Whether or not diamine oxidase (DAO) and histaminase are identical enzymes has been vigorously debated (1-6) and we have previously contributed to the controversy with the finding that DAO and histaminase values of plasmas from many primates do not correlate ( 7 ) . In further studies (8) we showed that acrylamide gel electrophoresis of plasmas from man and all of many primates examined separated an oxidase fraction which readily utilized histamine as a substrate, but not putrescine (as is characteristic of DAO), further supporting the view that two genetically different enzymes may exist.Since this electrophoretic fraction with histaminase activity was found at a position in the gel comparable to ceruloplasmin (Cp) , the copper-carrying plasma protein known to have oxidase activity, it raised the question of whether or not this fraction was in fact ceruloplasmin (8). The present study indicates that Cp and this histaminase fraction are the same but it does not resolve the issue first raised, i.e., whether or not DAO and histaminase are identical.Materials and Methods. Enzymic oxidative deamination of either putrescine or histamine results in the production of H 2 0 2 (1, 4, 5 ) . Thus, the method of Gordon and Peters (9) , based on secondary oxidation of o-dianisidine, was modified to locate such activity zones in acrylamide gels. Rhesus monkey (Macaca mulatta) plasma (heparinized) was used due to its high DAO and histaminase activities. Also, heparinized plasmas from pregnant (3rd trimester) and nonpregnant humans were examined. All plasma had been stored frozen before use. Electrophoresis was carried out as described by Ornstein (10) and Davis (1 1) for the separation of sera. Following electrophoresis, the gels were incubated overnight in a solution consisting of 0.03 M substrate, 0.1 "/o peroxidase (Calbiochem, horseradish, B grade), 0.01 % o-dianisidine and 0.01 26 M Tris buffer (pH 7.0).
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