A Method for the Determination of Soluble Arabinoxylan Released from Insoluble Substrates by Xylanases

Abstract

The propensity for xylanase to convert insoluble to soluble arabinoxylan is an important parameter in many applications. Current methods for determining xylanase activity on insoluble substrates are nonspecific or utilize artificial substrates which may provide much different results from native substrates. Therefore, a new method for the determination of xylanase activity on insoluble substrates was developed. This method involved incubation of the enzyme with a material containing insoluble arabinoxylan. Arabinoxylan released by the enzyme was quantified as total pentose sugars colorimetrically by reaction with phloroglucinol upon heating in acetic–hydrochloric acid. Absorbance was determined at 552 nm, and interfering hexoses were accounted for by subtracting the absorbance at 510 nm. Because the method measured total pentose sugars released by the enzyme, (arabino)xylanase activity, rather than xylanase activity, was recommended for expressing results. The method was tested using two xylanases and six insoluble arabinoxylan-containing substrates. Sodium acetate and sodium citrate buffers (50 mM) were suitable for the reaction; sodium phosphate buffer substantially interfered with quantification of reaction products by reducing color development. The enzymic release of soluble arabinoxylan was linear for at least 5 min under all reaction conditions tested. Contaminating amylase and cellulase activity did not influence the results, despite the presence of starch and cellulose in many substrate sources. Relative standard deviations were <5% between reactions assayed on different days. Activity on substrates from different botanical origin differed by up to 100-fold, emphasizing the need for the use of application-specific substrates to obtain accurate estimations of enzyme activity.