Enviornmental Toxicology Chapter 3: absorbtion, Distribution and Excretion of Toxins.

BIOL 4316

How do you know if the toxixcity of a chemical is low?

Low rate of absorption, accumulates in organs other than the target, is bio transformed into less toxic metabolites, is rapidly eliminated.

Barriers Toxicants must pass through:

Skin, lungs, Alimentary canal

Target of Hg, Pb

CNS, Kidney, hemetopoietic organs.

Benzene target

Hemtopoetic organs

CCl4 (carbon terachloride) target

liver damage

Toxicants are eliminated by the body by:

Bio-transformation → liver, Excretion → kidney, lungs and biliary, Storage → fat

Thickness

number of layers and kinds of cells.

Eg: statified epithelium →skin, simple epithelium → lungs, endothelium → blood vessles.

Structure of plasma membrane

Thickness = 7nm, bilayer of non-ridgid phospolipids and cholesterol (fluid mosaic model), protiens embedded in lipids → serve as transport protiens, channels, receptors, enzymes and form structures.

Rate of transport of toxicangt is dependant on:

partition coefficient (PC) and concentration graident across PM

How do small hydrophobic molecules diffisue through PM?

aqueous channels

large organic molecules difuse through PM through:

hydropbobic domains

Ethanol absorbtion:

lipid soluble, easily absorbed through the stomach.

How do toxicants ina solution exist?

either as ionized or unionized

Why are polar molecules unablt tocross PM?

they are ionized/ionic

why do non-polar molecules rapidly cross PM?

they are non-ionized/non-ionic

Diffusion of toxicanyts depends on:

lipid solubiblty

Fick’s law

the movement (flux) of mass (molecules) due to diffusion is based on the concentration of the substances and the area that the substance has to pass through.

Acorrding to Fick’s law, rate of diffucion is dependent on:

concentration of gradient across PM, thickness of PM, diffucion constant of toxicants, molecular weight of toxicants, surface area of PM.

many toxicants are (related to pH):

Weak acids/bases.

what is the amount of weak organic acids or bases in a solution dependent on?

their disassociation contant (K).

HA (acid) → H+ (disassociated proton) + A- (conjugate base); process is reversable and dependent on pH.

Toxiants which go through PM with simple difusion

TCDD (2,3,7,8-Tetrachlorodibenzodioxin) and DTT (Dithiothreitol)

Toxicant which go through PM with mediated channel transport

Terodotoxin → neurotoxin found in pufferfish which can cause rapid weakening and paralysis of muscles, including those of the respiratory tract, which can lead to respiratory arrest and death.

Toxins which cross PM with carrier mediated transport

Fe (Iron), 5-flurouracil (5-FU), paraquat, α-amantin, Ca (calcium), Pb (lead).

Toxins which cross PM with active transport

Penicillin (β-lactam antibiotics)

pH

-log[H+]

pKa

pH at which 50% of acid/base has been disassociated

Henderson-Hasselbalch equation weak acids

log[non-ionized form]/[ionized form] = pKa - pH

Henderson-Hasselbalch equation weak base

log [ionized form]/[non-ionized form] = pKa-pH

Acids with low pKa; acids with high pKa

strong; weak

Bases with low pKa; bases with high pka

weak; strong

pH formula

pH = pKa + log [A-]/[HA]

pKa alone cannot say whether compund is acidic/basic.

What greatly influences penetration of toxicants across PM?

partition coefficent (PC)

PC is related to solubiltiy of toxicants in lipids. What is the formula for calculating PC?

PC = concentration of toxicant in lipid/concentration of water

PC relation to lipid solubility

postitive correlation; higher PC = higher lipid solubility

solvent commenly used for PC determination

Octanol because it best mimics phospolipids of the PM

other solvents used for PC determination

chlorofom, ether, and organic olive oil

Filtration

movement of molecules through membranes of region with high hydrostatic pressure to region with low hydrostatic pressure. Includes speeration of solis from fluids; generally applies only to capillary walls.

example of filtration

renal golmeruli where molecules of 60,000 Da pass through

Glumerular cappilary filtation barriers

Endothelium, basement membrane, epithelial cells. (order from inside to outside)

Endothelium glomerular cappilary filtration barrier

fenestrated ( has multiple tiny holes)

basement membrane glomerular cappilary filtration barrier

negatively charged restriction site for protiens

Epithelial cells glomerular cappilary filtration barrier

restriction site for [pprotiens too, chracterized by foot-like processes called podocytes.

Active trasnport example

Na+ transport

facilitated difusion example

glucose from GI → plasma →RBCs

example of phagocytosis and pinocytosis

removal of perticulate material by macrophages

3 routes of absorbtion of toxicants

absorbtion by GI tract, lungs, and/or skin.

why is GI route absorbtion (genrally aclled absorbtion) important for toxicalogists

suicidial situaltion and children exposure

Examples of toxicants which are absorbed by GI

Nitorglycerin (sublingual), rectal supositories, most entry of tocicants (oral)

what is GI absorption if toxicant dependant on?

ionic species of the toxicant (wheher ionized or unionized)

Stomach pH in relation to intestinal pH

Stomach pH = highly acidic ( pH = 2); intesinal pH = near neutral (pH = 6)

What can help detremine posibility of absorbtion of toxicants through GI?

pKa value

3 specific transport systems of mamalia Gi for absorbtion of substances

2-step absorbtion, active transport, facillitated difusion

2 step absorbtion example

Fe2+ which is rapid through mucosa and slow into blood

Active transport GI absorption example

very few substances n/a

Facilitated difusion GI absorbtion example

azo dyes

where does most absorbtion of aspirin occur? stomach or intenstine?

The pKa of aspirin = 3.5. So, most aspirin is mostly absorbed in the stomach

Steps to find if ratio favors absorbtion (with benezoic acid pH = 4 in stomach)

pka-pH = log10 [non-ionized]/[ionized]

4 (pH of toxicant) - 2 (pH of stomach) = log10 [non-ionized]/[ionized]

2 (difference of 2 from 4) = log10 [non-ionized]/[ionized]

10² (10 is put to power of 2 or diffence of pHs to cencel out log) = [non-ionized]/[ionized]

100 (10 squared equals to 100) = [non-ionized]/[ionized]

100/1 (100 is non-ionized and 1 is ionized) = [non-ionized]/[ionized]

Ratio favors absorbtion in Stomach!

Steps to find if ratio favors absorbtion (with benezoic acid pH = 4 in stomach)

pka-pH = log10 [non-ionized]/[ionized]

4 (pH of toxicant) - 6 (pH of small intestine) = log10 [non-ionized]/[ionized]

-2 (difference of 6 from 4) = log10 [non-ionized]/[ionized]

10^-2 (10 is put to power of -2 or diffence of pHs to cencel out log) = [non-ionized]/[ionized]

1/100 (10 negatively squared equals to 1/100) = [non-ionized]/[ionized]

1/100 (1 is non-ionized and 100 is ionized) = [non-ionized]/[ionized]

Ratio does not favor absorbtion in small intestine.

Restisatnce of toxicants to what things affect absorption through GI

pH, enzymes, mircoflora.

Eg: sname venome is least toxic through oral route but fatal through IV and DDT is converted to DDE (safer metabolite) when consumed orally by gut bacteria.

Chleating agents

Ethalene diamine tetracetic acid (EDTA) increased membrane permeability increasing absorbtion

How does GI mobility ag=fect absorbtion?

higher GI motility higher absorbtion

How do other metals affect other metal absorbtion?

Cadmium (Cd) decreases absorbtion of Zinc (Zn)

Zn decreases absorbtion of Copper (Cu)

How does old age affect absorbtion

old age decreases absorbtion

How does starvation affect absorbtion

starvation increases absorbtion

Toxicants absorbed by lungs are usually:

gases and vapors such as CO, NO2 and SO2

Gasses and vapors in the atmostpher are in ________ with blood.

direct equilibrium

If gas is more soluble in blood it has a high absorbtiion rate. What is the exception to this rule?

Chlorofom gas

where in lungs are particles 2-5 µM deposoted?

tracheo-brochiolar region where later they are cleared by mucosa

Where are particles 1µM or smaller in lungs?

these are penetrate into aveolar sacs where overal removal of particles from aveoli is inefficent.

where are particles 5-30µM deposoted in lungs?

NOT deposited in lungs instead they hand arround in nose, mouth and/or pharynx

What examples of chemicals enter through skin?

nerva gas like sarin and CCL4

what is the major mechnsism of difucion through skin?

percutaneous absorbtion

Skin absorbtion phase 1

epthelium

skin absorbtion phase 2

dermis

skin absorbtion phase 3

blood and internal fluids

What example of chemical agent increases rate of penetration of chemicals through skin?

Dimethyl sulferoxide (DMSO) is belived to remove fat d=from skin and increase absorbtion.

Disrtibution of toxicants is very rapid. What factors does distribution depend on?

extent of blood supply or an organ, rate of diffiution or speacial stransport, PC

the site of accumulation may not be the sute of action so:

the toxin is inert till it reaches its target.

total body water is divided in3 compartments

plasma water, intersitital water, intracellular water

volume of distribution (VD)

porportion of a chemical in the body that is found in plasma

How is VD determined

disolving known amount of dye in an unknown volume of water and then calculating for VD.

VD formula

VD = amountof dye added/ concentration in water

VD formula in vivo

amounto fchemical administered /concentration in plasma

Toxicants stored in tissues have what volume of diribution?

restricted

toxicants are often ______ in tissues

concntrated; some can achieve a high concemtration at the site of action like CO in the hemoglobin (Hb) and paraquat in the lungs.

Storage sites of toxicants in the bosy are called ______

storage deposists which are protective mechanisms. Concentration in storage deposits are in equilibrium with those in the plasma and the biological half life (t 1/2) of stored toxicants is very long.

What are the storage deposits of the body?

plasma protiens, liver, kidney, fat, and bone.

Albumin (plasma protien stoage depot)

binds to Ca2+, Cu2+, Zn2+, bilirubin, uric acid, penicillin, salicylates, histamine, and barbituates.

trasnferrin (Plasam protien storgae depot)

transfers Fe2+

Lipoprotiens (plasma protien storage depot)

binds to lipid soluble compounds

Are protien bound toxicants available for distribution?

No.

How can a substance perviously bound to a plasma protien be diplaced?

If another substance competed with binding to the plasma protien and displaces the previous substance.

Eg: Strong dose of sulfonamide (an antibiotic) can displace an antidiabetic drug which was bound b=perviously. When this happens the antidiabetic drug is able to be dirtbuted via the blood and can result in hypoglycemia.

Xenobiotic

a compound not originally in the human body. Can distplace normal physiological compounds bound to plasma protiens.

Examples: dieldrin, acrylonitrile

How does the liver and kidney work together as storage depots?

Both have high capacity to elimate toxins. The liver biotransforms toxicants while the kidney eliminates those toxins trhough the urine.

Ligandin

protien in the lievr cytoplasm; has high affinity for organic acids, carcinogens, and corticosteriods.

Metallothionein

protin on both the liver and kidney; binds to metal ions like Cd2+ and Zn2+. Also enables liver to store Pb2+.

Why is fat a great storage depot?

Many xenobiotics are fat solible (chloroform, DDT, and PCB) and alos are able to enter the fat through simple diffusion. A substance with high partition coefficent (PC) will accumulate in the fat. This is a reason why obese people are at risk for too much toxin storgage.

Eg: during WWII mnay solders were found to have DDT in thier fat because the fat was able to store DDT which had a high PC.

Bone as a storage depot

great for F, Pb, and Sr. Additionally, F- and OH- are redeily exchangeable so there is an equilibrium between the bone and blood.

Bone seeker

Ra2+, includes such metals which actively seeks the bone out and behaves like Ca2+. This makes it so that Ra2+ can actually be made into bone tissue which can cause severe bone diseases later on.

Blood brain barrier (BBB)

A selective semi-permeable membrane between the blood and the interstitium of the brain.