ProgesteroneReceptor:StabilityStudies and Correlationbetween Steroid BindingAssay and Enzyme lmmunoassay

We evaluated and compared Abbott Laboratories’ newly developed enzyme immunoassay(EIA) for measuring progesterone receptors (PgR) with that of DuPont’s steroidbindingassay (SBA). We also used both methodsto study the stability of PgR under various conditions. There were excellent correlations for all 59 cytosols compared (r = 0.94) and for the 44 cytosols containing PgR >10 fmol per milligram of protein (r = 0.93), but the correlation for cytosols containing <10 fmol of PgR per milligram was poor. We found PgR to be more stable as assayed by EIA than by SBA. The biological halt-lives of PgR at 30, 4, and -60 #{176}C were -3 h, 6 days, and 19 days, respectively. The effect of molybdate on PgR is complex. Its presence during tissue homogenization leads to analytical recovery of more PgR and may stabilize PgR during storage. Its presence during enzyme immunoassay is less critical. Unlike estrogen receptor, PgR is not protected by its ligand, R5020.