Reactivity of the cysteine and tyrosine residues of aspartate transaminase from chicken heart cytosol |
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Authors: | V.M. Kochkina Yu.M. Torchinskii |
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Affiliation: | Institute of Molecular Biology, Academy of Sciences, Moscow, USSR |
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Abstract: | Aspartate transaminase (EC 2.6.1.1) from chicken heart cytosol contains 4 thiol groups per subunit. Two of them are fully buried. One exposed SH group is readily modified by iodoacetamide, N-ethylmaleimide, tetranitromethane, 5,5′-dithio-bis(2-nitrobenzoate), 4,4′-dipyridyl disulfide and p-mercuribenzoate. A further SH group is semi-buried: while inaccessible for alkylating reagents and disulfides, it can be blocked by p-mercuribenzoate at pH about 5 (but not at pH 8). Treatment of the enzyme with tetranitromethane in the absence of substrates leads to nitration of maximally 0.8 tyrosine residue per subunit; in the presence of amino and keto substrate 1.65 eq of nitrotyrosine is formed, with a moderate decrease of enzymic activity. |
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