Several characteristics distinguish carrier-mediated transport from passive diffusion. Rates are generally faster than for passive diffusion, and transport is solute specific and shows a greater temperature variation (Q10). Transport is saturable, resembling Michaelis-Menten enzyme kinetics. Transport rates may not be the same in both directions across the membrane at a given substrate concentration. Transport may be inhibitable by competitive transport substrates or by noncompetitive inhibitors acting at other sites. Transport may be regulated by cell state (e.g., by phosphorylation, induction, or repression of transporter molecules) or by gene copy number. Transport is tissue specific because it depends on the expression of particular transporters that do not occur in all membranes. Active transport is a special form of carrier-mediated transport in which solute concentration is mechanistically linked to energetically favorable reactions (Equation 14.1). Distinction between primary pumps and secondary transporters may be made on the basis of cosubstrate dependence (e.g., oxida-tive substrate, adenosine triphosphate, or phosphe-nolpyruvate requirement) or of the effects of various ionophores, uncouplers, and inhibitors of primary pumps.
Mechanisms of drug transport in vivo have been better established in bacterial systems than in mammalian systems, owing to greater experimental control and ability to genetically manipulate properties of the bacterial systems. Table 14.3 lists examples of drugs for which the transport in bacteria is dominated by the indicated mechanisms (7).
TABLE 14.3 Transport Mechanisms in Bacteria"
Transport mechanism Example
Passive diffusion across lipid bilayer Fluoroquinolones
Tetracyclines (hydrophobic) Facilitated diffusion (nonselective)l b-Lactams
Tetracyclines (hydrophilic) Mediated transport (selective) Imipenem
Cycloserine a Adapted from Table 1 in Hancock REW. Bacterial transport as an import mechanism and target for antimicrobials. In: Georgopapadakou NH, ed. Drug transport in antimicrobial and anticancer chemotherapy. New York: Marcel Dekker, Inc.; 1995. p. 289-306.
The distinction between facilitated diffusion through channels and carrier-mediated transport is somewhat artificial, but may be justified on the basis of specificity. For example, b-lactams in general can pass through nonselective bacterial outer membrane porin (e.g., OmpF) channels via passive diffusion, whereas imipenem (and related zwitterionic carbapenems) can also utilize OprD channels, which preferentially recognize basic amino acids and dipeptides. The identification of mutants that selectively confer imipenem resistance suggests that more intimate protein-drug associations are involved in carrier-mediated transport than in facilitated diffusion, which may be limited only by pore diameter.
The tetracyclines provide an interesting example in that bacterial uptake is passive (by both non-mediated and carrier-mediated pathways), efflux is active, and their transport is subject to pH, membrane potential, and metal ion gradient effects (21). Tetracycline is both a weak base (pKa1 = 3.3) and a weak acid (pKa2 = 7.7, pKa3 = 9.7) and is subject to pH trapping. Furthermore, the anions can chelate divalent cations such as magnesium, forming metal chelates that have altered solubility. Uptake across the outer membrane of gram-negative bacteria is nonmediated for hydrophobic tetracyclines and carrier mediated via porins (e.g., OmpF) for hydrophilic homologs. Nonmediated diffusion via the lipopolysac-charide depends on the uncharged species, whereas carrier-mediated diffusion via the porins favors the magnesium-bound anion (net positive charge) and is enhanced by the Donnan membrane potential. In contrast to most mammalian membranes, passive diffusion across the lipopolysaccharide outer membrane of Escherichia coli is slower for more hydrophobic analogs and may account for their lower antimicrobial activity. Uptake across the cytoplasmic membrane is by nonmediated passive diffusion of the neutral species, and is thermodynamically driven by the pH gradient across the inner membrane (pH 7.8 inside, pH 6.1 outside, for cells grown at a nominal pH 7.0). On the other hand, efflux of tetracycline is due to active transport via TetA, which catalyzes antiport of the [Mg-anion chelate]1+ (out) in exchange for a proton (in).
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