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Plant Physiology 50:228-234 (1972)
© 1972 American Society of Plant Biologists

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Articles

Amino Acid Biosynthesis by Isolated Chloroplasts during Photosynthesis 1

Paul R. Kirk2 and Rachel M. Leech

a Department of Biology, University of York, Heslington, York, England.

The pool sizes of the common amino acids in purified intact chloroplasts from Vicia faba L. were measured (nanomoles per milligram chlorophyll). The three amino acids present in the highest concentrations were glutamate, aspartate, and threonine. Alanine, serine, and glycine were each present at levels between 15 and 20 nanomoles per milligram chlorophyll and 13 other amino acids were detectable at levels below 10.

Only aspartate, alanine, glycine, serine, threonine, and lysine became labeled during photosynthetic 14CO2 fixation by isolated chloroplasts: the label in aspartate represented over 60% of the total 14C found in the amino acids. Glutamate dehydrogenase, glutamate-oxaloacetate, and glutamate-pyruvate aminotransferases were present in the chloroplasts, but no other transferase activities from glutamate could be detected. The chloroplasts were able to synthesize a total of 17 other protein amino acids from either alanine or aspartate, but no synthesis of leucine by aminotransferase reactions could be detected. The synthesis of aspartate was studied in more detail. The enzyme systems required for the generation of oxaloacetate from triose phosphate were virtually absent from the chloroplasts but present in the leaf cytoplasmic fraction. Addition of either a leaf "cytoplasmic" fraction or an oxaloacetate generating system resulted in an increased proportion of the total 14C fixed being found in the amino acid fraction during photosynthetic 14CO2 fixation.

It is suggested that the supply of oxaloacetate from the cytoplasm is one of the important factors controlling the synthesis of amino acids by the chloroplast.

2 Present address: Department of Biological Sciences, The Polytechnic, Wolverhampton, G.B.

1 This work was supported by the Agricultural Research Council, Great Britain.






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ASPB Publications PLANT PHYSIOLOGY® THE PLANT CELL
Copyright © 1972 by the American Society of Plant Biologists