Increasing the Supply of Women Physicians in Rural Areas: Outcomes of a Medical School Rural Program

The membrane permeation characteristics of 59-deoxythymidine (59-ddThd) and 59-azido-59-deoxythymidine (59-N3-59-ddThd) were investigated in human erythrocytes, with an inhibitor-stop assay, at 20 degrees. Uptake of both nucleoside analogs occurred without metabolism, was nonconcentrative, and was partially inhibited by nucleosides or inhibitors of nucleoside transport at micromolar permeant concentrations. At higher permeant concentrations (greater than 1.0 mM), the influx rate of each analog was linearly dependent on concentration and insensitive to inhibition by nucleosides, inhibitors of nucleoside transport, and nucleobases. Kinetic analyses using nonlinear regression revealed that a saturable component of 59-ddThd influx (Km = 200 microM) was competitively inhibited by thymidine (dThd) (Ki = 86 microM) or 5-iodo-29-deoxyuridine (Ki = 84 microM). Similarly, a saturable component of 59-N3-59-ddThd influx (Km = 220 microM) was competitively inhibited by 2-chloroadenosine (Ki = 18 microM). The Ki values for these nucleoside inhibitors were similar to their reported Km values as permeants of the nucleoside transporter. Both 59-ddThd and 59-N3-59-ddThd competitively inhibited the influx of dThd (Km = 60 microM), with similar Ki values (150 and 200 microM, respectively). We conclude that these two 59-modified dThd analogs enter human erythrocytes both by nonfacilitated diffusion and by the nucleoside transporter. The absence of the 59-hydroxyl group of dThd (59-ddThd) resulted in a large increase in the octanol/buffer partition coefficient, in an ability to permeate human erythrocytes by nonfacilitated diffusion, and in a 3-fold diminished binding to the nucleoside transporter. The 59-azido group (59-N3-59-ddThd) resulted in an additional 1.4-fold increase in the octanol/buffer partition coefficient and in a 2-fold increase in the rate of nonfacilitated diffusion.