A simple enzymatic synthesis of 4-nitrophenyl β-1,4-d-xylobioside, a chromogenic substrate for assay and differentiation of endoxylanases

A simple procedure has been elaborated for preparation of 4-nitrophenyl β-d-xylopyranosyl-1,4-β-d-xylopyranoside (NPX2), a chromogenic substrate of some endo-β-1,4-xylanases. The procedure is based on a self-transfer reaction from 4-nitrophenyl β-d-xylopyranoside catalyzed by an Aureobasidium pullulans and Aspergillus niger β-xylosidases. Both enzymes catalyzed only the formation of 4-nitrophenyl glycosides of β-1,4-xylobiose with a small admixture of 4-nitrophenyl glycoside of β-1,3-xylobiose. The highest yields of the NPX2 (19.4%) was obtained at pH 5.5. The removal of the β-1,3-isomer from NPX2 is not necessary for quantification of endo-β-1,4-xylanase activity since it is not attacked by endo-β-1,4-xylanases. In contrast to GH family 5 xylanase from Erwinia chrysanthemi, which did not attack NPX2, all family 10 and 11 xylanases cleaved the chromogenic substrate exclusively between xylobiose and the aromatic aglycone. Significant differences in the Km values of GH10 and GH11 xylanases suggested that activities of these enzymes could be selectively quantified in the mixtures using various concentrations of NPX2. Moreover, NPX2 could serve as an ideal substrate to follow the interaction of endo-β-1,4-xylanases with various xylanase inhibitors.