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Identification of Essential Amino-Acid Residues in Azotobacter vinelandii Isocitrate Dehydrogenase by Radical Anions and H Atoms

Jack Schubert, Albert E. Chung and Sheffield Gordon
Radiation Research
Vol. 80, No. 3 (Dec., 1979), pp. 440-446
DOI: 10.2307/3574986
Stable URL: http://www.jstor.org/stable/3574986
Page Count: 7
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Identification of Essential Amino-Acid Residues in Azotobacter vinelandii Isocitrate Dehydrogenase by Radical Anions and H Atoms
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Abstract

Pure ${\rm TPN}^{+}\text{-specific}$ isocitrate dehydrogenase from Azotobacter vinelandii was irradiated with H atoms generated in a γ-irradiated solution at pH 6.5. A G(-activity) = 0.12 ± 0.01 was found. At the same time no corresponding loss in free sulfhydryls was observed. These results confirmed the essentiality of methionine for the enzymatic activity as known from previous studies. Irradiation with the radical anions, $({\rm CNS})_{2}{}^{-}$ and ${\rm Br}{}_{2}{}^{-}$ generated in γ-irradiated solutions at pH 6.5, strongly inactivated isocitrate dehydrogenase with yields of G(-activity) of 2.1 and 3.9, respectively. Part of the inactivating effect, however, is due to oxidation of sulfhydryl groups. These results lead to the conclusion that tryptophan is an essential amino-acid residue in isocitrate dehydrogenase from A. vinelandii. The presence of tryptophan in the enzyme was demonstrated by pulse radiolysis.

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