Effect of methionine sulfoximine on asparaginase activity and ammonium levels in pea leaves

In developing leaves of Pisum sativum the levels of ammonium did not change during the light-dark photoperiod even though asparaginase (EC 3.5.1.1) did; asparaginase activity in detached leaves doubled during the first 2.5 hours in the light. When these leaves were supplied with 1 millimolar methionine sulfoximine (MSX, an inhibitor of glutamine synthetase, GS, activity) at the beginning of the photoperiod, levels of ammonium increased 8-to 10-fold, GS activity was inhibited 95%, and the light-stimulated increase in asparaginase activity was completely prevented, and declined to less than initial levels. When high concentrations of ammonium were supplied to leaves, the light-stimulated increase of asparaginase was partially prevented. However, it was also possible to prevent asparaginase increase, in the absence of ammonium accumulation, by the addition of MSX together with aminooxyacetate (AOA, which inhibits transamination and some other reactions of photorespiratory nitrogen cycling). AOA alone did not prevent light-stimulated asparaginase increase; neither MSX, AOA, or elevated ammonium levels inhibited the activity of asparaginase in vitro. These results suggest that the effect of MSX on asparaginase increase is not due solely to interference with photorespiratory cycling (since AOA also prevents cycling, but has no effect alone), nor to the production of high ammonium concentration or its subsequent effect on photosynthetic mechanisms. MSX must have further inhibitory effects on metabolism. It is concluded that accumulation of ammonium in the presence of MSX may underestimate rates of ammonium turnover, since liberation of ammonium from systems such as asparaginase is reduced by the effects of MSX.     

Effect of methionine sulfoximine on methylation of guanine residues in astroglial transfer ribonucleic acids

Culture-grown astrocytes derived from 3-day-old rat brain were incubated in the presence of [³H]guanosine and of the convulsant agentl-methionine-dl-sulfoximine (MSO). The resulting [³H]tRNA was purified from control and MSO-exposed cells at several time points during the incubation and was hydrolyzed to [³H]guanine and four [³H]methyl guanines which were separated by high pressure liquid chromatography. Three of the four [³H]methyl guanines were more highly labeled in the [³H]tRNA of the MSO-exposed cells, relative to that of the control cells throughout the entire incubation period. The findings extend to cultured astrocytes, the stimulatory effect of MSO on the methylation of neural tRNA guanines, previouly observed both in vitro using [¹⁴C]S-adenosyl-l-methionine and in vivo using [methyl ³-H]l-methionine.     

Influence of methionine sulfoximine on the photosynthesis of isolated chloroplasts

Methionine sulfoximine provided at a concentration which inhibits photosynthesis in intact leaves (10 mM) had no significant influence on the rate of photosynthesis of isolated pea leaf chloroplasts. In contrast, ammonium, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and D,L-glyceraldehyde all strongly inhibited the photosynthesis of isolated chloroplasts. We conclude that low concentrations of methionine sulfoximine (up to 10 mM) have no direct effect on the photosynthetic process.     

Mechanism of inhibition of pea chloroplast glutamine synthetase by methionine sulfoximine

In the presence of ATP and Mg2+ L-methionine sulfoximine irreversibly inhibits homogeneous glutamine synthetase (EC 6.3.1.2) from pea chloroplasts (I0.5 = 1.0 x 10(-7) M; Ki = 6.25 . 10(-8) M. Glutamate (but not NH4Cl) exerts a protective effect, which is enhanced when glutamate and NH4Cl are simultaneously present in the reaction mixture. The inhibiting action of L-methionine sulfoximine with respect to glutamate is of a mixed type. ATP and Mg-ATP produce the same non-competitive protective effect on L-methionine sulfoximine. The data obtained suggest that the formation of a quaternary complex (or a transition state) between the enzyme and all its substrates is essential for the catalysis.     

Ammonia accumulation and inhibition of photosynthesis in methionine sulfoximine treated spinach

Ammonia accumulation and photosynthetic rate inhibition took place when spinach leaf tissue was supplied with methionine sulfoximine (MSO), an inhibitor of glutamine synthetase. This effect was observed in the absence of significant inorganic nitrogen reduction or an exogenous source of ammonia. Both the time lag prior to the initial photosynthetic rate decrease and the rate of that decrease depend on the O₂ and MSO concentrations supplied to the leaf tissue. However, the total rate of ammonia accumulation was similar at both 20% and 2.2% O₂. The decline in photosynthetic rate was not caused by stomatal closure but may be a result of ammonia toxicity. The data point out the importance of glutamine synthetase in preventing the poisoning of leaf metabolism by ammonia generated internally through processes not involved in net nitrogen assimilation. The rapidity of the action of MSO in suppressing photosynthesis was unexpected and should not be overlooked in interpreting data from other experiments involving that inhibitor. MSO shows promise as a tool for investigating C-N flow, particularly during photorespiration.     

The effect of methionine on the uptake,distribution, and binding of the convulsant methionine sulfoximine in the rat

The effect of methionine on the uptake, distribution, and binding of the convulsant methionine sulfoximine (MSO) in 7 rat brain regions, the spinal cord, the liver, and the kidney was investigated. The administration of methionine decreased the uptake of MSO in all brain regions. The uptake of MSO by and its distribution in the nervous tissue was uniform and failed to result in any preferential accumulation of the drug. Methionine decreased the amount of MSO bound to cerebral structures and to the spinal cord. MSO bound to the spinal cord was less susceptible to release by Triton X-100 than was brain-bound MSO.