The pure isomers L-buthionine-S-sulfoximine and L-buthionine-R- sulfoximine are provided. The L-S-isomer has utility, for example, in causing the depletion of glutathione, a major protectant molecule in tumors and certain parasites. The L-R-isomer has utility as a compound decreasing further uptake and thus effect of the L-S-isomer and for causing glutathione depletion specific to the kidney. The isolation of pure isomers from L-buthionine-SR-sulfoximine enables treatment at lower dosages without cross effects.PPThe pure L-buthionine-S- sulfoximine isomer is obtained from L-buthionine-SR-sulfoximine by recrystallization preferably by forming a solution of L-buthionine-SR- sulfoximine in water at a concentration of 0.8M and cooling to form crystals enriched in the L-R-isomer and a filtrate enriched in L-S-isomer, drying the filtrate to obtain a solid, dissolving the solid in ethanol, adding trifluoroacetic acid to form acid salt and provide solvent, then adding hexane to cause crystal formation, then converting the crystals to the free zwitterion using ion exchange resin.PPBoth isomers can be separated concurrently by reverse phase chromatography in Cu.sup.++ /D-proline buffers.
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