Color Test for Pentoses

Abstract

One drop (0.05 cc.) of a pentose solution (1-arabinose, d-arabinose, xylose or ribose) containing 0.05 mg. or more of the sugar is placed in a test tube. 0.5 cc. of benzidine solution (1 gm. benzidine in 25 cc. glacial acetic acid. This solution keeps for 4 days) is added and the mixture is brought to vigorous boiling. The test tube is then put in cold water. Within a few seconds a very stable cherry red color develops. Glucose, fructose and galactose give yellow to brown colors with the benzidine solution. Even large amounts of hexoses, however, do not interfere with the test. There is only a slight delay in the formation of the red color (0.1 mg. of arabinose and one mg. of glucose). As little as 10 gamma of pentose may be detected. For instance urine specimens containing 0.1 mg. of arabinose per cc. gave a distinct red color when 0.1 cc. of urine was added to 0.5 cc. of benzidine solution. Normal and abnormal urinary constituents do not interfere with the test. Too large amounts of proteins may be removed from pathological urines by adding an equal volume of 10% trichlor acetic acid, mixing, and heating to 95°. The filtered urine may then be tested.

When testing for pentose in urine, the urine (0.1 cc.) and benzidine solution (0.5 cc.) is brought to vigorous boiling. The mixture is cooled under tap water and 1 cc. of distilled water is added. In the presence of pentose a pink to red color is shown immediately, whereas if pentose is absent the mixture has a yellowish brown color.

This color test is also given by vitamin B₂ (riboflavin), by the yellow oxidation enzyme of Warburg and Christian, and by nucleic acids owing to the ribose content of all of these compounds. It is negative, however, with gum arabic since this pentosan is not hydrolyzed by the benzidine-acetic acid solution. The test is highly specific for pentoses.