Endoxylanase, for which the optimum temperature is 60 degreesC (optimum pH 7), is labile to heat. Because the isoelectric point (pl) value of this xylanase is 10.6, the net charge of this enzyme is positive at pH 7. Thus, ions are likely to influence its enzyme structure and the thermal stability of endoxylanase may improve. Among the various ions tested, orthophosphate anion (HPO42-) was found to significantly improve not only the stability but the activity of xylanase. When K2HPO4 concentration was increased from 50 mM to 1.2 M, the T-m value of xylanase was increased from 60.0 degreesC to 74.5 degreesC. The affinity of xylanase on xylan also increased along with K2HPO4 concentration. Thus, the xylanase activity at 0.6 M K2HPO4 was 2.3-fold higher than that at 50 mM K2HPO4, and 120.2-fold higher than that in 40 mM MOPS buffer. This enhanced activity in the presence of K2HPO4 probably takes place because the orthophosphate anion affects the binding and catalytic residues of endoxylanase. (C) 2001 John Wiley & Sons, Inc.