The equilibrium and kinetic drug binding properties of the mouse P-gp1a and P-gp1b P-glycoproteins are similar

Abstract

SummaryThe gene encoding the multidrug resistance P-glycoprotein (P-gp) is duplicated in rodent species and the functional basis for this remains unresolved. Despite a high sequence similarity, the mouse P-gp1a and P-gp1b isoforms show distinct patterns of tissue distribution which suggest a specific role of the P-gp1b isoform in steroid transport. In the present study possible biochemical differences between the isoforms were directly investigated at the level of drug interaction. There was no detectable difference in the affinity or binding capacity of the two isoforms towards [ 3 H]vinblastine at equilibrium. Similarly, the rate at which [ 3 H]vinblastine associates with P-gp was indistinguishable between the two isoforms. Some modest differences were observed in the relative abilities of the multidrug-resistant (MDR) reversing agents CP100-356, nicardipine and verapamil to displace equilibrium [ 3 H]vinblastine binding to P-gp1a and P-gp1b. The steroid hormone progesterone displayed a low affinity (K i = 1.2 ± 0.2 µM for P-gp1a and 3.5 ± 0.5 µM for P-gp1b), suggesting an unlikely role as a physiological substrate. Thus the mouse isoforms do not appear to exhibit functional differences at the level of initial substrate interaction with protein.

Sertraline Alters Multidrug Resistance Phosphoglycoprotein Activity in the Mouse Placenta and Fetal Blood–Brain Barrier

“…[40][41][42][43] However, these earlier studies used indirect tests to assess substrate specificity and their significance has been questioned by more recent studies, which have reported no difference in the affinity of the 2 isoforms toward vinblastine. 38 It was concluded in these more recent studies that if any difference in transport exists between the 2 P-gp isoforms, it must occur at a step subsequent to binding, such as the translocation step or linkage between drug binding and ATP hydrolysis. 38 It is possible that sertraline can differentially affect the expression of Abcb1a and Abcb1b in the placenta and BBB.…”

Stereoselective transport of hydrophilic quaternary drugs by human MDR1 and rat Mdr1b P‐glycoproteins

“…Despite their high sequence similarity (80% at amino acid level), the observation that functional dierences exist is not unexpected. It has been shown that the two murine drug transporting isoforms, Mdr1a and Mdr1b, display overlapping, but distinct drug resistance phenotypes (Tang-Wai et al, 1995), and transport [ 3 H]vinblastine with dierent eciencies (Yang et al, 1990), in spite of their equal anity and binding capacity towards this substrate (Taylor et al, 1999). Moreover, as summarized by Ambudkar et al (1999), studies with mutated Pgps revealed that a change of only one amino acid may already dramatically aect the initial rate of drug uptake in accumulation assays with transfected cells.…”

Expression of mdr isoforms in mice during estrous cycle and under hormone stimulation

“…mdr3 is referred to as the most effective isoform in detoxification, and mdr1 as the isoform preferably associated with the transport of steroid hormones (Yang et al, 1989;Gottesman and Pastan, 1993). Interestingly, Taylor et al (1999) found no significant differences between the mdr3 and mdr1 isoforms in the nature of drug-binding sites and suggested that the presence of multiple isoforms of Pgp alMdr expression in estrous cycle 759 Mice which are homozygous for a disruption of mdr1 or mdr3 are apparently healthy (Borst et al, 1993). In 1997, Schinkel et al obtained mice which, although homozygously deficient for the mdr1 and mdr3 genes combined, were healthy and fertile.…”