Aspartate aminotransferase forms large well-formed crystals on slow concentration by vapor diffusion against solutions of polyethylene glycol. Native enzyme and enzyme blocked at one or both of its surface sulfhydryls with small uncharged substituents were crystallized under the same conditions with methylmercuric chloride, p-chloromercuribenzoate or mercuric acetate in attempts to provide heavy atom derivative crystals. Methylmercury binds to cysteine-191 and cysteine-82 in both native AAT and the enzyme blocked with N-ethylmaleimide. Mercuribenzoate binds to cysteine-82 and cysteine-45 in the native enzyme. Native AAT has been crystallized with 2 equivalents per monomer of methylmercuric chloride or p-chloromercuribenzoate to provide two heavy atom derivative crystals for the crystallographic structure determination;Pyridoxal 5'-sulfate reacts at the active site of aspartate apoaminotransferase to modify lysine-258. Three peptides containing lysine-258 modified by pyridoxal sulfate have been isolated and characterized. The second nucleophilic group involved in the reaction of pyridoxal sulfate with the enzyme remains unidentified. It is most likely the hydroxyl of serine-257, but the guanidino group of arginine-386 and a hydroxide from the solvent are also possibilities;The 5'trans-carboxyethenyl analog of pyridoxal phosphate appears to modify the same active site peptides as pyridoxal sulfate.