membranes [8].
FIGURE 3.1 Mate transporters across species and localization. (a) Phylogenic tree of MATE transporters in mammals according to three subclasses: class I (red), class II (black), and class III (blue).
Adapted from [22].
(b) Transepithelial transport by OCT and MATE proteins in liver (left) and kidneys (right).
3.3 TRANSPORT ACTIVITY
While MATE transporters are the final step in organic cation secretion in renal tubules and hepatocytes, most studies evaluating MATE activity and function have been performed in overexpressing cells that operate in an “uptake mode.” This is accomplished by creating a proton gradient that favors MATE uptake of organic cations from the external face of the plasma membrane. Evaluation of MATE transport in an uptake mode overcomes limitations with quantifying efflux from cells that may not be otherwise permeable to organic cation entry. The energetics and kinetics of transport by MATEs are considered similar while operating in either the uptake or efflux mode [25].
FIGURE 3.2 Examples of MATE inhibitors. Structures of chemical inhibitors were downloaded from public database. https://www.ebi.ac.uk/chebi/init.do.
3.3.1 Energetics of Transport
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.
3.3.2 Substrates
MATE proteins transport a wide array of chemicals (Tables 3.1 and 3.2). Medication substrates include antidiabetic drugs (metformin), antihistamines (cimetidine), antiarrhythmic drugs (procainamide), antivirals (acyclovir and ganciclovir), anticancer drugs (topotecan, imatinib), antibiotics (cephalexin) [12, 16, 27]. Endogenous molecules and nutrients transported by MATEs include creatinine, N 1‐methyladenosine, N‐methyl nicotinamide (NMN), guanidine, and thiamine. Notably, not all of these molecules are cations with cephalexin being a zwitterion [16, 27]. Mice lacking Mate1 have elevated accumulation of cephalexin in their kidneys confirming it is a Mate substrate [28]. Likewise, anions such as estrone sulfate, acyclovir, and ganciclovir can also be transported by rMate1 [27]. Thus, while MATEs are often considered the major carriers of organic cations in the liver and kidneys, they are able to transport a wider range of substrates.
While there is significant overlap in substrates between MATE1 and MATE2‐K, there are select compounds that are differentially transported by the two isoforms. For example, cephalexin and cephradine are transported by hMATE1, but not hMATE2 [27]. In addition, cimetidine has a greater affinity for rabbit Mate1 (denoted as rbMate1) compared with rbMate2‐K [20], whereas there is a high affinity for both hMATE1 and hMATE2‐K [29]. The opposite was observed for choline, which has greater affinity for rbMate2‐K [20].
TABLE 3.1 Prototypical and endogenous substrates of MATE transporters
| Substrates | Isoforms | Transport kinetics | References |
|---|---|---|---|
| 4‐(4‐(Dimethyl amino)styryl)‐N‐methylpyridinium iodide (ASP+) | hMATE1 | K m = 3.2 ± 1.8 μM | [129] |
| hMATE2‐K | K m = 5.4 ± 1.7 μM | [129] | |
| Dopamine | hMATE1 | K m = 0.56 ± 0.18 mM V max = 3.71 ± 0.15 nmol/mg protein/min | [51] |
| hMATE2‐K | K m = 2.48 ± 0.65 mM V max = 7.69 ± 1.12 nmol/mg protein/min | [51] | |
| mMate1 | K m = 0.53 ± 0.08 mM V max = 8.73 ± 0.08 nmol/mg protein/min | [51] | |
| Estrone sulfate | hMATE1 | K m = 0.47 ± 0.02 mM V max = 0.53 ± 0.06 nmol/mg protein/2 min | [27] |
| hMATE2‐K | K m = 0.85 ± 0.17 mM V max = 0.85 ± 0.14 nmol/mg protein/2 min | [27] | |
| Na‐methyladenosine | mMate1 | K m = 246 ± 14 μM V max = 76.7 ± 3.0 pmol/mg protein/min | [50] |
| hMATE2‐K | N.R. | [50] | |
| 1‐Methyl‐4‐phenylpyridinium (MPP+) | hMATE1 | K m = 0.10 ± 0.02 mM V max = 1.47 ± 0.13 nmol/mg protein/2 min | [27] |
| K m = 34.5 ± 12.9 μM | [59] | ||
|
hMATE2‐K
|