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Selenium Metabolism in Neptunia amplexicaulis.

J N Burnell1

  • 1School of Environmental and Life Sciences, Murdoch University, Murdoch, Western Australia 6155.

Plant Physiology
|February 1, 1981
PubMed
Summary

Neptunia amplexicaulis enzymes ATP sulfurylase, cysteinyl-tRNA synthetase, and methionyl-tRNA synthetase were purified. Selenium analogs are utilized by synthetases, but selenate incorporation into proteins is prevented by later stages in protein synthesis.

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Area of Science:

  • Biochemistry
  • Plant Science
  • Molecular Biology

Background:

  • ATP sulfurylase (EC 2.7.7.4), cysteinyl-tRNA synthetase (EC 6.1.1.16), and methionyl-tRNA synthetase (EC 6.1.1.10) are key enzymes in sulfur and amino acid metabolism.
  • Neptunia amplexicaulis is a plant species with unique metabolic pathways.

Purpose of the Study:

  • To purify and characterize key enzymes involved in sulfur metabolism and protein synthesis from Neptunia amplexicaulis.
  • To investigate the substrate specificity of these enzymes, particularly concerning selenium analogs.
  • To elucidate the mechanism by which selenium is excluded from protein synthesis in this plant.

Main Methods:

  • Purification of ATP sulfurylase, cysteinyl-tRNA synthetase, and methionyl-tRNA synthetase using biochemical assays.
  • Enzymatic assays to determine substrate incorporation and activity, including ATP-pyrophosphate exchange and aminoacylation.
  • Analysis of reactions involving sulfate, selenate, and their derivatives.

Main Results:

  • ATP sulfurylase was purified and shown to incorporate sulfate into adenosine 5'-phosphosulfate, with evidence for analogous selenate incorporation.
  • Crude extracts showed sulfate-dependent NADH oxidation but not selenate-dependent oxidation in the adenosine 5'-phosphosulfate kinase assay.
  • Purified cysteinyl-tRNA synthetase and methionyl-tRNA synthetase accepted selenium-containing analogs as substrates.

Conclusions:

  • Selenium-containing amino acids appear to be excluded from proteins via mechanisms downstream of amino acid activation.
  • The plant's protein synthesis machinery possesses a regulatory step that prevents the incorporation of selenomethionine and selenocysteine.
  • Further research is needed to identify the specific exclusion mechanisms in Neptunia amplexicaulis protein synthesis.

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