Proteolysis of hen's egg yolk incubated in acidic condition

Proteolysis of egg yolk in an acid solution was investigated. Egg yolk solutions (yolk 1 : water 4), adjusted to pH 4.0 by acetic acid, were kept at 40 deg C for 3 hours and at 5 deg C for 3 weeks. Tyrosine contents of 5% trichloroacetic acid filtrates increased in both conditions to give 20 micro g/ml tyrosine after 3 hours at 40 deg C and 50 micro g/ml after 3 weeks at 5 deg C respectively. According to SDS-PAGE analysis, the patterns changed with incubation time, and especially the protein of relative molecular mass of about 200 KDa decreased clearly. Addition of pepstatin A prevented these changes. It may be concluded from these results that the proteolysis of egg yolk under acidic conditions is caused by egg yolk aspartic proteinase.