Two Characteristic and Sensitive Color Reactions Between Sulfhydryl Compounds and Thymonucleic Acid. ∗

Thymonucleic acid reacts with cysteine in H₂S0₄ at 40° in two different ways depending upon which of the two reactants is in excess. The reaction products are colored and can be used for the detection of cysteine in a dilution of 1:10⁶ and of thymonucleic acid in 1:10⁴. Glutathione also gives a positive reaction in a dilution of 1:300,000. The procedure for the two reactions is as follows:

I. To 1 cc of a solution of Na thymonucleate (0.01 to 0.1%) are added, with cooling, 0.04 cc of a 5% solution of cysteine hydrochloride and 5 cc of a mixture of 190 cc of H₂O and 450 cc of a 5% solution of cysteine hydrochloride and 5 cc of a mixture of 190 cc of H₂O and 450 cc of concentrated H₂SO₄. The mixture is put into a water bath at 40° for 5 minutes. A deep pink color appears, which is stable for hours. It shows a sharp absorption maximum at 500 μ. Cystine, homocystine, methionine, and dithiodiglycolic acid do not react even in high concentrations. Glutathione reacts like cysteine in equivalent concentrations. Thioglycolic acid reacts also, though a little less strongly than cysteine. Propyl mercaptan also gives the reactions, but pyruvic acid and acetone do not. When the desoxyribose is destroyed by heating the thymonucleic acid in 2% H₂SO₄ for 2 hours at 100°, the reaction with cysteine disappears completely. All these facts indicate that the colored substance is the product of a reaction between the desoxyribose of the thymonucleic acid and the sufhydryl group of the cysteine (glutathione, thioglycolic acid.)

The reaction can be used for the detection and probably for the colorimetric determination of small amounts of thymonucleic acid.