Lecture 3.1b - Alcohol metabolism and oxidative stress.pdf

Reactive oxygen species: ◦What are ROS? - metabolic by-products of oxygen metabolism mainly produced in the mitochondria (most important organelle that uses oxygen) ◦Other ROS sources - lipoxygenases (LOX) and cyclooxygenases (COX) Reactive oxygen species - superoxide: ◦Super...

Reactive oxygen species: ◦What are ROS? - metabolic by-products of oxygen metabolism mainly produced in the mitochondria (most important organelle that uses oxygen) ◦Other ROS sources - lipoxygenases (LOX) and cyclooxygenases (COX) Reactive oxygen species - superoxide: ◦Superoxide radicals (O2*-), hydrogen peroxide (H2O2), hydroxyl radicals (*OH), and singlet oxygen (1O2), nitric oxide and peroxynitrite ◦Superoxide: Other ROS: ◦Hydroxyl radicals: ‣ Can be produced by ionising radiation - UV light, X-rays, gamma rays (increases the amount of hydroxyl radicals) ‣ Very damaging to cell membrane (causes haemolysis in RBCs) and DNA ‣ Cannot be eliminated by an enzyme system Defence against ROS - endogenous: ◦The body has several mechanisms to counteract ROS by producing antioxidants, either naturally generally in situ (endogenous antioxidants), or externally supplies through foods (exogenous antioxidants) ◦Antioxidants are to neutralise the excess of free radicals Superoxide dismutase/catalase: ◦The enzyme superoxide dismutase (SOD) counters the damaging effects of superoxide by catalysing its conversion to hydrogen peroxide and oxygen (detoxify) ◦Hydrogen peroxide is a powerful oxidising agent itself and it is rapidly broken down to molecular oxygen and water by the enzyme catalase.  Glutathione: ‣ Glutathione (GSH) is a tripeptide (Gly-Cys-Glu) Glutathione is one of the first lines of antioxidant defence ‣ Glutathione is synthesised by the body as an antioxidant to protect against oxidative damage. ‣ The thiol group of the Cys residue donates an electron to ROS and reacts with another GSH forming a disulphide bond (GSSG) between the cysteine molecules Defence against ROS - exogenous: ROS in health: ◦Under controlled levels, ROS can be a positive factor in health ◦Phagocytes synthesise and store free radicals and protect them against pathogenic microbes. ◦Involved in a number of cellular signalling pathways for example nitric oxide. ◦Induction of a mitogenic response ◦Gene transcription ◦Skin cells differentiation ◦Spermatic function and fertilisation ROS in disease - oxidative stress: ◦Oxidative stress is a phenomenon caused by an imbalance between production and accumulation of oxygen reactive species (ROS) in cells and tissues and the ability of a biological system to detoxify these reactive products. ◦When ROS production increases, they start showing harmful effects on important cellular structures like proteins lipids and nucleic acids - can lead to changes in cells, causing disease ‣ Oxidative phosphorylation ‣ Radiation ‣ Lipid peroxidation - damage to cell membrane ‣ Can damage DNA - leading to mutation ‣ Damage to proteins - can lead to protein modification (e.g. modifying the function of enzymes -> disease)  Conditions associated with oxidative stress: Clinical context: ◦Oxidative burst: ‣ NADPH oxidase ‣ Rapid release of superoxide and hydrogen peroxide from cells - usually leukocytes e.g. neutrophils and monocytes ‣ Membrane bound enzyme in cell membrane and phagosomes ‣ Rapid production of ROS kills pathogens in the locality. Hyperchloride (bleach) can protect against infection and kill bacteria Chronic granulomatous disease: ◦Chronic granulomatous disease is a genetic disease ◦In CGD, mutations in any one of five different genes can cause a defect in an enzyme called phagocyte NADPH oxidase. ◦X-linked genetic disorder - immune defence is weak ◦Signs and symptoms: ‣ Infections that are serious, show up suddenly and keep coming back ‣ Granulomas, which are hard lumps that build up in the tissue and usually appear in the bladder, intestines, lungs, stomach and skin - granulomas try to contain the pathogens ‣ Abdominal (stomach area) pain, diarrhoea and weight loss G6PDH deficiency: ◦Glucose-6 phosphate dehydrogenase deficiency: ‣ NADPH oxidase requires the restoration of NADPH from NADP that is being consumed in the process ‣ The enzyme hexose kinase works by trapping glucose inside the cell by phosphorylation  This is where G6PDH returns the levels of NADPH back to normal ‣ Also useful for maintaining the levels of glutathione in the right form ◦Production of NADPH is limited and won't return to its normal levels ◦Reduced the ability to recycle oxidised glutathione (GSSG) back to its protective reduced form (GSH) ◦G6PD deficiency is an X-linked disorder ◦Neonatal jaundice and/or acute haemolytic anaemia ◦Triggers include: ‣ Illness, such as bacterial and viral infections, some painkillers and fever-lowering drugs, some antibiotics (sulfonamides), some antimalarial drugs (primaquine), fava beans (also called broad beans), Naphthalene ◦Accumulation of Heinz bodies Paracetamol (acetaminophen): ◦Acetaminophen is widely used for its analgesic and antipyretic properties ◦Usual therapeutic adult dose of 1-2 g/day (adults) and 50-75 mg/kg/day in children ◦High oval bioavailability (88%) (first pass metabolism - goes straight to liver) and a plasma half-life of 1.5-2.5 hours ◦Metabolism - liver, and to a lesser extent the kidney and intestine ◦~10g is toxic ◦Acetaminophen is oxidised to N-acetyl-p-benzo-quinone imine (NAPQI) which is toxic to hepatocytes ◦Glutathione prevents NAPQI toxicity ◦Accumulation of NAPQI depletes glutathione leading to oxidative stress ◦PATIENT SAFETY - If it is confirmed that a patient has taken an overdose of paracetamol, rapid treatment with N-acetylcysteine is required. The sooner this is administered the better. If given within two hours of overdose, prognosis is excellent. If treatment is delayed over 8 hours then death from liver failure may be inevitable  Paracteamol metabolism: