Logit-log evaluation of planar yeast estrogen screens

Receptor assays like the yeast estrogen screen (YES) performed in microtiter plates normally provide dose-response curves with a sigmoidal shape in semi-log plots. Such sigmoidal plots can be linearized by the logit function resulting in logit-log plots, as mainly known for the evaluation of enzyme-linked immunosorbent assays and radioimmunoassays. Since the planar yeast estrogen screen (pYES) represents the transfer of the receptor assay YES to high-performance thin-layer chromatography (HPTLC), it was assumed to obtain sigmoidal shaped dose-response curves from the measured signals, which subsequently could be used to generate logit-log plots. However, it was observed that typical sigmoidal curves were not obtained, when peak areas were plotted against the applied amount on a logarithmic scale (log amount). Therefore, peak heights were examined in the present study, which revealed proper dose-response curves when plotted against the log amount. The presence of sigmoidal dose-response curves from HPTLC-pYES made it possible to transform the signals into logits and, therefore, to create logit-log plots with linear correlations. The logit-log plots for the estrogen active compounds (EAC) 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) provided a working range up to 500pg/zone. Applying logit-log plots, mean recovery rates for E2 and EE2 from spiked water samples (2-20ng/L) were determined to 90% and 108%, respectively, with ≤24% RSD. Moreover, the linear graphs allowed an easy determination of the half maximal effect dose (ED₅₀) of EAC, since the intersection of the graph with the abscissa represents the ED₅₀. Additionally, with the knowledge of the ED₅₀ values, the estrogenic potential of EAC in terms of estradiol equivalent factors (EEF) could be determined, resulting in 0.64 for EE2.