EXAM 5 - Phase 3 Elimination Flashcards
If compounds are hydrophilic/polar and trapped inside the cell…
Phase 3 transport proteins facilitate “efflux” (pump out)
Explain the reasoning behind Phase 3 elimination.
- lipophilic drugs/xenobiotics readily diffuse into the cells
- Phase 2 metabolism increases polarity of the compounds –> reduced ability to diffuse across lipid bilayer of the cell membrane (trapped in cell)
- polar conjugates must rely on transport processes to facilitate efflux from the cell (AKA Phase 3 elimination)
- Liver and kidney
Explain the significance of the liver and kidney in phase 3 elimination.
- hepatocyte: hepatobiliary transport - major cell in the liver (interface between the bile and the blood)
- kidney: tubular secretion - transporters transport chemicals between the blood and the urine through the renal proximal tubule cells (RPTs)
Explain the various ways chemicals move through the membrane.
Passive transport
* high to low concentration gradient
facilitated diffusion
* high to low concentration gradient w/ pore
* passive
active transport
* against the concentration gradient
primary active transport - ATP pushes the chemical against the gradient
secondary active transport - something else is used to drive the transport of the chemical
* symport - compound moves in the same direction as the chemical that diffuses across the pore.
* antiport - compound moves in opposite directions of the chemical its aided by
Identify the 2 broad classes of transport proteins.
ABC transporters: ATP-Binding Cassette transporters
* primary active transport
SLC transporters: solute carrier transporters
* facilitated diffusion
* secondary active transport (symport + antiport)
Describe ABC transporters.
- dimer
- each monomer has a transmembrane domain and an intracellular binding domain
- binding domain is always intracellular (bc thats where the ATP is)
- hydrolysis of ATP induces conformational change
Name some of the ABC transporter subfamilies that we will discuss.
ABCA - 12
* name: ABCA1
ABCB - 11
* name: MDR; ABCB1
ABCC - 13
* name: MRP; ABCC7
Decscribe ABCA (12) subfamily.
[ABCA1]
* contains some of the largest transporters
* ABCA1 is.a major regulator of cellular cholesterol and phospholipid homeostasis
Describe ABCB (11) subfamily.
[MDR; ABCB1]
* consists of 4 full and 7 half transporters
* multi-drug resistance protein
* ABCB1 is an ATP-dependent drug efflux pump for xenobiotic compounds w/ broad substrate specificity.
* responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs
Describe ABCC (13).
[MRP;ABCC7]
* 13 full transporters
* multidrug resistance-associated proteins
* used in ion transport, cell-surface receptors, toxin secretion
Explain the ligand export mechanism of ABC transporters
- ligand binds to intracellular side of the TMD
- ATP binding changes protein conformation
- ligand is secreted into extracellular space
- ATP hydrolysis
- ADP release restores protein to open dimer
Describe SLC (solute carrier) transporters.
- don’t require ATP
- can be either facilitated or secondary active transporters
- can either import or export
- over 300 transporters grouped into 51 families
Describe the pathway of chemicals in the liver.
- portal vein brings blood from the intestinal tract to the central vein –> chemicals are exposed to the hepatocytes
- hepatocytes have basolateral (next to blood) and canaliculi side (next to bile)
Describe transporter proteins in the liver based on the image.
- The chemical (yellow) can (1) passively diffuse into the hepatocyte or (2) move through the transported (purple) from the blood
- In the hepatocyte, Phase 1 reaction that puts on a hydroxyl group (blue circle)
- Phase 2 puts large hydrophilic molecule (red) –> polar
- now a substrate for phase 3 transporter –> (1) gets transported into the bile and excreted or (2) transported back into the blood
Explain the Phase 3 transporters that are responsible for hepatocyte efflux.
Efflux of phase 1+2 metabolites from the liver are dependent on ABC transporters
* MRP1,3,4,5,6 transporters - orient to the blood
* MDR1 (and MRP2) - orient towards the bile
Explain the Phase 3 transporters that are responsible for hepatocyte influx.
Influx driven by solute carrier transporters (SLC) (and also diffusion of lipophilic compounds)
Describe multidrug resistance-associated proteins (MRP, ABCC family).
Multidurg resistance-associated proteins (MRP) transport various conjugates of xenobiotics + unconjugated xenobiotics.
* MRP1 expressed in several tissues and basolateral membrane of hepatocytes
* MRP2 located on canalicular surface of hepatocytes - expressed in kidneys
* MRP3 is associated with basolateral membrane and other excretion organs.
* MRP4/5 are in hepatocytes and in extrahepatic tissues
Capable of transporting a variety of conjugates (including glutathione, glucuronic acid, and sulfate)
Explain p-glycoprotein group of ABC transporters.
- also expressed in intestinal epithelium, renal proximal tubular cells, and BBB
- MDR1 (ABCB1) - first identified (multi-drug resistance protein)
- MDR1 has very broad substrate specificity (can transport many chemicals)
- transports from liver into bile
Explain Bcrp - breast cancer resistant protein (ABCG2).
ATP binding cassette transporter in the liver
* transports from hepatocyte to bile
Explain Bsep - Bile salt export pump (ABCB11).
ATP binding cassette transporter in the liver
* transports from hepatocyte to bile
Describe the outcome of glucuronides depending on their molecular weight.
MW > ~400 –> bile
* through MRP2 and MDR1
MW < ~300 –> blood
* transported by other MRPs
Explain SLC examples.
SLC22 family - contains OCTs (organic cation transport) and OATs (organic anion transport)
* transporting chemicals from the blood into the liver cell
Describe renal proximal tubule (RPT) cells.
- line the proximal tubule
- Very important in regulating ion movement between the urine and blood
Explain the significance of ABC and SLC transporters in the kidney.
- blood surrounds the glomerulus
- urine is located inside the proximal tubule
- SLC transport the ions from the blood to the RPT cell (co-transporting a-KG –> antiport)
- ABC proteins transporting anions out of the cell into the urine
Describe organic anion transport in the kidney.
Endogenous anions
* prostaglandins
* folic acid
Xenobiotics
* NSAIDS
* Penicillans
important for regulating levels of anions in blood, urine, cells
Describe organic cation transport in the kidney.
Endogenous cations
* dopamine
* choline
Xenobiotics
* nicotine
* metformin
Explain the big picture of kidney tubular secretion.
- SLC proteins transporting from the blood to the RPT and back (possibly from urine)
- SLC proteins go both ways
- ABC transporters - from RPT cell into the urine
Draw the overall pathway of xenobiotic metabolism.
