The development and application of a direct radioimmunoassay for prostaglandin E2 utilising a gamma-labelled ligand

A direct radioimmunoassay for urinary PGE2 has been developed, using a Pasteur Institute antibody and a gamma-labelled ligand. The assay does not require preliminary solvent extraction of the sample or any chromatographic steps. Gamma-labelled PGE2 of high specific activity was prepared by direct iodination of a PGE2-histamine derivative. Results obtained by the direct assay after serial dilution of urine samples were parallel to the standard curve. Accuracy of the method was determined by the recovery of added amounts of PGE2 to diluted urine (r = 0.094, y = 1.039x-0.535, n = 20). The lowest concentration of PGE2 distinguishable from zero was 0.063 pg/ml. The assay curve covered the range of 0.063-500 pg/ml. The assay was desensitized by increasing the concentration of both label and antiserum to obtain a curve covering the range of 2-1250 pg/ml. Intra-assay and interassay coefficient of variation were 8.6% and 12.5% respectively. Values obtained by the direct method were lower than those obtained by gas chromatography mass spectrometry (r = 0.43, y = 1.57x-3.52, n = 12) and by radioimmunoassay using 3H-PGE2 (r = 0.76, y = 0.93x+2.87, n = 12). Antibodies to PGE2 were produced in rabbits immunised with a conjugate of PGE2 covalently linked to thyroglobulin by carbodiimide reaction. The method was used to study the effect of urine flow and sodium intake on PGE2 excretion levels in normal volunteers. PGE2 excretion was flow dependent but did not vary with sodium excretion.