Ammonia assimilation inhibitors

Although there are many cautionary tales regarding the use of inhibitors they have their uses, provided that they are reasonably specific and the results are critically assessed. Fortunately there are some well-known compounds which have proved useful in ammonia assimilation. The most common one is L-methionine-5-sulphoximine (MSO) this compound has been shown to be an irreversible Inhibitor of glutamine synthetase (Ronzio et al. 1969 Tate and Meister, 1973). In the presence of ATP and metal ions the MSO becomes tightly bound to the enzyme in the form of MSO-phosphate and this is analogous to the formation of y-glutamyl phosphate by the en-... 

Inhibitor studies (Leaet al., 1979) have shown that the synthesis of protein from glutamine by isolated pea cotyledons is unaffected by MSO but inhibited by azaserine, consistent with the action of glutamate synthase but not very conclusive because of the side effects of azaserine. Little if any labeling data on ammonia assimilation has so far been produced using developing seeds. 

As stated previously, the rate of ammonia production in the glycine serine conversion reaction during photorespiration in a C3 plant leaf is approximately 10 times the normal rate of nitrate assimilation (Keys et al., 1978). There is therefore a major requirement for the plant to reassimilate the ammonia as rapidly as possible. Experiments with inhibitors of and mutants lacking the key enzymes have clearly shown that plants die very rapidly if ammonia assimilation is blocked (Lea and Ridley, 1989 Blackwell et al., 1988b). 

There is thus good evidence from the inhibitor studies that glutamine synthetase plays a key role in the process of photorespiration. Possible explanations for the rapid fall in photosynthetic rates will be discussed in the following section, which describes the properties of mutants lacking enzymes of the ammonia assimilation pathway. 

However, models which assume that ammonia is assimilated solely via GDH cannot account in quantitative terms for the observed rates of NH/ incorporation (Rhodes et al, 1980 Fentem et ai, 1983a). Rhodes et ai, (1989) modeled the kinetics of NH4 incorporation in tobacco cell cultures and suggest that up to 30% of the ammonia assimilated may enter via the GDH reaction. However, the relative sizes of the glutamate and glutamine pools made it difficult to distinguish between the simultaneous operation of alternative pathways and Rhodes et al. were unable to verify the model when inhibitors of GS and GOGAT were used to block the glutamate synthase cycle. 

Inhibition patterns of [ N]ammonia accumulation by strains JL907 and JB822 (GDH") are shown in Figure 4. It is clear that assimilation plays an important role in [ N] ammonia accumulation in these strains. Strain JB822 is resistant to methionine sulfoximine due to a defect in methionine transport (data not shown). The inhibition by metabolic inhibitors (Figure 4) was not unexpected, since assimilation and all possible accumulation mechanisms (Table II) are energy-dependent. It is presently unclear whether N-ethylmaleimide inhibition was due to inactivation of a sulfhydryl-containing surface receptor or to inhibition of electron transport. 

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