1. HLTH 340 Lecture A3 Toxicokinetic processes: absorption (part-2) carrier-mediated transport
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2. Carrier-mediated absorption using membrane transport channels
large glycoprotein molecules embedded in the phospholipid membrane
act as channels (pores) that allow specific hydrophilic solutes (e.g. metal ions) to cross membrane barriers
enables transcellular absorption of many hydrophilic ions and polar molecules
2 types of membrane transporter channels
active transport channels
facilitated diffusion channels
active transport
increases rate of absorption
can concentrate (or remove) substance in tissues against a concentration gradient of a given solute (non-equilibrium; pumps solutes ‘uphill’)
requires energy source (e.g. ATP or other source of chemical energy)
facilitated diffusion (passive transport)
does not concentrate or eliminate substance in tissues (equilibrium only; no pumping)
no energy source required

3. Types of transcellular membrane transport

4. Absorption of inorganic ions through membrane ion transport channels
each type of channel has a selective preference for transporting certain ions
specific solutes are preferred, but not absolutely specific
factors determining ion transporter specificity include
• ionic radius(size)
• positive or negative charge
• valency of the ionic charge
• class of metal ion (alkaline metals, transition metals, heavy metals)
size and ionic charge determines selective transport thru ion channels
cation transport channels (K+, Na+, Ca++, etc.)
transport positively charged ions (e.g. metals)
monovalent (+1), divalent (+2), trivalent (+3), etc.
anion transport channels (Cl-, I- , ClO3- etc.)
transport negatively charged ions
monovalent (-1), divalent (-2), trivalent (-3), etc.
factors affecting rate of transport
affinity (high-affinity ==> rapid transport) <-- determined mainly by ionic charge and the effective ionic radius
saturation (rate-limiting factor for absorption speed)
competition (2 substances compete for the same transporter channel)
regulation (up-regulation or down-regulation by other physiological factors)

5. Metal speciation
metals can exist in different physico-chemical states (species)
elemental (metallic)
oxidized (metal oxides and salts)
inorganic or organic compounds
metal speciation
inorganic metals can have several different oxidation states
elemental (metallic) species, Meo
neutral, no electric charge
solid metal (e.g. Pbo) or metal vapor (e.g. Hgo )
oxidation states: loss of electron (LEO) is chemical oxidation
Me+ loss of 1 electron is valence state (I)
Me++ loss of 2 electrons is valence state (II)
Me+++ loss of 3 electrons is valence state (II)
different species of the same metal can vary widely in toxicity
example: chromium: Cr-III (chromium three) -- oxidation--> Cr-VI (chromium six) low toxicity high toxicity
organometallic compounds
metal atom covalent linked to organic (carbon chain) group(s)
frequently very toxic
lipophilic organometal compounds are especially hazardous
example: mercury
mercuric ion (Hg2+) --> methyl mercury (organic)
low toxicity high toxicity

6. Metal ions - ‘heavy metal’ ions compete for membrane transport via ionic mimicry
precious metals (toxic, but rare)
essential metals
trivalent metals
(too ionic or insoluble to be absorbed)
toxic metals
heavy metals (ionic mimicry)
transition metals (absorbed via transport channels)
alkali metals (absorbed via transport channels) 6

7. Intestinal calcium transport channels: calcium (Ca++) uptake competes with lead (Pb++)
intestinal epithelium contains selective ion transport channels for different divalent cations (Ca++, Fe++, Zn++)
each serves as a transcellular transport channel of divalent cations from the intestinal lumen into the bloodstream
example: calcium (Ca++ )
calcium is an essential metal nutrient
paracellular uptake by passive diffusion is sufficient at high dietary calcium levels(strong conc gradient)
transcellular carrier-mediated transport is required for low dietary concentrations of calcium
Ca++ uptake requires transcellular transport via a selective divalent cation calcium channel
requires ATP energy to run the active transport system (pump)
selective calcium absorption occurs from many foods (milk, dairy products, vegetables, fruits, fish)
TRPV6 (ECaC2) calcium channel
Ca++ is absorbed from the intestine to blood using indirect active transport via the TRPV6 calcium channel
epithelial calcium channel type 2 (ECaC2; now termed TRPV6)
carrier-mediated uptake by TRPV6 is up-regulated by active vitamin D 1, 25-(OH)2 D3
• sunlight / UV synthesis / melanization • vitamin D supplements in milk and other diary products
also sustained by estrogen hormones (up-regulation)** **ocurs only in females
lead (Pb++ )
lead exists as a divalent cation (Pb++) -- the ion structure of lead mimics calcium ion (ionic mimicry)
Pb++ is absorbed from the intestine into the blood using same Ca++ transport channel (TRPV6 )
saturation-competition occurs with calcium versus lead for saturable TRPV6 channels in intestine 7

8. Carrier-mediated transport is a saturable process that has a limited capacity to move solutes
paracellular intestinal
calcium absorption
at high Ca++ conc (passive diffusion)
TRPV6 transport channel becomes saturated at high Ca++ conc
TRPV6 intestinal
calcium absorption
at low Ca++ conc (carrier-mediated) 8

9. Calcium ion transport channel embedded in epithelial cell membrane (TRPV6)
Pb2+ 9

10. Organized water - ionic groups and polar groups
effective ionic radius - ionic molecules and metal ions form electrostatic bonds with the surrounding water molecules, so that the ions actually have a much larger sphere of hydration than the notional size of the unhydrated metal ion -- the radius of the sphere of hydration determines the degree of penetration of the metal ion through a specific carrier-mediated transport channel
hydrogen bonding - in hydrophilic polar molecules, the hydrogen in one or more functional groups (e.g. -OH) has a slight positive charge and is therefore attracted by the partly negatively charged oxygen of a nearby water molecule, with the result that a hydrogen bond is formed.
The dissolved (solute) molecules and the water molecules thus become linked to one another. When non-ionic polar molecules form hydrogen bonds with the surrounding water molecules, the polar molecules are ‘glued’ to the water solvent layer -- this organized water layer must be stripped away before the solute molecule can penetrate through a narrow carrier-mediated transport channel 10

11. Ion transport channels - the effective ionic radius of hydration sphere for divalent cations
effective ionic radius - for ionic radius of metal divalent cations, the smaller ions have higher ionic potentials and form stronger bonds with water molecules, so that smaller ions actually have a larger sphere of hydration when entering into a specific ion transport channel 11

12. Structure of the TRPV6 (ECaC2) tetrameric calcium channel using molecular modeling
IG. 10. Structure of the TRPV5/6 pore. A: alignment of the pore regions of TRPV4, -5, and -6 and the potassium channel KcsA (accession numbers AAG17543 for TRPV4, CAB40138 for TRPV5, AAD47636 for TRPV6, and S60172 for KcsA). The boxed sequences represent the putative selectivity of all channels. The amino-terminal part of this pore structure likely represents a putative pore helix. Band C: model for the TRPV6 pore region, based on the KcsA structure. Views of the structure are shown, looking sideways at two opposite subunits ( left ) or looking down from the external solution to the complete homotetrameric channel ( right ). At the narrowest point, formed by the acidic side chain of Asp-541 (orange), the pore has a diameter of Blue residues correspond to the residues in TM5 and TM6 (TM1 and TM2 in KcsA), whose accessibility was not tested in this study. Amino acids that were subjected to SCAM analysis (residues P526 to D547) are colored in green, yellow, red, or gray: residues in red reacted rapidly to Ag +(reaction rate >5 x10 6M–1 · s –1 ), residues in yellow reacted with Ag +at a rate <5 x10 6M–1 · s –1 , and residues in green did not show significant reactivity to Ag +. Residues where cysteine substitution resulted in nonfunctional channels are marked in gray. The figure was prepared using DS ViewerPro 5.0 (Accelrys), based on the coordinates of KcsA. 12

13. ATP-dependent channel
Cation transport channel (TRPV6) for calcium ion: absorption in the intestinal epithelium
antiporter channel
Pb++
ATP-dependent channel
FIG. 1. Mechanism of epithelial Ca 2+ transport. Epithelia can absorb Ca 2+ by paracellular and transcellular transport. Passive and paracellular Ca 2+ transport takes place across the tight junctions and is driven by the electrochemical gradient for Ca 2+ (blue arrow). The active form of vitamin D, 1,25-dihydroxyvitamin D 3[1,25-(OH) 2D3], stimulates the individual steps of transcellular Ca 2+ transport by increasing the expression levels of the luminal Ca 2+ channels, calbindins, and the extrusion systems. Active and transcellular Ca 2+ transport is carried out as a three-step process. Following entry of Ca 2+ through the (hetero)tetrameric epithelial Ca 2+ channels, TRPV5 and TRPV6, Ca 2+ bound to calbindin diffuses to the basolateral membrane. At the basolateral membrane, Ca 2+ is extruded via a ATP-dependent Ca 2+ -ATPase (PMCA1b) and a Na +/Ca 2+ exchanger (NCX1). In this way, there is net Ca 2+ absorption from the luminal space to the extracellular compartment 13

14. Absorption pathways for lead permeation from the intestinal lumen through the enterocytes
TRPV6
(Ca++) ??
DMT-1 (Fe++)
transcellular
FIG. 1. Mechanism of epithelial Ca 2+ transport. Epithelia can absorb Ca 2+ by paracellular and transcellular transport. Passive and paracellular Ca 2+ transport takes place across the tight junctions and is driven by the electrochemical gradient for Ca 2+ (blue arrow). The active form of vitamin D, 1,25-dihydroxyvitamin D 3[1,25-(OH) 2D3], stimulates the individual steps of transcellular Ca 2+ transport by increasing the expression levels of the luminal Ca 2+ channels, calbindins, and the extrusion systems. Active and transcellular Ca 2+ transport is carried out as a three-step process. Following entry of Ca 2+ through the (hetero)tetrameric epithelial Ca 2+ channels, TRPV5 and TRPV6, Ca 2+ bound to calbindin diffuses to the basolateral membrane. At the basolateral membrane, Ca 2+ is extruded via a ATP-dependent Ca 2+ -ATPase (PMCA1b) and a Na +/Ca 2+ exchanger (NCX1). In this way, there is net Ca 2+ absorption from the luminal space to the extracellular compartment
ZIP-1 (Zn++)
paracellular 14

15. Lead (Pb): a heavy metal toxicant that is a common environmental health hazard
4 major toxic heavy metals are of common concern in environmental health
lead
mercury
cadmium
arsenic
major environmental sources of lead
leaded paints (lead oxide PbO) -- houses built before 1978
auto exhaust in soils (PbCO3 from leaded gasoline) -- leaded gasoline additive (tetraethyl lead) until 1976
pica in young children (PbO in house dust, paint chips)
lead contamination dust (industrial and lead smelter neighborhoods)
lead drinking water pipes in older neighborhoods/houses
chronic exposure to very low levels of lead (~10 ug/dL??) can cause permanent neurologic damage in children
IQ reduction and subtle mental retardation
hyperactivity and behavioral disorders (maybe??)
prevention of chronic lead poisoning -- (1) infants (2) pregnant women (3) elderly
calcium supplements (BEWARE!)
diet with dairy products, high protein intake, adequate vitamin D
estrogen supplements in post-menopausal women??

16. Pathological features of lead poisoning in adults and children
In the adult, the brain and central nervous system are relatively less vulnerable to the toxic effects of lead because the adult brain is biologically matured and does not undergo developmental retardation as is the case for infants and young children.
Therefore many adults may experience other health effects such as chronic anemia, hypertension, or kidney malfunction.
In children the main effect is neurological (impaired brain development). 16

17. Speciation of lead (Pb) in the environment

18. Causal pathways of lead exposure and health effects - DPSEEA framework
S “stressor” E “exposure”
E “effect” A 18

19. Lead pipe replacement programs can lead to elevated lead in home drinking water supplies

20. Effects of lead poisoning in children related to blood lead levels (ug/DL)20

21. Potential effect of chronic lead poisoning on the intellectual development of young children21

22. Effect of ethnicity (race) on percentage of children with blood lead conc > 10 ug/dL

23. Relationship between population poverty and % living in high-risk housing for lead exposure23

24. Lead exposure patterns in neighborhoods close or distant to arterial roads24

25. regulatory lead levels
Effect of phase-out of leaded gasoline in the 1970’s (United States)
regulatory lead levels
(ug/dL in blood)
observed lead levels
< 2 ug/dL? 25