Lecture 5

• FMO3

  • Microsomal enzyme — FAD prosthetic group
  • Important in oxidation reactions at nitrogen, sulphur + phosphorus centres
  • 5 diff forms — FMO3 major hepatic isoform
• FMO3 deficiency


  • Trimethylamine (TMA) — Unpleasant smelling compound
  • Breakdown product of no. of precursors (L-cartinine)
  • Normal — conversion to TMA-N-oxide (TMAO) by FMO3
  • Lack enzyme and suffer fish odour syndrome — ability to convert TMA → TMAO
  • 1 in 100 individuals heterozygous for this defect → some impairment in TMA metabolism
• Fish odour syndrome - molecular basis

  • FMO3 gene sequenced in individuals with fish odour syndrome
  • Most common mutation — Pro153Leu → complete loss of enzyme activity
• FMO3 common polymorphisms + mutations

  • N61S → Associated with lack of TMA oxidation but no effect on methimazole metabolism
  • E158K/E308G → compound polymorphism quite common — DECREASED ACTIVITY (mild FOS in children)
• Sulindac metabolism


  • NSAID — chemoprevention of colon cancer
  • Better outcome of those with polymorphism — slower metabolism (prodrug activated by gut flora prior to absorption)
• Paraoxonase 1


  • Arylesterase → hydrolyses organophosphates in liver + serum
  • Referred to as PON1 → related genes (PON2+3) may have diff function
  • PON1 ass with HDL in serum + protect against atherosclerosis — Hydrolyses oxidised LDL-associated cholesterol
• Paraoxonase 1 polymorphism

  • Arg192Gln substitution
  • 50% Europeans — low activity of paraoxon hydrolysis in serum — homozygous for Gln
  • 10% North Africans RR homozygotes, 15.8% South African population QQ
  • Relevance — use and choice of pesticides; however range of enzyme activities
  • Paraoxon — low activity with Gln form
  • Phenylacetate — no effect
  • Diazoxon — higher activity w/ Gln form
• Some evidence for role of PON1 in atherosclerosis/CHD, but associations not strong
• Relationship between PON1 + atherosclerosis remain controversial
• Cholinesterase

  • Plasma esterase → hydrolyses succinylcholine (suxamethionium)
  • 5% of population heterozygous for poor hydrolysis alleles
• Cholinesterase phenotyping

  • Most individuals with deficient hydrolysis of succinylcholine show normal hydrolysis of other substrates (benzoylcholine)
  • Can detect with inhibition pattern of benzoylcholine hydrolysis — dibucaine or fluoride
• Dibucaine


  • Measurement of dibucaine number — Inhibition of benzoylcholine hydrolysis by dibucaine
  • Atypical — less inhibition than wild type
• Genotypic basis of cholinesterase deficiency


  • Asp70Gly — atypical form
  • Fluoride-sensitive forms — several variants
  • Silent forms — no enzyme produced (frameshift)
  • Other forms — aa substitutions associated with decreased succinylcholine hydrolysis
• Relevance of cholinesterase deficiency


  • Prolonged ventilation after succinylcholine (apnoea)
  • Could avoid — lower doses
  • Screening not usually performed
  • Could have sensitivity to pesticides — unclear
• Carboxylesterase CES1


  • Oseltamavir (Tamiflu)
  • Polymorphism of functional significance
  • Gly143Glu — low activity (4% pop. freq.)
  • Asp260fs — no activity (rare)
• Carboxylesterase CES2


  • Aspirin, irinotecan
  • No functionally important polymorphisms described as yet
• Dihydropyrimidine dehydrogenase (DYPD)


  • Converts uracil → thymine → dihydro metabolites → further metabolism to aas
  • 5-Fluorouracil (5-FU) — anticancer drug — metabolised by DYPD
  • Rare inborn disorders — completely lack DYPD → suffer epilepsy + mental retardation
• DYPD reaction


  Uracil → thymine → dihydro metabolites → further metabolism to aas
• Molecular basis of DYPD deficiency


  • Recessive trait — heterozygotes (2-3% Europeans) show lower enzyme levels than normal
  • Most common variant — G→A (intron 4 — position 1) → skipping exon 14 + truncated protein
  • Heterozygotes → toxicity if given 5-FU
• Aldehyde dehydrogenase (ALDH)


  • Conversion of acetylaldehyde → acetate from ethanol consumption
  • ALDH1 — Cytosolic, Higher Km (=100uM)
  • ALDH2 — Mitochondrial, Lower Km (=2-3uM)
• ALDH2 Deficiency


  • East Asian flush + suffer nausea following ethanol consumption
  • Due to acetylaldehyde accumulation
  • Glu487Lys at C-terminal end of protein
  • Unstable protein produced
  • Dominant inheritence — consisting of 4 identical subunits
  • Proteins containing single ALDH2*2 are catalytically inactive
• Consequences of ALDH2 deficiency