Amino acids interference on the quantification of reducing sugars by the 3,5-dinitrosalicylic acid assay mislead carbohydrase activity measurements

[Display omitted] ► The interference of several amino acids on the phenol-free DNS assay was proved. ► The interference is modulated by both sugar and the amino acid concentrations. ► Likewise phenol, cysteine enhances the reduction of DNS. ► The amino acids interference can mislead carbohydrase activity measurements. ► Cysteine is a potential candidate to substitute phenol in the DNS formulation. This study evaluated the interference of the amino acids tryptophan, cysteine, histidine, tyrosine, hydroxyproline, leucine, proline, serine, glycine, valine, glutamic acid, phenylalanine, and methionine on the measurement of reducing sugars using a phenol-free 3,5-dinitrosalicylic acid (DNS) reagent. It was found that in reaction mixtures containing 20mM of either tryptophan, cysteine, histidine, tyrosine, or hydroxyproline the measurement of 3.7mM glucose was overestimated by 76%, 50%, 35%, 18%, and 10%, respectively. The amino acids valine, glutamic acid, and phenylalanine did not affect the DNS reaction, while methionine decreased the color development by 5%. The measurement of glucose, xylose, arabinose, and cellobiose at the 3.7–12.4mM range in the presence of 20mM cysteine resulted in an overestimated concentration of 34.8–50%. Enzymatic assays for measuring xylanolytic and filter paper activity (FPAse) were conducted in the presence of 20–60mM cysteine, and compared to cysteine-free assays. In the presence of cysteine, the measured xylanase activity increased threefold and the FPAse activity increased twofold due to the overestimation of the reducing sugar concentrations in the assays. The interference from cysteine was reduced to a maximum of 8.6% when a DNS reagent containing phenol was used.