We report on the conformation of reduced glutathione in solutions at low and physiological pH, examined with 1H and 13C n.m.r. spectroscopy. The tripeptide in 1H2O was shown to interconvert rapidly between an array of conformers; in addition, the carbon backbone of the glutamyl was more rigid than anticipated if the residue were freely mobile. This restricted motion results from interaction of the α‐amino and α‐carboxyl groups on the glutamyl, with the γ‐Glu‐Cys peptide‐carbonyl and amino, respectively. Our results support theoretical predictions of the conformation but they are at variance with previous ultraviolet spectroscopic and lower field n.m.r. studies.

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Summary.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: K^ (± SD) = 0-6 ± 0'3 mmol/1 aqueous cell volume and V^,^ (± SD) =12 + 4 iimo\/\ packed red blood cells (RBC) min"'. 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.

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The first application of inversion-recovery spin-echo proton nuclear magnetic resonance spectroscopy to the monitoring of reactions in rat brain preparations is presented. The initial report of the assignment of proton spin-echo nuclear magnetic resonance spectra from rabbit brain homogenates (C. R. Middlehurst et al., J. Neurochem. 42, 878-879, 1984) was used to assist in the assignment of spectra acquired from rat brain homogenates that were obtained from animals killed by cervical fracture or focussed microwave irradiation. Microwave-irradiated brains were divided into four major anatomical regions. Differences in metabolite levels were detected when spectra from fresh tissue and from various regions were compared. The in situ steady-state kinetics of prolidase in whole brain homogenate was determined. The procedure relies on the spectral differences between enzyme substrates and reaction products. The concentration dependence of the rate of hydrolysis of glycyl-L-proline was discribable by the Michaelis-Menten expression with a Michaelis constant of 1.90 mmol L-1 and a maximal velocity of 9.30 mumol min-1 mg-1 protein. The reactions catalysed by glutaminase and acetylcholinesterase in the brain were also monitored.