Читать книгу Drug Transporters - Группа авторов - Страница 81

Tetraethylammonium (TEA) and 1‐methyl‐4‐phenylpyridinium (MPP) are routinely used as substrates of MATE in vitro and in preclinical studies. Heterologous expression of MATE transporters in cells has revealed that the uptake of TEA, metformin, MPP, cimetidine, and procainamide is saturable and dependent upon pH [5, 6]. MATE1 utilizes an outwardly directed H⁺ gradient to enable antiport uptake of ¹⁴C [TEA]. Notably, lower extracellular pH (~6.0) reduces activity of hMATE1, mMate1, and rMate1, and maximal activity is observed between pH values of 8.0 and 8.5 [5, 15, 16]. Similarly, extracellular pH between 6.0 and 9.0 increases TEA uptake by hMATE2‐K and rbMate2‐K [6, 20]. Further analysis confirmed that the H⁺ gradient, and not just the environmental pH, is the driving force required for rMate1 activity [26]. Using membrane vesicles isolated from rMate1‐expressing cells, it was demonstrated that a high intravesicular H⁺ concentration stimulated ¹⁴C‐TEA uptake that was not observed in HEK‐pcDNA control cells [26].

Using inhibitors that disrupt proton conduction and pH gradients, it was demonstrated that hMATE1 utilizes H⁺/TEA antiport exchange [5]. Incubation of mMate1‐expressing cells and rMate1‐expressing membrane vesicles with inhibitors of membrane depolarization (e.g., valinomycin) had no effect on transporter activity [15, 26]. Taken together, these findings suggested that the exchange of cations and protons did not involve a net flux of electric charge and was electroneutral [15]. This observation contrasted the inside‐negative membrane potential that drives the uptake of cations by OCT transporters.