Download CELL MEMBRANES (Cassaret and Doull`s) Toxicants usually pass

CELL MEMBRANES
(Cassaret and Doull’s)
Toxicants usually pass through a number of cells, such as the stratified epithelium of the skin,
the thin cell layers of the lungs or the gastrointestinal (GI) tract, capillary endothelium, and
ultimately the cells of the target organ. The plasma membranes surrounding all these cells are
remarkably similar. The basic unit of the cell membrane is a phospholipid bilayer composed
primarily of phosphatidylcholine and phosphatidylethanolamine. Phospholipids are amphiphilic,
consisting of a hydrophilic polar head and a hydrophobic lipid tail. In membranes, polar head
groups are oriented toward the outer and inner surfaces of the membrane, whereas the
hydrophobic tails are oriented inward and face each other to form a continuous hydrophobic
inner space. The thickness of the cell membrane is about 7–9 nm. Numerous proteins are
inserted or embedded in the bilayer, and some transmembrane proteins traverse the entire
lipid bilayer, functioning as important biological receptors or allowing the formation of aqueous
pores and ion channels (Fig. 5-2). Some cell membranes (eukaryotic) have an outer coat or
glycocalyx consisting of glycoproteins and glycolipids. The fatty acids of the membrane do not
have a rigid crystalline structure but are semifluid at physiologic temperatures. The fluid
character of membranes is determined largely by the structure and relative abundance of
unsaturated fatty acids. The more unsaturated fatty acids the membranes contain, the more
fluid-like they are, facilitating more rapid active or passive transport.
Toxicants cross membranes either by passive processes in which the cell expends no energy or
by mechanisms in which the cell provides energy to translocate the toxicant across its
membrane.
Passive Transport
Simple Diffusion Most toxicants cross membranes by simple diffusion, following the principles of
Fick’s law which establishes that chemicals traverse from regions of higher concentration to
regions of lower concentration without any energy expenditure. Small hydrophilic molecules (up
to about 600 Da) permeate membranes through aqueous pores (Benz et al., 1980), in a process
termed paracellular diffusion, whereas hydrophobic molecules diffuse across the lipid domain of
membranes (transcellular diffusion). The smaller a hydrophilic molecule is, the more readily it
traverses membranes by simple diffusion through aqueous pores. Consequently, a small, watersoluble compound such as ethanol is rapidly absorbed into the blood from the GI tract and is
distributed just as rapidly throughout the body by simple diffusion from blood into all tissues.
Many toxicants are larger organic molecules with differing degrees of lipid solubility. For such
compounds, the rate of transport across membranes correlates with lipid solubility, which is
frequently expressed as octanol/water partition coefficients of the uncharged molecules, or log
P as listed in Table 5-1. The log P is an extremely informative physicochemical parameter
relative to assessing potential membrane permeability, with positive values associated with high
lipid solubility. Thus, the amino acids such as glycine are water soluble and have a negative log
P, whereas the environmental contaminants DDT and TCDD are very lipid soluble and have a
high, positive log P. Many chemicals are weak organic acids or bases which in solution are
ionized according to Arrhenius’ theory. The ionized form usually has low lipid solubility and thus
does not permeate readily through the lipid domain of a membrane. There may be some
transport of organic anions and cations (depending on their molecular weight) through the
aqueous pores, but this is a slow and inefficient process. In contrast, the nonionized form of
weak organic acids and bases is to some extent lipid soluble, resulting in diffusion across the
lipid domain of a membrane. The rate of transport of the nonionized form is proportional to its
lipid solubility. The molar ratio of ionized to nonionized molecules of a weak organic acid or
base in solution depends on the ionization constant. The ionization constant provides a
measure for the weakness of organic acids and bases. The pH at which a weak organic acid or
base is 50% ionized is called its pKa or pKb. Like pH, both pKa and pKb are defined as the
negative logarithm of the ionization constant of a weak organic acid or base.
With the equation pKa = 14 − pKb, pKa can also be calculated for weak organic bases. An
organic acid with a low pKa is relatively a strong acid, and one with a high pKa is a weak acid.
The opposite is true for bases. The numerical value of pKa does not indicate whether a
chemical is an organic acid or a base. Knowledge of the chemical structure is required to
distinguish between organic acids and bases.
The degree of ionization of a chemical depends on its pKa and on the pH of the solution. The
relationship between pKa and pH is described by the Henderson–Hasselbalch equations.
The effect of pH on the degree of ionization of an organic acid (benzoic acid) and an organic
base (aniline) is shown in Fig. 5-3. According to the Brönsted–Lowry acid–base theory, an acid
is a proton (H+) donor and a base is a proton acceptor. Thus, the ionized and nonionized forms
of an organic acid represent an acid–base pair, with the nonionized moiety being the acid and
the ionized moiety being the base. At a low pH, a weak organic acid such as benzoic acid is
largely nonionized. At pH 4, exactly 50% of benzoic acid is ionized and 50% is nonionized
because this is the pKa of the compound. As the pH increases, more and more protons are
neutralized by hydroxyl groups, and benzoic acid continues to dissociate until almost all of it is
in the ionized form. For an organic base such as aniline, the inverse is true. At a low pH, when
protons are abundant, almost all of aniline is protonated, i.e., ionized. This form of aniline is an
acid because it can donate protons. As the pH increases, ions from aniline continue to
dissociate until almost all the aniline is in the nonionized form, which is the aniline base. As
transmembrane passage is largely restricted to the nonionized form, benzoic acid is more
readily translocated through a membrane from an acidic environment, whereas more aniline is
transferred from an alkaline environment.
Filtration:
When water flows in bulk across a porous membrane, any solute small enough to pass through
the pores flows with it. Passage through these channels is called filtration, as it involves bulk
flow of water caused by hydrostatic or osmotic force. One of the main differences between
various membranes is the size of these channels. In renal glomeruli, a primary site of filtration,
these pores are relatively large (about 70 nm) allowing molecules smaller than albumin
(approximately 60 kDa) to pass through. In contrast, there are no aqueous pores at cellular tight
junctions, and channels in most cells are much smaller (3–6 Armstrongs), thereby only
permitting substantial passage of molecules with molecular weights of no more than a few
hundred daltons (Schanker, 1962; Lin, 2006).
Special Transport
There are numerous compounds whose movement across membranes cannot be explained by
simple diffusion or filtration. Some compounds are too large to pass through aqueous pores or
too insoluble in lipids to diffuse across the lipid domains of membranes. Nevertheless, they are
often transported very rapidly across membranes, even against concentration gradients. To
explain these phenomena, specialized transport systems have been identified. These systems
are responsible for the transport (both influx and efflux) across cell membranes of many
nutrients, such as sugars and amino and nucleic acids, along with some foreign compounds.
Based on the sequencing of the human genome, there are at least 500 genes whose putative
function involves membrane transport (Venter et al., 2001). However, not all of these genes
contribute to the disposition of toxicants. Throughout this chapter, transporters known to
contribute to the disposition and subsequent effects of xenobiotics will be emphasized.
Importantly, the role of xenobiotic transporters in chemical disposition is an emerging research
field, and new information regarding their function, molecular regulation, and genetic
polymorphisms is likely to modify traditional concepts in toxicology. Active Transport Active
transport is characterized by: (1) movement of chemicals against electrochemical or
concentration gradients, (2) saturability at high substrate concentrations, (3) selectivity for
certain structural features of chemicals, (4) competitive inhibition by chemical cogeners or
compounds that are carried by the same transporter, and (5) requirement for expenditure of
energy, so that metabolic inhibitors block the transport process. Substances actively transported
across cell membranes presumably form a complex with a membrane-bound macromolecular
carrier on one side of the membrane. The complex subsequently traverses to the other side of
the membrane, where the substance is released. Afterward, the carrier returns to the original
surface to repeat the transport cycle.
Facilitated Diffusion
Facilitated diffusion applies to carriermediated transport that exhibits the properties of active
transport except that the substrate is not moved against an electrochemical or concentration
gradient, and the transport process does not require the input of energy; that is, metabolic
poisons do not interfere with this transport. The transport of glucose from the GI tract across the
basolateral membrane of the intestinal epithelium, from plasma into red blood cells, and from
blood into the central nervous system (CNS) occurs by facilitated diffusion. As noted earlier,
OCTs (Oct-mediated transport is the electrochemical gradient of the transported catión), which
function in the uptake of organic cations particularly in the liver and kidney, mediate cation
movement by facilitated diffusion.
10.2.2 HENDERSON–HASSELBACH EQUATION
(Clinical Toxicology 2005 pp 90- 93)
The relationship between the ionization of a weakly acidic drug, its pK a, and the pH of the
solution is given by the Henderson-Hasselbach equation. The equation allows for the prediction
of the nonionic and ionic state of a compound at a given pH. For acids and bases, the formulas
are:
The equations are derived from the logarithmic expression of the dissociation constant formula
above. Small changes in pH near the pKa of a weakly acidic or basic
drug markedly affect its degree of ionization. This is more clearly shown with rearrangement of
the Henderson-Hasselbach equations, such that:
For an acidic compound present in the stomach (e.g., pKa = 4, in an average pH = 2), inserting
the numbers into the Henderson-Hasselbach equation results in a relative ratio of nonionized to
ionized species of 100:1. This transforms the compound predominantly to the nonionic form
within the acidic environment of the stomach, rendering it more lipophilic. Lipophilic compounds
have a greater tendency for absorption within that compartment. *
In the proximal small intestine areas of the duodenum and jejunum, where the pH is
approximately 8, the same compound will be predominantly in the ionized state. The relative
ratio of nonionized to ionized species is reversed (1:104). Thus, within the weakly basic
environment of the proximal intestine, a strongly acidic drug is less lipophilic, more ionized, and
slower to be absorbed.
Conversely, for a strong basic compound with a pKa = 4 (pKb =10) in the stomach, the
Henderson-Hasselbach equation predicts that the ratio of ionized to nonionized species equals
100:1. Thus, a basic compound is more ionized, less lipophilic, and slower to be absorbed in the
stomach. In the basic environment of the proximal intestine (pH = 8), however, the ionized to
nonionized species is 1:104, rendering it more lipophilic and imparting a greater propensity for
absorption.
The knowledge of the pKa of a therapeutic drug is useful in predicting its absorption in these
compartments. In addition, as explained below, this information is helpful in determining
distribution and elimination functions. Table 10.1 and Table 10.2 summarize the chemical
properties and behavior of strong acidic and basic drugs, such as aspirin and amphetamine
hydrochloride, respectively. Based on the extremely acidic environment of an empty stomach,
such compounds are either completely nonionized and highly lipophilic or ionized and
hydrophilic. In the basic environment of the small intestine, although some ionization is present,
amphetamine absorption is favored over that of aspirin. Conversely, the behavior of weakly
basic or acidic drugs, such as morphine sulfate and sodium phenobarbital, respectively, can be
predicted depending on the pH of the environment (Table 10.3 and Table 10.4). It should be
noted that some of the pharmacokinetic principles that govern drug absorption are not always
significant factors in the determination of toxic effects. This is primarily due to the circumstances
surrounding toxic chemical exposure. Consequently, some of the circumstances that influence
drug absorption in a therapeutic setting will not be considered here. These factors include:
formulation and physical characteristics of the drug product; drug interactions; presence of food
in the intestinal tract; gastric emptying time; and concurrent presence of gastrointestinal
diseases. Individual categories of toxic substances and special circumstances that alter their
effects will be considered in their respective chapters.

It should be noted that the absorption rate of the stomach is limited and is secondary to its digestion
and churning functions.