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Prolidases I and II were highly purified from human erythrocytes. The effects of various amino acids, MnCl2 and mercaptoethanol, on these two enzymes were investigated. Normal prolidase II was very labile in the absence of MnCl2 or mercaptoethanol. The activity of prolidase II was maintained at about 76% by pre-incubation with MnCl2; it was then activated up to 140% by treatment with mercaptoethanol for 60 minutes at 37 °C. Normal prolidases I and II showed the highest activity against glycylproline or methionylproline in the presence of MnCl2. The activity of prolidase I against glycylproline was enhanced strongly by glycine and MnCl2, but not activated in the absence of MnCl2. The activity of prolidase II against methionylproline was enhanced three-fold in the presence of glycine and MnCl2, but its activity against glycylproline was very low even in the presence of MnCl2. A stronger enhancement of this activity was found in normal erythrocytes, and a lower level of this activity was found in erythrocytes of patients treated with glycine, MnCl2 and mercaptoethanol compared to those treated with glycine and MnCl2. The activity of prolidase II against methionylproline in all erythrocytes, of normal humans and of patients, was strongly activated by the addition of glycine with MnCl2 but suppressed by the addition of mercaptoethanol. [ABSTRACT FROM AUTHOR]