Tumor and placental histaminase, II A procedure for radioimmunoassay

Histaminase (diamine oxidase) is an enzyme associated with pregnancy and with a number of human cancers. In pregnancy, the enzyme is produced by the decidual cells of the placenta. Histaminase of the placenta is biochemically and immunologically identical to that of cancer, Based on this, a radioimmunoassay procedure for histaminase has been developed. The high affinity monospecific antiserum for the assay was obtained from rabbits by injecting with homogeneous histaminase purified from placenta by affinity chromatography. Radioactive labeling of histaminase was carried out by iodination with 125I using chloramine T as the oxidizing agent. The iodination yielded [125I]-histamine of high specific radioactivity (20 microCi/microgram protein) with no apparent affect on the immunologic affinity. For separating the antibody-antigen complex from the unbound antigen, a second antibody bound to polyacrylamide beads was most effective at high antiserum dilutions. The assay had a working range of 0.3 to 80 microgram/ml and a minimal detectable quantity of 0.15 ng/ml. Compared to the enzymatic assay with [14C] putrescine as substrate, the radioimmunoassay procedure is about 70 times more sensitive. Measurements of histamine in placental extracts and malignant effusions using both radioimmunoassay and enzyme assay demonstrated that the two methods were highly correlated, thus providing evidence for the specificity of the radioimmunoassay. This procedure will be useful in future studies of histaminase as a biochemical marker for human cancer and for the elucidation of the significance of this enzyme is pregnancy and in neoplasia.