from the literature. Details of the kinetics were used to develop a comprehensive dynamic model of choline metabolism by erythrocytes. The scheme is as follows; phosphatidylcholine associated with high density lipoproteins exchanges with the erythrocyte membrane phospholipids, the neutral phospholipids undergo two dimensional translational and rotational motion and also flip between each layer of the bilayer thus becoming exposed to an intracellularly-located phospholipase-D, whereupon the choline is hydrolysed and released into the intracellular milieu. A choline transport protein, which is able to be inhibited by lithium, mediates the influx and efflux of choline. The differential equations that describe reactant flux in this scheme were integrated numerically and the choline accumulation profiles under various conditions of transport and enzyme inhibition are presented. Computer solution of the model, by using as input values plasma lithium levels in the upper limit of the therapeutic range, required that the red cell life-time be reduced in order to explain the previously observed negative association between choline and increasing lithium levels. The results of the computer simulations under varying initial conditions of plasma and erythrocyte lithium and choline concentrations permit, for the first time, a comprehensive description of those factors affecting erythrocyte choline levels.