transporters

• what are transporters?

  are proteins localised within the membrane bilayer - ‘integral transmembrane proteins’ - embedded
• what is the function of transporters?

  translocation of substances across the plasma membrane
• name two types of transporters...

  • Solute Carrier (SLC) transporters
  • ATP-Binding Cassette (ABC) transporters
• What are the types of transport for molecules?

  • passive transport (simple and passive) - down a concentration gradient
  • active transport (ATP) - against a concentration gradient (may be a problem when treating cancer and delivering certain drugs)
• In the intestine, what is the use of enzymes and transporters?

  Aid absorption of nutrients. Large macromolecules (proteins, complex carbohydrates) can not be absorbed. So enzymes break down macromolecules into smaller, absorbable molecules.
• sucrose is broken down into what and by which enzyme?

  breaks into glucose and fructose via the enzyme sucrase
• lactose is broken down into what and by which enzyme?

  Glucose and galactose, by the enzyme lactase.
• maltose is broken down into what and by which enzyme?

  Glucose by the enzyme maltase
• how is glucose absorbed in the intestines?

  • Glucose is absorbed by the Na+ - dependent glucose transporter-1 SGLT1 or SLC5A1
  • Glucose uptake is energised by the movement of Na+ ions down their electrochemical gradient into the enterocyte.
• what is the structure of SGLT1/SLC5A1 like? (glucose dependent transporter)

  • SGLT1/SLC5A1 possesses 14 membrane-spanning regions that fold to form a pore
• how are proteins digested?

  • Proteins are digested into small peptides & amino acids
  • Small peptides transported into the cell by PepT1/SLC15A1 transporter
  • Peptide uptake is energised by the movement of H+ down their electrochemical gradient into the enterocyte.
• how many membrane-spanning regions do PepT1/SLC15A1 possess?

  12
• • Absorption via SLC transporters is extremely efficient
  • And so it is exploited by drug designers
  • They design drugs based on natural products, e.g. peptide-based drugs (PepT1)
  • and so can hijack the method of protein transport to deliver drugs this way
• examples of PepT1 substrates?

  • Antibiotics (e.g. ampicillin, cefixime, cefadroxil)
  • Angiotensin-converting enzyme (ACE) inhibitors (e.g. captopril, enalapril)
  • Valacyclovir (prodrug of acyclovir) and valganciclorvir (prodrug of ganciclorvir)
  • these prodrugs are the parent drugs with valine attached
• how is absorption via SLC transporters exploited by drug designers to aid drug absorption?

  • Absorption via SLC transporters is extremely efficient
  • And it can be exploited by drug designers
  • They design drugs based on natural products, e.g. peptide-based drugs (PepT1)
  • which can hijack the method of protein transport to deliver drugs this way
• what is an OATP1B1 transporter?

  An organic anion transporting polypeptide - (OATP1B1/SLCO1B1)

  It's involved in the uptake of statins into the lining of the intestines
• what do statins inhibit?

  Liver HMG-CoA reductase

  • Inhibition reduces cholesterol synthesis in the liver
  • Indication: reduce plasma cholesterol level
• Some SLC transporters aid drug absorption, but some ABC transporters may hinder absorption.
• what are ABC transporters?

  ATP Binding Cassette transporters - they hydrolyse ATP to energise the transport of substrates against a concentration gradient - so pump drugs out of the cell (which can be a problem)

  They are found throughout the body.
• what do ABC transporters usually do?

  transport substrates out of cells (efflux)
• name two ABC transporters...

  P-glycoprotein - (P-gp)/ABCB1

  Breast Cancer Resistance Protein (BCRP)/ABCG2
• describe the structure of the P-glycoprotein transporter...

  • 2 Transmembrane domains (TMDs), each consisting of 6 helices
  • 2 Nucleotide-binding domains (NBDs) in the cytoplasmic region (intracellular facing) that hydrolyse ATP to allow for efflux
  • Forms a pore in the membrane
• describe the structure of the breast cancer resistance protein (ABCG2 transporter)...

  • 1 Transmembrane domain (TMD), consisting of 6 helices
  • 1 Nucleotide-binding domain (NBD) in the cytoplasmic region (intracellular facing) that hydrolyses ATP.
  • Two ABCG2 proteins combine → Form a pore in the membrane
• what therapeutic drugs does ABCG2 transport? (breast cancer resistance protein)

  • Rosuvastatin
  • Cimetidine
  • Prazosin
  • Sulfasalazine
  • Topotecan
  • Doxorubicin
• where are ABCB1 (P-gp) and ABCG2 expressed?

  in the apical (membrane facing the fluid of the GI tract) membrane of intestinal enterocytes

  • they can reduce the absorption of therapeutic drugs
• what are some examples of ABCB1 (P-gp) substrates?

  • Ritonavir (HIV PI)
  • Cyclosporin A, tacrolimus (immunosuppressants)
  • Erythromycin (antibiotic)
  • Digoxin (cardiotonic)
  • Loperamide (mu opioid receptor antagonist, diarrhoea)
  • Simvastatin, Atorvastatin, Lovastatin (hypercholesterolaemia)
• many anticancer drugs are ABCB1 substrates, name some...

  • Doxorubicin - is administrated via infusion as there are transporters in the intestines so can not be orally
  • this can be a problem if Doxorubicin Extravasation occurs - if the drug leaks out of the blood vessels, can damage and kill surrounding cells (as it’s a toxic drug). If it doesn’t leak, it’s diluted and carried away.
  • Daunorubicin
  • Etoposide
  • Vinblastine
  • Vincristine
  • Paclitaxel
  • ABC transporters can pump anti-cancer drugs out of the cell
• what happens if the ABCB1 activity is inhibited?

  This reduces the amount of drug substrate the transporter can efflux/pump out of the cell.
• P-glycoprotein/ ABCB1 inhibitors include...

  • Quinidine
  • Itraconazole
  • Verapamil
• Intestinal P-glycoprotein/ ABCB1 and Digoxin...

  1. Digoxin treatment alone

  • it gets pumped back out

  1. Polypharmacy: Digoxin and Quinidine → Quinidine inhibits P-glycoprotein

  • digoxin is less likely to bind to the transporter so less likely to be pumped back out of the cell
  • more digoxin reaches blood (absorbed) → high concentration - high plasma levels → potential toxicity
  • digoxin has quite a narrow therapeutic window
• Inhibitors of transporters like ABCB1 (P-glycoprotein) reduce the clearance of the drug from the body as these transporters are present in the liver and kidneys as well