Objective • This test is for sulphur containing amino acids cysteine and cystine. Methionine does not respond to this test.
Principle • Sulphur containing amino acids, such as cysteine and cystine upon boiling with sodium hydroxide (hot alkali) yield sodium sulphide. This reaction is due to partial conversion of the organic sulphur to inorganic sulphide, which can detected by precipitating it to lead sulphide, using lead acetate solution.
Cysteine or cystine respond to this test but the sulphur present in the methionine doesn’t respond to this test. • Casein and Gelatin which contain metheonine give negative test
Reagents • NaOH 40% solution in H2O • Lead Acetate solution
Procedure • Take 2 ml of give solution in test tube. • Add 2ml of 40% NaOH solution. • Boil for 2 minutes • Add ml of lead acetate solution.
Interpretation • Brown or black ppts formed if cysteine or cysteine containing protein is present in the solution