Fractionation and characterization of cystine aminopeptidase (oxytocinase) and arylamidase of the human placenta

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Abstract
Three activity peaks hydrolyzing L-cystine-di-.beta.-naphthylamide (CysNA) and 2 activities hydrolyzing L-leucine-.beta.-naphthylamide (LeuNA) were separated by gel filtration on Sepharose 6B from human placental tissue. The enzyme activities in the void volume and the solubilized enzyme activities with both substrates apparently are bound and free forms of the same enzymes (I) since detergent treatment caused a total disappearance of the activities in the void volume. The 2nd distinct enzyme (II) was highly soluble and detected only with CysNA. The particle-bound enzyme(s) had a pH optimum at 6.5 with CysNA and at about 7.5 with LeuNA. They were highly sensitive to EDTA, could be reactivated by Co2+ and Zn2+ and were more sensitive to Ni2+ and L-methionine than the soluble enzyme II. The former enzyme(s) tolerated thermal treatment better than the soluble enzyme II. The solubilized free enzyme(s) I had a MW of about 309,000. The soluble enzyme II was resistant to EDTA. Its optimum was at pH 6.0 and an estimate of 76,000 for the MW was obtained. [Homogenates of human placenta inactivate oxytocin hormone. The inactivation is apparently enzymic in nature.].

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