EN
Frog liver acid phosphatase hydrolyzes phosphotyrosine at acidic pH optimum. Mn2+, Ca2+ and Mg2+ (but not Zn2+) ions modulate this activity by shifting its pH optimum to physiological pH. This effect is not observed when p-nitrophenylphos-phate is used as a substrate. Phosphoserine and phosphothreonine are not hydrolyzed under the same conditions.