Spermidine oxidase in human pregnancy serum. Probable identity with diamine oxidase

Abstract

Diamine oxidase was previously measured in human pregnancy serum with putrescine or histamine as substrate. The presence of spermidine oxidase activity in pregnancy serum was documented by means of a specific radioactive assay with [14C]spermidine as substrate and Dowex 50 cation-exchange chromatography to separate products from substrate. The apparent Km of a partially purified preparation of this enzyme for spermidine was 10.9 .mu.M and the Ki [inhibition constant] for aminoguanidine was 0.8 .mu.M. The pH optimum (pH 9.0) and temperature optimum (55.degree. C) were identical with those for diamine oxidase. Spermidine oxidase activity and diamine oxidase activity eluted in a concerted fashion when pregnancy serum was subjected to cadaverine-Sepharose chromatography, gel filtration and ion-exchange chromatography. Spermidine oxidase became detectable in serum during pregnancy in the human 8 wk after the last menstrual period and increased with gestational age in concert with the increase in diamine oxidase activity, reaching a plateau at 20 wk of gestation. Fetal-cord serum displayed virtually no activity of either enzyme. A 400-fold purified preparation of diamine oxidase retained the same diamine oxidase/spermidine oxidase ratio as exhibited by crude pregnancy serum. Apparently in pregnancy serum, unlike fetal bovine serum, spermidine oxidase and diamine oxidase activity may be a single enzyme protein.