Respiratory Biochemistry PDF

Metabolism and bioactivation of toxins in pulmonary tissue; α1-antitrypsin deficiency syndrome.  Biotransformation in lungs The lung has the potential of metabolizing many foreign compounds. Biotransformation is the process by which cells modify xenobiotics they en...

Metabolism and bioactivation of toxins in pulmonary tissue; α1-antitrypsin deficiency syndrome.  Biotransformation in lungs The lung has the potential of metabolizing many foreign compounds. Biotransformation is the process by which cells modify xenobiotics they enter into contact, with the goal of facilitating the elimination of lipophilic substances, which otherwise would accumulate in cell’s lipids. This is achieved by a set of broad specificity enzymes capable of introducing new functional groups (Phase I reactions), or conjugating with internal cell’s molecules, to increase its water solubility (Phase II reactions).  Xenobiotic-metabolising enzymes in human lung Most Phase I reactions consists in oxidation reactions catalyzed by cytochrome P450-depending monooxygenases and flavin monooxygenases and reductions catalyzed by P450 reductase Phase II reactions are catalyzed by: GST: glutathione S-transferase; UDP-glucuronyltransferase; EH: epoxide hydrolase; ST: sulphotransferase; NAT: N-acetyltransferase. Scheme of xenobiotic detoxification  METABOLIC BIOACTIVATION In general, biotransformation reactions are beneficial in that they help the pulmonary tissues to eliminate foreign compounds. Sometimes, however, these enzymes transform an otherwise harmless substance into a reactive form - metabolic bioactivation. A classic example is the activation of benzo(a)pyrene, which is a constituent of tobacco smoke and combustion products, into reactive forms capable of binding to DNA and leading to cancer formation. Role of P450 in the bioactivation of the pre-carcinogen benzo[a]pyrene. Benzo[a]pyrene (BP) is metabolised by P450 enzymes into BP-7,8-epoxide, which is subsequently converted into BP-7,8-diol by epoxide hydrolase (EH), and into BP-7,8-diol-9,10-epoxide BPDE) by CYP1A1. BP-7,8- diol-9,10-epoxide is a reactive metabolite that covalently Interacts with DNA.  Expression of P450 enzymes in pulmonary tissue The proteins encoded by the CYP superfamily (P450 monooxygenases) are responsible for the oxidative metabolism of most xenobiotics (drugs, environmental pollutants, carcinogens), as well as endogenous compounds. P450 enzymes act as monooxygenases, i.e. use one atom of molecular oxygen to oxidize xenobiotics, and require the aid of NADPH-cytochrome P450 reductase, to provide the electrons required for the reduction of the second oxygen atom to H2O. At least CYP1A1 (in smokers), CYP1B1, CYP2B6, CYP2E1, CYP2J2, and CYP3A5 proteins are expressed in human lung, and other CYP forms are likely to be expressed. Xenobiotic-metabolizing CYP enzymes are mostly expressed in the bronchial and bronchiolar epithelium, Clara cells, type II pneumocytes, and alveolar macrophages in the human lung, although individual CYP forms have different patterns of localization in pulmonary tissues.  Expression of phase II enzymes in pulmonary tissue The lung also contains Phase II enzymes that play an active role in the elimination metabolites formed during Phase I reactions. Glutathione S-transferases (GSTs), UDP-glucuronyltransferases (UGTs) and epoxide hydrolases have been identified in pulmonary tissue. Glutathione transferases (GSTs) are a family of enzymes that catalyze the conjugation of glutathione to a wide variety of endogenous and exogenous electrophilic compounds.  Diseases of respiratory system Noninfective diseases of the respiratory system are grouped into obstructive and restrictive, although patients can suffer from one or both. Obstructive lung diseases, as the name implies, include medical conditions where airflow is impaired by some manner. Classic obstructive diseases of the respiratory system include emphysema, chronic bronchitis, bronchiectasis, and asthma. Restrictive lung diseases – restriction of complete lung expansion. This may be due to fibrosis or scarring damage to the lung tissue, mechanical problems such as severe scoliosis, chest wall deformities, and/or increased abdominal pressure.  What is Emphysema? ⚫ Loss of elastic recoil ⚫ This loss of recoil leads to increased compliance and inability to expel gas out of the alveoli ⚫ Leading to trapped air in the lung ⚫ Alveoli cluster together forming “blebs”. ⚫ Emphysema is one of several irreversible lung diseases that diminish the ability to exhale. This group of diseases is called chronic obstructive pulmonary disease (COPD). The two major diseases in this category are emphysema and chronic bronchitis, which often develop together.  Classification of Emphysema Emphysema is divided into panacinar and types because of its location but, more centroacinar importantly, because of the biochemical basis of the disease. A third type of emphysema, congenital lobar, seen in some newborns, results from the overgrowth of a lung or pulmonary lobe and the resulting compression and bronchial narrowing seen on other parts of the respiratory system, including the pulmonary vasculature.  Panacinar emphysema Panacinar emphysema involves all portionsof the acinus and secondary pulmonary lobule uniformly. It predominates in the lower lobes and is the form of emphysema associated with1-antitrypsin deficiency  Emphysema and α1-antitrypsin Elastin is a lung protein. The enzyme elastase secreted from neutrophils destroys the elastin in the lungs. a1-antitrypsin is an enzyme (secreted from the liver) that inhibits elastase and prevents the destruction of elastin. Thus, deficiency of α1-antitrypsin especially in smokers leads to degradation of the lung and destruction of the lung (emphysema).  Centroacinar emphysema The second major type of emphysema is centroacinar, affecting cells mainly at the end of the bronchioles (i.e., the center of the acinus) with less impact on the more external alveolar structures. Cigarette smoke, specifically, the effects of nicotine on neutrophils and the neutrophil elastase enzyme, is a major cause of centroacinar emphysema in susceptible patients. Nicotine serves as a major attractant for neutrophils via increased production of several metabolites (nuclear factor-kappa b, tumor necrosis factor (TNF) and interleukin (IL)-80 INCREASES protein tyrosine kinase (PTK) activity. PTK phosphorylations promote neutrophil chemotaxis and cell adhesion for the accumulation of neutrophils as well as the activation of neutrophil elastase, leading to the destruction of the lung architecture.  BRONCHITIS Bronchitis, inflammation of the bronchial mucus membranes, can be divided into acute and chronic forms. Acute bronchitis is usually caused by a temporary irritant such as an infective agent [virus (∼90% of all cases) or bacteria] or a temporary environmental irritant. Cough with possible production of excessive mucus is the major symptom that resolves as soon as the agent is eliminated.  ASTHMA Asthma, a chronic inflammatory condition of the bronchi. Asthma leads to an intermittent variable and partly reversible constriction of the bronchi smooth muscles known as bronchospasm and airway obstruction with breathing difficulties, including the characteristic asthmatic wheeze of an asthma exacerbation. In very severe cases, obstruction becomes so severe and airflow so limited that wheezes become inaudible. Pathological growth and enlargement of smooth muscles and mucus-producing cells as well as pro-inflammatory cytokines, and elevated immunoglobulin (Ig) E  ASTHMA Regardless of the precipitating factor(s), asthma is associated with an inflammatory disease resulting in the overproduction of IgE, which may block β2-receptors on bronchi smooth muscle cells. This “β-adrenergic theory of asthma,” first proposed in 1968, has led to an improved understanding of the biochemical mechanism behind asthma and allergies in general as well as the development of specific medicines to counteract these effects.  ASTHMA Genetic and other variables make cells in the bronchi “hypersensitive” to particular environmental triggers. Environmental triggers are ingested by antigen-presenting cells (e.g., macrophages, B-cells, and dendritic cells), which interact with immature, helper T cells (TH0) as part of a normal antigen-mediated immune response. Normally, no inflammatory response would result. However, in patients with a predilection to asthma, a B-cell/helper T cell (Th2) response is elicited with the production of antibodies to the trigger. Repeated exposure to this trigger results in further activation of the immune system via a type I hypersensitivity response. This type I response produces IgE from plasma cells. The IgE interacts with mast cells and basophils causing them to secrete a variety of molecules.  What is ARDS? Acute respiratory distress syndrome ARDS is a respiratory condition characterized by hypoxemia, and stiff lungs, without mechanical ventilation most patients would die. ARDS represents a response to many different insults/injuries and evolves through different phases. The pathology of ARDS can progress through three overlapping stages: exudative, proliferative, fibrotic.  Phases of ARDS Exudative Phase. This phase is the acute inflammatory stage of ARDS. It occurs by the release of pro-inflammatory cytokines, influx of neutrophils, impaired endothelial cell barrier function, and decrease in surfactant production by type II epithelial cells. Proliferative Phase. This phase is characterized by: decrease of type II pneumocytes, early fibrotic changes, and thickening of the alveolar capillaries. Fibrotic Phase. The last phase is associated with increased collagen deposition and decreased compliance.  Lung Injuries and their symptoms Direct Signs and Symptoms of lung injuries ⦿ Pneumonia ⦿ Breathing in smoke and harmful irritants The first signs and symptoms which the ⦿ Ventilators patient develops rapidly progressive dyspnea ⦿ Nearly drowning (shortness of breath ), tachypnea (fast Indirect breathing), and hypoxemia (low oxygen content in blood). ⦿ Sepsis ⦿ Blood transfusions In some cases, some people will be ⦿ Severe injury to the chest and the head hypotensive, confused, and will have extreme ⦿ Severe bleeding from an injury tiredness, and may develop cyanosis. ⦿ Pancreatitis ⦿ Fat embolism ⦿ Drug reactions  Cyanosis Cyanosis refers to a bluish cast to the skin and mucous membranes. Peripheral cyanosis is when there is a bluish discoloration of the hands or feet. It's usually caused by low oxygen levels in the red blood cells or problems getting oxygenated blood. Cyanosis occurs when oxygen-depleted (deoxygenated) blood, which is bluish rather than red, circulates through the skin. Cyanosis can be caused by many types of severe lung or heart disease that cause levels of oxygen in the blood to be low.  Biochemical basis of lung injuries The biochemical basis of the disease also shares the hallmarks of an initial release of cytokines leading to increased vascular permeability and inflammation with neutrophils, monocytes, and macrophages present. The inflammation results in damage to the alveoli, including swelling, which increases the space between the alveolar membrane and the surrounding capillaries, worsening O2–CO2 exchange and causing respiratory acidosis, shortness of breath, increased breathing rate, severe decrease in oxygenation, and often respiratory failure requiring mechanical ventilatory support. Further inflammation leads to the deposition of fibrous (hyaline membrane) material. A noted reduction in pulmonary surfactant production, due to dysfunction of type II alveolar cells, may result in the complete collapse of alveoli and may also impact pulmonary compliance and work of breathing  Cystic fibrosis (CF) A multisystem disease. Autosomal recessive inheritance. Cause: mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Clinical features of Cystic Fibrosis Chronic Sino-Pulmonary Disease. Nutritional deficiency/GI abnormality. Electrolyte abnormality.  Who is affected by CF Autosomal recessive genetic disorder clinically characterized by chronic lung disease and pancreatic insufficiency. Median survival is 25 years, with an increasing number of patients surviving into their 30’s. Affects about one in 2,500 persons of European ancestry. One in 25 persons of European ancestry is a carrier, having one normal and one abnormal cystic fibrosis gene. Autosomal recessive inheritance in CF Let C= normal CFTR With mom and dad carriers, then: Let c= mutant CFTR 50% chance of having child who is a carrier If mom and dad are both carriers 25% chance of child being affected then: 25% of child with no mutant copies C c of CFTR C CC Cc c Cc cc Cystic fibrosis transmembrane conductance regulator (CFTR) gene The CFTR gene is located on the long arm of chromosome 7. There are 1522 mutations in CFTR listed on the CFTR mutation database The most common mutation is Δ F508---70% CF alleles in Caucasians.  The cystic fibrosis transmembrane conductance regulator (CFTR) The CFTR protein is made up of 1,480 amino acids. Once the CFTR protein chain is made, it is folded into a specific 3-D shape. The CFTR protein is shaped like a tube that goes through the membrane surrounding the cell, like a straw goes through the plastic top of a cup. It is a member of the ATP binding cassette (ABC) superfamily of proteins which includes several clinically important proteins  What Does the CFTR Protein Do? The CFTR protein is an ion channel. An ion channel moves atoms or molecules that have an electrical charge from inside the cell to outside, or from outside the cell to inside. In the lung, the CFTR ion channel moves chloride ions from inside the cell to outside the cell. To get out of the cell, the chloride ions move through the centre of the tube formed by the CFTR protein. Once the chloride ions are outside the cell, they attract a layer of water. This water layer is important because it allows tiny hairs on the surface of the lung cells, called cilia, to sweep back and forth. This sweeping motion moves mucus up and out of the airways.  How Do Problems With the CFTR Protein Cause CF? When any of these problems occur, the chloride ions are trapped inside the cell, and water is no longer attracted to the space outside the cell. When there is less water outside the cells, the mucus in the airways becomes dehydrated and thickens, causing it to flatten the cilia. The cilia can't sweep properly when thick, sticky mucus weighs them down. Because the cilia can't move properly, mucus gets stuck in the airways, making it difficult to breathe. In addition, germs caught in the mucus are no longer expelled from the airway, allowing them to multiply and cause infections. Normal and abnormal CFTR proteins. Computer illustration of a normally functioning cystic fibrosis transmembrane conductance regulator (CFTR) protein (left) and a malfunctioning mutant CFTR protein (right) in a cell membrane (red). CFTR is an ion-channel that moves chloride and thiocyanate ions across epithelial cell membranes. Functional irregularities of these proteins, caused by mutations of the CFTR gene, lead to malfunctioning of epithelial fluid transport, causing mucous (beige, upper right) to build up outside the cells in the lung, pancreas, and other organs, resulting in cystic fibrosis.  Airway surface liquid low volume hypothesis Mucus---helps clear airway of bacteria Clearance of mucus depends on Ciliary function Mucin secretion Volume of airway surface liquid (ASL) Forms periciliary liquid layer Dilutes mucus--- facilitates entrapment of bacteria and clearance Optimal volume of ASL regulated by Na+ absorption and Cl- secretion  Airway surface liquid low volume hypothesis and CFTR Normal CFTR inhibits a sodium channel (ENaC) Mutant CFTR----ENaC not inhibited Sodium absorption is increased Water follows sodium ASL volume decreases Normal CFTR will cause Cl- ions to be secreted if the ASL fluid is low Mutant CFTR - Cl- ions will not be secreted  Airway surface liquid low volume hypothesis and consequences Cilia do not beat well when PCL volume is depleted Mucins are not diluted and cannot be easily swept up the airway Mucus becomes concentrated Results in increased adhesion to airway surface Promotes chronic infection  CF Lung Pathophysiology Diminished host defenses Common bacterial results in chronic bacterialpathogens: infections of the lung Pseudomonas which results in: aeruginosa (most › Inflammation common) › Infection Staphylococcus aureus › Obstruction Haemophilus influenzae Burkholderia cepacia Inflammation + Infection + Obstruction + Bacterial Toxins = Impaired Gas Exchange  CF Symptoms Pulmonary Cough Increased sputum production Recurrent pneumonia Digital Clubbing When the lungs aren’t working well, the body has trouble pushing enough oxygen through the body, the fingers, and toes, may become deprived of oxygen and it results in changes in the nails and nail beds and bulbous swelling at the end of fingers. CF Gastrointestinal Pathophysiology Pancreatic enzyme insufficiency: trypsin, chymotrypsin, carboxypeptidase, amylase, and lipase Fat soluble vitamin deficiency- A, D, E, K Malnutrition due to fat malabsorption  Diagnosis-Sweat chloride ⦿ Technique first described by Gibson and Cooke in 1950s › Chemical that stimulates sweating placed under electrode pad; saline under another electrode pad on arm › Mild electric current is passed between electrodes › Sweat collected › Positive Sweat chloride: 60-165 meq/L › Borderine sweat chloride: 40-60 meq/L › Normal sweat chloride: 0-40  Prenatal screening American College of Obstetricians and Gynecologists recommended offering patients option of prenatal screening for CF Carrier testing of 23 most common mutations Sensitivity of prenatal screening for CF among the white population