The choice of buffer depends on the pH optima of the endo-xylanase enzyme being measured and therefore the buffer that is closest in pH to the enzyme pH optima should be used.
For endo-xylanase with a pH optimum of approx. pH 4.5 choose buffer B, 0.1M Sodium acetate pH 4.5 plus 0.5 mg/mL BSA.
For endo-xylanase with a pH optimum of approx. pH 6.0 choose buffer D, 0.1M Sodium phosphate pH 6.0 plus 0.5 mg/mL BSA.
The supplied endo-xylanase enzyme in bottle 2, is from trichoderma source and has a pH optimum of pH 6. Hence extraction buffer/dilution buffer D is required for this enzyme.