Chapter 2 (exam 1) Flashcards
Absorption factors
Variables in Fick’s rate of diffusion
- polarity, ionization (Kow)
- surface area
- # and thickness of membranes
- Conc gradient (high - low)
DMSO
Dimethyl sulfoxide
- Moves across skin barrier rapidly
- Dissolve toxicant in DMSO for rapid dermal uptake
Partition Coefficient
measures polarity
- Kow = conc in the organic layer/conc in the water layer
- high Kow is highly nonpolar and crosses lipid membranes readily
- determine polarity based on structure
Ionization
Neutral compounds diffuse across membranes better than charged ions
-dependent on the pH of the environment
pKa and environment pH example
Aspirin pKa of -COOH = 3.49, stomach pH =2, GI pH =6
-uptake greater in stomach because at a lower pH HA will be dominant over A- and the more neutral compound will diffuse better over the nonpolar lipid membrane
Why do pharmaceuticals come with HCL
- Forms a salt, which is easier to transport and package
- The toxin has little effect on the pH of the environment
Concentration Gradient
- more probably that high conc molecules will hit the membrane and cross over
- based on random motion
Passive diffusion
gradient dependent
- probability of hitting the membrane
- conc in blood is very close to zero, fast motion removes the substance quickly
- “hitchhiker” uptake from Pb2+ looking like Ca2+
Active Transport
- burns ATP
- couple with nonspontaneous
- against the gradient
- use transport proteins and enzymes as carriers
Filtration
passive diffusion of polar/charged substance through small pores
Endocytosis/pinocytosis
particles into the cells by forming vesicle from membrane
-nanoparticles
GIT
epithelial cells continuous with exterior high surface area pH - stomach(2) intestine(6) blood(7.4) mostly passive diffusion
Lungs
- thin membrane
- facilitate exchange
- difference between upper and lower
Particle Size in Lungs
Large are filtered out by the URT
Small penetrate to the LRT
Gases and Aerosols in Lungs
- some particulate filtered out by URT
- permanent gases make it to the LRT
- PM2.5 - particle smaller than 2.5 um gets to LRT, less than 1 um gets to alveoli
- water soluble gases dissolve into nasal fluid
- low water solubility go to LRT
Reactivity in Lungs
- highly reactive effect the URT
- low reactivity effect the LRT
- Ozone - 30% into URT even though low water solubility. High reactivity
Brick and Mortar model of skin
Keratinocytes/corneocytes are polar
surrounded by nonpolar lipids
-Tortuosity describes the indirect pathway
-no active transport because cells are dead
Solvents through dermis
nonpolar increase fluidity of the lipids or remove it. Less barrier to move through
Perfusion
blood flow to an organ or tissue. More exposure to toxicant in the higher perfused tissue
high- brain, liver , kidneys
low- bone, adipose,
Depots
accumulation of a chemical in a specific tissue based on physical or chemical properties
- nonpolar compounds in fat
- Sr2+ and Pb2+ on bone
- albumin
Albumin
- blood plasma protein
- attracts nonpolar and ionic compounds to minimize the surface area exposed to water
- common for antibiotics
Deports (reversibly bound)
Follow LeChatelier’s principle
- when blood conc decreases, more is released
- lead in kids, removing them from the lead environment will cause it to be released from depots
- does not apply to irreversible binding
- graphs
Distribution barrier
Blood brain barrier
-blood vessels have smaller/fewer pores, more specific transporters, tighter junctions between cells
Blood placenta barrier
-not as selective as blood brain barrier
Metabolism Goal
- facilitate excretion
- increase aqueous solubility
Nonpolar risks
- increased uptake
- increased storage in depots and accumulation
- orange baby noses from accumulation of beta-keratine in carrots and sweet potatoes
Pro/Con to modifying chemical structure
pro-detoxification
-decrease receptor compatability
con - metabolite more toxic than parent compound
- highly reactive metabolite
Membrane thickness
GIT - 30um
Respiratory - 0.4 - 1.5 um
Skin - 100-200 um
injection - 0 um
Phase II metabolism (general)
increase solubility by adding small polar molecule (conjugation reaction)
- sugar, polypeptides add -OH groups and some proteins can be ionized
- often occurs at the site of phase I functional group
- reliably detoxifies
- creates large polar region to increase excretion
- relies on enzyme
Phase I metabolism (general)
generally oxidation that increases aq solubility by adding a polar functional group
- OH, -COOH (ionizable), R=O
- single functional group
- more reactive than phase II
Phase I enzymes
-cytochrome P450s
Flavin containing monooxygenases (FMO)
-Dehydrogenases
P450 general
- enzymes for phase I
- 2000+ vary by substrate specificity and organism
- found in portals of entry (skin, nasal mucosa, lungs, GIT, liver, kidney)
- metabolize xenobiotics and endogenous materials
P450 as microsomal enzyme
- “small bodies” organelles in a cell
- Primarily in smooth endoplasmic reticulum
- isolate by lysing cell and centrifuge
- -s9 fraction is the percentage of microsomes and cytosolic enzymes
P450 general reaction
RH + O2 + NADPH + H+ = ROH + H2O + NADP+
-NADPH to help the O2 oxidize C-H bond
P450 catalytic cycle
RH present at all times -Fe3+ from heme and NADPH e- = Fe2+ -Fe2+ and O2 = Fe3+O2- -NADPH e- = Fe3+[O2]2- - 2H+ leave as water = [Fe-O]3+ -Fe3+ + ROH (can all occur in vitro)
P450 example
benzo(a)pyrene is a polycyclic carbon
- product of incomplete combustion and cig smoke
- P450 produces an epoxide
- epoxide hydrolase produces diol
- P450 and diol make diol epoxide(toxic) that is not a substrate of P450
O-dealkylation
- P450 reaction
- chlorfenvinfos - organophosphate pesticide
- remove alkyl group and leave -OH
- as EN atoms, ONSP
Flavin-containing Monooxygenases (FMO)
- microsomal enzyme for phase I
- NADPH and O2 required
- goes between quinone(oxidized =O) and hydroquinone(reduced -OH)
- less reactive than P450
Alcohol dehydrogenase (ADH)
-in cytosol, of liver, kidney, lungs
-effective on primary and secondary(slower) alcohols
-Produces aldehydes and ketones, which are more toxic (activation)
RCH2OH + NAD+ = RCHO + NADH + H+
Fusel Oil
nonethanol alcohols
Aldehyde Dehydrogenase (ALDH)
- in cytosol and endoplasmic reticulum, same tissue as ADH
- product is less toxic carboxylic acid
Antabuse
- treats alcoholism
- inhibits ALDH
- aldehyde accumulation and causes illness like hangover without other effects of EtOH
- resembles -COO- and ALDH is occupied. Retains high affinity
Phase I hydrolysis
Catalyzed by carboxylesterases in tissues and cholinesterases in plasma
- RCOOR’ + H2O = RCOOH + R’OH
- Similar for S and N, replace carbonyl O, same enzyme but fastest with carboxyester
- know the arrow pushing
- nonzero rate without enzyme, can occur abiotically
Reductive Dehalogenation
RX = RH + HX
- decrease persistence of organochlorine/bromine
- C-Cl bonds are extremely stable
- reductive dehalogenation is extremely SLOW
- microbially mediated - anerobic (human gut, sediment under water)
PCBs
polychlorinated biphenyls
- used in electrical insulators
- persist for decades
- 209 congeners
- low Kow
Reductive dehalogenation conditions
anerobic
human gut
sediment-under water, lower rate of oxygen diffusion (consumption>supply)
Accumulation of organics in the sediment
PBDE
polybrominated diphenyl ethers
flame retardants
low Kow
similar to PCB in reductive dehalogenation process
Global Distillation
- organohalogens have a low volatility but it is not zero
- evaporate over time and circulate
- Deposit in arctic because air is cold
- high accumulation is polar bear
- high fat content in polar bear milk and gets passed to cubs
- native women are warned against breastfeeding
Glucuronidation facts
- glucuronosyl transferase puts glucuronic acid on a polar functional group (-OH, NH2, COOH)
- enzyme in microsomal fraction of liver, kidney, GIT
- products subject to elimination with polar waste or active transport into bile
Glucuronidation reaction
- glucuronic acid carrier by an energetic compound (uridine-5-diphosphoglucuronic acid (UDPGA)
- ROH + UDPGA = RO-GA + UDP
- reversed by abiotic acid hydrolysis
- beta-glucuronidase: enzyme produced by gut flora
- enterohepatic circulation (increase half life)
Sulfate conjugation
similar to glucuronidation
- ROH + PAPS + SULT = RO-SO3- + H+ + PAP
- PAPS - 3 phosphoadenosine-5 phosphosulfate
- SULT - sulfotransferase
- elimination with polar waste or active transport in kidney
Phase I to Phase II info
Phase I - powerful oxidation that produces reactive metabolites
Phase II - ready to works and is used immediately
- energized mechanism
- must work immediately because phase I metabolite is reactive
Gluthione conjugation
must have a reactive center on the substrate(electrophilic carbon)
gluthione is a nucleophile
Gluthione(GSH)
polypeptide
gamma-glutamyl-cysteinyl-glycine - high conc in liver, up to 10% of cellular protein, in cytoplasm
SH nucleophilic center
doesnt need enzyme
Rate increases with glutathione-S-transferase (GST)
Gluthione mechanism
RX + GSH = RSG
mercapturic acid is excreted
remove glutamate and glycine, then acetylate the N
Acetylation and Methyltransferases
Acetylation catalyzed by N-acetyltransferase
-adds CH3CO to N
Methyltransferases
-N, O and S methyltransferases
-Co-Substrate is S-adenosylmethionine
-HG2+ + MeHg+ = Me2Hg is nonpolar and accumulates lethal at extremely small doses
-methylate 3 amine to make 4+ amine and increase solubility
Ultra short lived metabolites
- bind to the enzyme that created them
- suicide substrate - piperonyl butoxide binds to P450 and decreases their activity, not toxic alone
- enzyme degradation
- new enzyme synthesis
Short Lived metabolites
-react close to where they are generated, dont make it beyond the cell or tissue
acetaminophen metabolites
- metabolism occurs by P450, sulfation, and gluceronidation
- P45O metabolite from acetaminophen is NAPQI which has toxic reaction with proteins and nucleic acids
- GSH conjugation metabolizes NAPQI
- Chronic EtOH abuse increases P450 activity and NAPQI increases, but GSH is limiting reagent and gets locally depleted
- liver cells die and release contents which can be toxic to other cells and kill them
Long lived metabolites
- transported systemically
- methanol CH3OH - CH2O - CHOOH (formic acid)
- formic acid travels to the eye and causes edema which causes blindness
Enzyme induction
increases enzyme production
- DNA-RNA-ribosome-proteins
- transcription-translation-synthesis
- inducers are often substrates for P450 and increase P450 production
- planar and nonpolar
- EtOH, benzo(a)pyrene, PCBs, PBDEs, dioxins
AhR
- Aryl hydrocarbon receptor
- cytosol
- complexes inducer
ARNT
- aryl hydrocarbon receptor nuclear translocator
- brings complex into the nucleus
XRE
xenobiotic response element (drug response element)
- on the P450 gene
- binding promotes transcription
- transcription leads to more P450s
Ramifications of Induction
- good - facilitates excretion of nonpolar contaminants that would accumulate
- bad - produces more reactive metabolites (diol epoxides)
Biomarkers
measurements that indicates an exposure or effect in an organism
EROD assay
ethoxyresorufin-O-deethylase assay
- measure P450 activity
- ethyoxyresorufin is not fluorescent, but resorufin is
- loe fluorescent levels are detectable advantage over UV-Vis
- dealkylation of ethyoxyresorufin to resorufin
Bioavailability
fraction of a total exposure that can be taken up by an organism
How to measure toxicant in environment
- EROD assay measures effect, not just exposure, P450s are active
- integrates exposure over time
- in vitro test with a liver sample
- Blood/fat samples tell amount but not effects
Renal elimination
- water soluble compounds
- Phase II metabolites
- -active transporters
- -anion/acid transporters (ionized carboxylic acids)
- -cation/base transporters (ionized amine bases)
Liver elimination
- bile acts as a surfactant (hydrophobic and hydrophilic sides)
- active transport can remove chemcials from blood into bile, inot intestine and feces
- compounds can be subject to enterohepatic circulation
Respiration elimination
- relevant for permanent gases and compounds with high vapor pressure
- EtOH, solvents
Milk
Human - 4% (same as cow)
Polar Bear - 30-35%
Human health hazard for persistant organic pollutants (POPs)
Non-mammalian pathways
- skin shedding or molting
- -birds and reptiles
- leaf fall and fruit production
