Genetics of Non CYP450 mediated phase I metabolism Flashcards
what is Flavin linked monooxygenases
Microsomal enzymes with FAD as prosthetic group, important in oxidation reactions at nitrogen, sulphur and phosphorous centres
what is Trimethylamine (TMA)
an unpleasant smelling compound that is a breakdown product of a number of precursors (such as L-carnitine) present in a range of foods – gut bacteria are involved in TMA formation
how many different forms of Flavin linked mono-oxygenases
At least 5 different forms but FMO3 is the major hepatic isoform in humans
what normally happends to TMA ?
Normally undergoes conversion to trimethylamine N-oxide (TMAO) by FMO3
- TMAO is less volatile and loses odur
Who suffers from fish odour syndrome and why
individuals who lack functional FMO3 as TMA isn’t converted to TMAO
what is the occurrence rate of subjects with fish odour syndrome
1/100 approx are heterozygous and shows defects in TMA metabolism
what is the most common mutation which can cause Fish odour syndrome
Pro153Leu (complete loss of activity)
Proline has a structural role in proteins by “breaking up” areas of alpha helix. Amino acid change to leucine is therefore functionally significant
what is the other mutation can be responsible for lack TMA oxidation
N61S
True or false - P153L + N61S have an effect on methimazole metabolism
Flase - there is no effects
what mutation is responsible for mild fish odour syndrome in children
E158K/E308G in FMO3 polymorphism
how does polymorphism in FMO3 effect sulindac metabolism
- Better outcome in those with polymorphism due to slower metabolism
- sulindac is a prodrug but is activated by gut flora prior to absorption)
- Produced sulindac sulphide which is active, The inactive metabolite of sulindac (the sulfone) is subject to enterohepatic recirculation.
- Both S and R epimers show anti cancer activity
- Low FMO3 higher conc in the gut and active metabolite = better effects in good
what hydrolysis organophosphates in the liver and serum
Arylesterase - paraoxonase
what is PON1 associated with
high density lipoprotein (HDL) in serum and may protect against atherosclerosis. Hydrolyses oxidised LDL-associated cholesterol, with potential protective effects
what % of europeans have low activity for paraoxon hydrolysis in serum and why?
50, due to a polymorphism at codon 192 with a Arg to Gln amino acid substitution
the effects of PON1 polymorphisms are _ dependent
substrate
Paraoxon: low activity with Gln form. Phenylacetate: no effect. Diazoxon: higher activity with Gln form
what are the variation of PON1 genotypes between continents
- 50% of Europeans are homozygous for Gln (low activity with paraoxon)
- 10% of North Africans were RR homozygotes,
- whilst a mixed ancestry South African population were 15.8% QQ
- 15% of Japanese are Gln homozygotes
what type of role does PON1 genotype have in CHD
Protective ? (some evidence)
- Low PON1 activity was associated with increased risk of CHD, but this cannot be linked to a particular polymorphism
-Low PON1 activity was associated with polymorphisms in the promoter region in the patient group, but there was no association between these polymorphisms and CHD
What is Butyrylcholinesterase
Plasma esterase which hydrolyses the muscle relaxant succinylcholine - high levels in plasma
what % of the population is heterogenous + poor hydrolysis of succs
5
butyrylcholinesterase polymorphisms is _ specific. therefore most individuals with deficient hydrolysis of succinylcholine show _ hydrolysis of other substrates
substrate
normal
what is used to detect individuals with deficiency phenotypically by inhibition pattern of benzoylcholine hydrolysis
dibucaine number
how do you measure the dibucaine number
- take plasma samples with varying levels of dibucaine and add benzoylcholine
-Dibucaine number is % inhibition of benzoylcholine by 10 micromolar dibucaine. - The majority of people show approx 80% inhibition.
- The atypical reaction shows 10-20% inhibition.
what is the muattion responsible for the atypical form assocated with abnormal dibucaine inhibition
Asp70Gly
what are the 3 main forms of cholinesterase deficiency
Atypical
fluoride-sensitive
silent
what is the difference between CES1 and CES2
CES1 - high levels in liver which is important in metabolism (Tamiflu)
CES2 - mainly expressed in intestine and in other extra hepatic tissues
what are the polymorphism involved in CES1
- Gly143Glu = low activity
- Asp260fS = No activity (Rare)
what is Dihydropyrimidine dehydrogenase (DPYD)
Metabolic enzyme that converts uracil and thymine to dihydro metabolites facilitating further metabolism to amino acids
what drug is metabolised by Dihydropyrimidine dehydrogenase
5-fluorouracil (5FU)is an important anticancer drug
Dihydropyrimidine dehydrogenase deficiency is caused by what, normally ?
Most common variant allele associated with deficiency has G to A base change in intron 14 (position 1) resulting in skipping of exon 14 and truncated protein
if an individual is heterozygous for DPYD mutation then they are at risk of what ?
what is the occurance of heterozygous ?
- toxicity if 5FU is given
- 2-3% of europeans
what does aldehyde dehydrogenese do ?
converts acetyaldehyde to acetate (from ethanol)
what are the two forms of aldehyde dehydrogenase
ALDH1 (cytosolic) and ALDH2 (mitochondrial enzymes)
why do east asians suffer from flushing/nausea when they consume alcohol
- amino acid substitution in ALDH2 of Glu487Lys at C-terminal end of protein
- results in unstable protien
- Dominant inheritance means subjects can be heterozygous
- flushing/nausea is due to accumulation of acetaldehyde
