MONOCARBOXYLATE TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 148

7.1. INTRODUCTION

7.1.1. SLC16 Gene Family: MCTs

The solute carrier 16 (SLC16) gene family is composed of 14 sequence-related iso-
forms, known as the monocarboxylate transporters (MCTs). 1 DNA and protein se-
quence analysis indicates that the members of the monocarboxylate transporter family
possess many similar sequence motifs, consistent with the structure of proteins em-
bedded in a lipid bilayer membrane. The topology predicted includes the presence
of 12 transmembrane domains, intracellular N- and C-termini, and a large intracel-
lular loop between transmembrane domains 6 and 7. The identification of the first
MCT came after the discovery of a mutant protein in Chinese hamster ovary cells
that preferentially transported mevalonate. 2 Subsequent cloning and expression of
the wild-type gene in a breast cancer cell line showed that pyruvate was the preferred
substrate. 3 Of the now 14 identified SLC16 gene family members, only MCT1 to
MCT4, MCT8, and TAT1 have been functionally characterized and their substrates
identified. Within the known MCT family members only MCT1-4 are known to
transport monocarboxylates such as lactate, pyruvate, butyrate, and the ketone bodies

-hydroxybutyrate and acetoacetate. 4 In contrast, MCT8, formerly XPCT, is known
to facilitate the proton-independent transport of thyroid hormones T3 and T4, 5 and
TAT1 is a transporter of neutral amino acids. MCT6 was recently shown to trans-
port the anion drug bumetanide in a pH- and membrane potential-sensitive manner,
although its physiological substrate was not identified. 6 Phylogenetic analysis reveals
that MCT1-4 have a high degree of sequence similarity (Figure 7.1). Therefore, it
is suggestive that MCT1-4 are the only MCTs likely to transport monocarboxylates.
Additionally, it is possible that due to their relatedness to TAT1 and MCT8, MCT9
and MCT14 may be involved in amino acid transport (Figure 7.1). The mechanism of
transport for MCT1-4 is electroneutral because a single proton is translocated across
the plasma membrane, along with each monocarboxylate anion. The process begins
with the binding of a proton followed by binding of the monocarboxylate anion. The
two substrates are then translocated across the plasma membrane and released in the
reverse order. 7
Through site-directed mutagenesis, many clues have been uncovered as to the
mechanism of monocarboxylate transport by rat MCT1. Several critical amino acids
have been identified that are important for both substrate recognition and transport
activity. Arginine at position 306 (R306) is conserved among almost all MCT family
members and is probably involved in binding of the monocarboxylate anion. 8 D302
and E369 are thought to be involved in H + binding. 8 It is hypothesized that the binding
of the monocarboxylate anion to R306 may cause the H + to move from D302 to E369.
The binding of the H + to E369 then causes a conformational change that results in
the release of the monocarboxylate anion on the opposite side of the membrane. 9
In addition to the residues that may interact physically with the substrates, there
are several other residues that are critical for transport activity. Mutation of R143,
G153, and F360 results in a loss of transport activity or inability to be inserted into
the plasma membrane. 9 The loss of MCT1 expression at the cell surface suggests