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. 1977 Oct;60(4):504–508. doi: 10.1104/pp.60.4.504

Polarographic Study of Ammonia Assimilation by Isolated Chloroplasts 1

John W Anderson a,2, James Done a,3
PMCID: PMC542651  PMID: 16660125

Abstract

Illuminated pea (Pisum sativum) chloroplasts catalyze (ammonia plus α-ketoglutarate [α-KG])-dependent O2 evolution at rates which are commensurate with other estimates of the flux of assimilated nitrogen (mean of eight determinations, 8.3 μmole per mg chlorophyll per hour, sd 2.4). The reaction was usually initiated with 1 mm ammonia after preincubating chloroplasts in the presence of α-KG, ADP, pyrophosphate, and MgCl2.

Progressive increases in ammonia concentration gave Vmax/2 at 0.2 mm (approximately) and Vmax at about 1 mm. Higher concentrations were inhibitory; at 7 mm the rate was again about Vmax/2. The highest ratio of O2 evolved per mol of ammonia supplied was 0.36.

The (ammonia plus α-KG)-dependent reaction was inhibited by methionine sulfoximine, azaserine, and aspartate in the presence of amino-oxyacetate but not by amino-oxyacetate alone and not by l-glutamate. The rate of O2 evolution in the presence of 1 mm ammonia and 2.5 mm α-KG was increased only slightly by addition of 5 mm glutamine. Similarly, the rate of O2 evolution in the presence of 5 mm glutamine and 2.5 mm α-KG was increased only slightly by addition of 1 mm ammonia.

The results are attributed to the incorporation of ammonia via glutamine synthetase and reductive transamination of the glutamine formed by photosynthetically coupled glutamate synthase using α-KG as the amino acceptor. Several lines of evidence rule out the possibility that photosynthetically coupled glutamate dehydrogenase is involved.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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