Optimization and Application of a Biotinylation Method for Quantification of Plasma Membrane Expression of Transporters in Cells

Quantitative proteomics, using LC-MS/MS, is increasingly used to quantify drug transporters present in tissues and cells. Most of these investigations quantify total transporter expression in the cells by utilizing a total membrane fraction, not only the plasma membrane. Here, we report development and optimization of a biotinylation method to quantify protein expression of transporters in the plasma membrane of cells. The Pierce cell surface isolation protocol was optimized for plasma membrane isolation. Incubation of OATP1B1-expressing CHO cells with 0.78 mg/mL of membrane impermeable biotinylation reagent (sulfo-NHS-SS-biotin) at 37°C for 1 h resulted in optimum isolation of the plasma membrane. Subsequently, the expression of transporters in the plasma membrane as a percent of the total was determined by quantitative proteomics using LC-MS/MS. Mean (±SD) plasma membrane expression of OATP1B1 in plated OATP1B1-expressing CHO, MDCKII, and HEK293 cells was found to be 79.7% (±4.7%), 67.7% (±12.2%), and 65.3% (±6.8%) of total cell OATP1B1 expression. Mean (±SD) plasma membrane expression of OATP1B3 in plated OATP1B3-expressing HEK293 cells, OATP2B1 in plated OATP2B1-expressing MDCKII cells, and sodium/taurocholate co-transporting polypeptide (NTCP) in plated NTCP-expressing CHO cells was 63.2% (±1.6%), 37.1% (±15.7%), and 71.7% (±1.2%), respectively. This method of quantifying transporter protein expression in the plasma membrane will be useful in the future to predict transporter-mediated drug disposition.