I was trying to measure aspartyl protease activity under acidic pH using 1% azocasein dissolved in Tris-HCl (pH 8.0). However, azocasein precipitates out in acidic pH when I add enzyme solution in acidic buffer (pH 3.5) to it or only acidic buffer in case of enzyme blank.

Any suggestion which assay (using same or different substrate ) should I follow to measure enzyme activity at acidic pH?