Speaker
Description
Background: Determining nonspecific binding in liver microsomes is critical for kinetic estimation but difficult for highly bound compounds due to methodological biases. This study compares three common dialysis techniques (equilibrium, pre-saturation, and flux) to assess how equilibration time and protein stability affect fraction unbound (fu) results, ultimately providing recommendations for accurate measurement.
Methods: Fourteen highly bound compounds were selected to evaluate fu using three methods: equilibrium dialysis (6 and 24 h), pre-saturation dialysis (6 h), and flux dialysis. Assays were conducted at three microsomal protein concentrations (0.1, 0.5, and 1 mg/mL). Both active and inactivated microsomes were compared. Protein stability over varying incubation times at 37 °C was assessed via native and SDS-PAGE. The study compared fu values across conditions to analyze potential sources of bias.
Results: Six-hour equilibrium dialysis underestimated fu for slow-equilibrating compounds, whereas pre-saturation and 24 h dialysis produced significantly higher values than flux dialysis. While room-temperature treatment caused only partial inactivation, 24 h incubation at 37 °C led to extensive protein degradation and conformational changes, falsely elevating fu. Flux dialysis generally showed consistent results between inactivated and non-inactivated microsomes, though non-inactivated samples appeared better to reflect the true binding state for certain compounds.
Conclusions: Extended incubation or treatments that degrade protein structure artificially elevate fu. Consequently, for highly bound compounds that fail to equilibrate within 4–6 h, we recommend flux dialysis using non-inactivated microsomes. This approach avoids stability artifacts and provides the most accurate binding estimates, offering a clear guide for standardizing microsomal binding assays.
