Purification and characterization of an acid protease from Monascus kaoliang

An acid protease from Monascus kaoliang was purified by consecutive applications of fractional acetone precipitation, batchwise CM-cellulose method and DEAE-cellulose column chromatography. The preparation was homogeneous on disc polyacrylamide gel electrophoresis at pH 4.5 and 7.5. The yield was about 30% with overall increase in specific activity of about 6-fold. The molecular weight as determined by SDS gel electrophoresis was about 34,000. The enzyme was a glycoprotease as indicated by specific carbohydrate staining on gels. It possessed the nature of an acid protease with a pH optimum at 3.0 toward heat-denatured casein and was stable over the range of pH 3.0 to 6.0. Reducing agents and thiol poisons had no effect on this enzyme, suggesting that free sulfhydryl groups were not required for enzyme activity. Diisopropyl fluorophosphate did not inactivate this protease, indicating the probable absence of serine residue in the active site. The enzyme was inactivated by reaction with the carboxy-group specific reagent, 1,2-epoxy-3-(p-nitrophenoxy) propane (EPNP). Pepstatin, a specific inhibitor for pepsin, was shown to inhibit this enzyme strongly. However, biacetyl (2,3-butadione) had little effect on this protease, although it inactivated pepsin to an 85% activity loss. Also, p-bromophenacyl bromide, another specific inhibitor of pepsin, failed to inactivate this acid protease.