THE EFFECT OF RAISED OXYGEN PRESSURE AND OF METHIONINE SULPHOXIMINE ON THE GLUTAMINE SYNTHETASE ACTIVITY OF RAT BRAIN

OXYGEN intoxication, like other pathological conditions, is associated with the liberation of ammonia in the brain (GERSHENOVICH and KRICHEVSKAYA, 1954). It may be expected that the excess of ammonia is removed by glutamine synthesis or by reductive amination of ketoglutaric acid. The synthesis of glutamine is an endothermal process requiring ATP and is catalysed by the enzyme glutamine synthetase (L-glutamate : ammonia ligase; 6:3.1.2). The inhibition of the glutamine synthetase by chloromercuribenzoate, and the elimination of the inhibition by cysteine and glutathione, suggest that this is an enzyme containing sulphydryl groups. Evidence of an increased reductive amination of ketoglutarate is given by the accumulation of glutamic acid and the decrease in brain respiration of animals subjected to oxygen pressures of 4 or 6 atmospheres. Oxygen at this pressure causes attacks of clonic and tonic spasms. The glutamine content and the activity of glutamine synthetase in the brain have previously been investigated in the preseizure stage, during the seizures, and during the terminal stage of oxygen intoxication (GERSHENOVICH and KRICHEVSKAYA, 1952, 1954, 1955, 1956). At the onset of the preseizure stage the amount of glutamine is somewhat higher than normal and the activity of glutamine synthetase is also greater; but if the preseizure stage lasts longer than 10 min, as in slow intoxication there is a rapid decrease in the glutamine content, which continues during the seizure and the terminal stage. The activity of glutamine synthetase during these periods of oxygen intoxication also decreases by 49-50 per cent on an average. The effect of oxygen under pressure on glutamine synthesis may be due to impaired utilization of ATP (BRONOVICKAYA, 1953) associated with oxidation of sulphydryl groups in the enzyme protein molecule. This is supported by the work of WIELAND et al. (1958) who demonstrated that an enzyme-ATP complex is formed in the presence of the sulphydryl group of the enzyme. This leads to the formation of ‘active’ glutamic acid to which ammonia is transferred. High oxygen pressure is known to oxidize the sulphydryl groups of enzymes (STADIE and HAUGARD 1946; BARRON, 1955). The action of oxygen at high pressure may be compared with the inhibition of glutamine synthetase by methionine sulphoximine. This substance also causes seizures. KOLOUSEK and JIRACEK (1959) showed that it inhibited the synthesis of glutamine and of protein, as indicated by the decrease in concentration of these substances in the rat brain after the administration of methionine sulphoximine. The action of methionine sulphoximine on protein synthesis is probably due to an inhibition of the activation of free methionine (formation of adenosyl methionine). It has