Liver Enzymes

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The liver is the largest internal organ. It is incredibly functionally complex. It carries out more than 500 different functions that include detoxification, protein synthesis and chemical synthesis to aid in digestion. Some of the livers main functions will be talked about. Bile produced by the liver aids in digestion in the small intestine. It breaks down and absorbs fats, cholesterol, and occasionally some vitamins. Bile is produced by the liver and is stored in the gallbladder. The liver also absorbs and metabolizes bilirubin. Bilirubin is caused by the breakdown of hemoglobin in red cells. This can either be extravascular or intravascular hemolysis and can either be normal breakdown as red cells only live for 120 days before being recycled or it can be not normal. The iron that is released by the bilirubin is stored in the liver or the bone marrow that is used to make new red cells. Bilirubin is metabolized to its direct conjugated form where it is excreted by the urine and feces. The liver produces clotting factors. Vitamin K is necessary for some clotting factors to be produced and in order for vitamin K to be absorbed from the diet the liver needs to produce bile. Without the bile clotting factors would not be produced. The liver aids in filtering the blood. It filters and removes different waste material produced from the body as well as exogenous compounds like alcohol and other drugs. The liver also has a immunological function in that it contains Kupffer cells. These Kupffer cells are the macrophages of the liver and are part of the mononuclear phagocyte system that destroy any foreign antigen that enters the liver. The liver produces albumin which is arguably the most important protein in the body. Albumin is used as a transport protein and maintains colloid pressure in the blood vessels. Angiotensinogen is produced by the liver that is used in the angiotensinogen-Renin system in the kidneys that raises blood pressure by vasoconstriction.

Regeneration is a unique feature of the liver. Its important to the body is unmatched and it has evolved the ability of regeneration. Regeneration is the ability for the organ to regrow rapidly as long as it is kept healthy. During this process of regeneration the function of the liver is not compromised. If needed there are a number of compounds that can aid in the regeneration process like hepatocyte growth factor, insulin, epidermal growth factor, IL-6, and even norepinephrine.

Analysis of bilirubin is based on the reaction of bilirubin with a diazotized sulfanilic acid. Three fractions of bilirubin is measured. Conjugated, unconjugated, and delta bilirubin. Delta bilirubin is bilirubin bound to albumin. A fasting sample is preferred and hemolyzed samples should be avoided. Bilirubin is sensitive to light so care should be taken to shield the sample from the light.

Liver enzymes are relatively nonspecific indicators that can indicate tissue destruction in several organs. Alkaline phosphatase is a group of isoenzymes that are found on the membranes of cells in almost every tissue. Alkaline phosphatase can become elevated in many different conditions including bone diseases, puberty, and even in late pregnancy. One important feature of alkaline phosphatase is that the isoenzyme found in bone is the most heat labile. Differentiation in the laboratory includes heating the sample and then remeasuring the alkaline phosphatase and measuring the difference.

Gamma-glutamyltransferase (GGT) is a membrane enzyme that is important in glutathione metabolism. It transfers glutamate to the amino acid peptide chain. GGT is among one of the first enzymes to become elevated in acute liver diseases such as hepatitis. Its important to note that GGT is normal in patients with bone diseases making it one of the most clinically specific of all the liver enzymes. Lactate dehydrogenase is a cytosolic enzyme that interconverts pyruvate and lactate. There are five isoenzymes. Isoenzymes 4 and 5 are increased in viral or toxic hepatitis, biliary obstructions and cirrhosis. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) convert aspartate and alanine to oxaloacetate and pyruvate respectively. ALT is very specific for liver pathology. AST is found in liver tissues, but also present in heart and muscle tissue. The AST/ALT ratio is an important factor to look at. A ratio less than 1.0 indicates viral hepatitis. This is an increase in ALT greater than the increase in AST. An AST/ALT ratio greater than 1.0 usually means that the ALT is elevated to a lesser degree than the AST and this can be found in cirrhosis, bile duct obstruction, or metastatic cancer of the liver.

-Caleb

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Viral Hepatitis

Viral hepatitis is acute or chronic symptomatic or even sometimes asymptomatic inflammation of the liver. Usually the infection is self-limiting and clears within 12 weeks, but in immunocompromised patients it can pose serious complications and development of systemic problems. All viral hepatitities can lead to liver cirrhosis if untreated. There are important serological markers involving different proteins of the virus that can tell us whether or not the infection is acute or chronic and whether or not the virus is actively replicating or not. The viruses replicate one week before detection and two weeks post-detectable levels. The most common types of hepatitis are A, B and C. Although Hep A and B are on the decline as vaccination campaigns have been more successful. Hep C is gradually declining, but as of recently there has been an increase in incidence. It should be noted that as of now there is no currently FDA approved vaccination for Hep C. There are some more less common Hep virus like E, and D, but the incidence is low and clinically not very relevant. One important distinction to point out is that a person much have Hep B to acquire Hep D. Hep D requires the viral envelope protein from Hep B to survive.

Hep A is a +ssRNA picornavirus. It is non-enveloped which means when it is shedding and replicating it causes lysis of the hepatocytes in the liver. The disease is spread by fecal/oral route and the virus is exceptionally hardy. It is very hard to inactivate and can survive in water or on surfaces for an extended period of time. The incubation period is 20-40 days and the disease itself is mild in nature. Clinical presentation includes variable prodromal symptoms usually fever, malaise, weakness, nausea, loss of appetite, and myalgia. Jaundice may or may not be present. The virus usually presents as asymptomatic, especially at a younger age as the immune system is able to hold it in check. Symptoms can actually be a good thing as it means that the immune system is recognizing the virus and trying to rid of it. Risk groups include those who have direct contact with persons who have Hep A such as a healthcare worker, travelers to endemic countries are at high risk even when good hygiene and sanitation measures are being applied. Less commonly users of injection and non-injection drugs, and people with clotting factor disorders. Diagnostic workout includes serologically workup and liver enzyme panel as well as bilirubin levels.

Hep B is a dsDNA-RT enveloped hepadnavirus. The Hep B genome is unique in that it virally replicates using an RNA intermediate and uses its reverse transcriptase mechanism to complete its DNA replication making the life cycle very complex. Transmission is through body fluids or sexually and is considered a sexually transmitted disease (STD). Activities involving percutaneous or mucosal contact such as heterosexual contact, injection drug-use with sharing of needles, birth to an infected mother or contact with blood or sores of an infected person. Medical professionals should take care to avoid any needle sticks or other exposures to those infected. Its estimated that 400 million people worldwide are infected with Hep B. The clinical presentation often presents with very generalized symptoms similar to Hep A with the addition of joint pain, jaundice, dark urine or clay-colored bowel movements. Chronic Hep B infection is not commonly talked about as Hep C is primarily dubbed as the chronic hepatitis infection, but 5% of adults become chronically infected with Hep B. They can live asymptomatically or they can display a spectrum of disease ranging from chronic hepatitis flares to cirrhosis or hepatocellular carcinoma.

The Hep B diagnosis is largely reliant on serology. The Hep B surface antigen (HBsAg) is a protein on the surface of HBV. It can be detected in high levels in the serum during acute or chronic HBV infection. It should be noted that individuals typically lose the expression of the HBsAg after 12 weeks when chronically infected. The presence of HBsAg means that person is infectious. Hep B surface antibody (Anti-HBs) present means that the person is recovering from an infection and that they now have immunity to the HBV. Anti-HBs also is developed when an individual is vaccinated against Hep B. IgM antibody to Hep B core antigen (IgM-Anti-HBc) indicates that an individual has an acute Hep B infection or has had an infection within the last 6 months. IgG-Anti-HBc indicates previous infection with Hep B and immunity towards the virus. The core antibody persists for life. Its important to note that when you run labs looking for hepatitis if the Anti-HBs antibody is present, but the Anti-HBc antibody is NOT present this indicates the individual has been vaccinated, not previously infected. The core antibody will not develop during vaccination. The Hep B (e) antigen is a secreted product of the nucleocapsid of HBV that is found in the serum during actively replicating infection. This indicates that the individual has an incredibly high viral burden. Anti-HBe antibody produced is an indicator of seroconversion and is used as a predictor of clearance of the Hep B virus for individuals undergoing treatment. Acute Hep B is usually not treated and the individual receives supportive therapy if needed. Chronic Hep B is treated using interferon or five different antiviral nucleoside analogs. The treatment is not curative, and only has a 20-50% response rate, but improves prognostic markers. Hep B is HIGHLY infective, 50-100x more infectious than HIV and 10x more infectious as HCV.

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Hep C is a +ssRNA enveloped flavivirus. Typical transmission is through exposure to infectious blood. This can either be through injection drug use of shared needles or through recipient of donated blood, or organs, or neonatal birth to an HCV infected mother. Sexual transmission can still occur, but not at the rate that Hep B is transmitted. Acute infection usually is asymptomatic or presents with only mild symptoms similar to that of HAV and HBV. The incubation period is 4-12 weeks. Chronic HCV is generally asymptomatic while some individuals present with mild chronic liver disease and others with cirrhosis or hepatocellular carcinoma. 15-25% of individuals who develop Hep C will be able to clear the disease. Of the 75-85% that develop chronic HCV 60-70% will develop chronic liver disease, 5-20% will develop cirrhosis, and 1-5% will develop HCC. Hep D diagnosis is dependent on screening tests for the HCV antibody. These screening methods include enzyme immunoassay (EIA) or enhanced chemiluminescence immunoassay (CIA). There are PCR tests that detect the HCV RNA and also that quantify the viral RNA to determine the disease burden. A diagnosis based solely on elevation of liver enzymes AST, ALT is not acceptable. There is no currently accepted vaccine, but there is research ongoing. Previous infection does not grant immunity for subsequent infections like one would think likely due to high rte of viral mutation. There are 3 characteristic classes of drugs for treatment of Hep C and one novel advancement of one. HCV protease, polymerase inhibitors and interferon are most commonly used. A new novel treatment called direct acting antivirals (DAA) is a combination therapy that is shown to be highly effective at suppressing viral replication and can actually result in a cure in as short as 12 weeks of treatment. The cure rate is 95%. This novel treatment protocol is showing unprecedented success, but it comes as cost. A cost that no insurance company wants to pay for and that most people can’t afford.

Chronic HBV and HCV are associated with an increased incidence of HCC. This is due to the continuous cycles of viral replication, the immune-mediated killing of the hepatocytes and inflammation. Inflammation induces oxidative damage and altered cellular metabolism. This creates an unhealthy environment in the liver.

-Caleb

Acid-Base Balance

An acid is any compound that can donate H+ when dissolved in water. A base is any compound that can donate OH- ions. A buffer system is a combination of a weak acid or base and its salt or conjugate that resists changes in pH. The human body has incredible mechanisms to maintain an acid-base balance. Changes in pH put the body in different physiological states that can cause an array of problems. Acidosis is when the pH falls below the reference range of 7.34. Alkalosis is when the pH increases above the reference range of 7.44.

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The most important buffer system in the body is the bicarbonate (HCO3)/carbonic acid (H2CO3) system. Carbonic acid works to allow the human body to rid of toxic CO2 via respiration to maintain a normal pH of 7.4. There normally is a 20:1 ratio of bicarbonate to carbonic acid.

The red cells pick up CO2 from tissues and throughout its travel through the blood vessel its converted to carbonic acid. That carbonic acid is then broken down into bicarbonate and hydrogen. The excess hydrogen ions are buffered by hemoglobin. Bicarbonate leaves the red cell and goes into circulation. Bicarbonate enters the plasma through an exchange mechanism with chloride to maintain a state of electroneutrality in the cell. When the red cells reach the lung the hemoglobin will release the excess hydrogen ions by the binding of oxygen to hemoglobin. The excess hydrogen ions bind to bicarbonate to form carbonic acid. Carbonic acid then dissociates into H20 and CO2 which is expelled.

As mentioned above, an individual can be in a state of acidosis or alkalosis. This can be caused by ventilation and is called respiratory acidosis or respiratory alkalosis or it can either be caused by HCO3-. This is called metabolic acidosis or alkalosis.

Respiratory acidosis is an increase in PCO2. Conversely respiratory alkalosis is a decrease in PCO2. Metabolic acidosis is a loss of HCO3- or an addition of H+. Metabolic alkalosis is a loss of H+ or an increase of HCO3-. The body will naturally compensate for the pH changes. Some of the compensatory mechanisms are increasing respiration in metabolic acidosis. Hyperventilation increases the amount of CO2 that is expelled and raising the pH. In respiratory acidosis the kidney will increase its reabsorption of HCO3-.

Metabolic acidosis can be caused by multiple different disease states. Excessive loss of HCO3- by diarrhea can cause metabolic acidosis. Diabetic ketoacidosis can cause it. Other causes are ingestion of acids or renal tubular failure where there is no renal reabsorption of HCO3-.

Metabolic alkalosis is caused by excess or an overdose of HCO3-. Excessive vomiting causes a loss of hydrochloric acid with the stomach contents. Vomiting also results in hypokalemia and hyponatremia which are both positively charged ions (acids) leading to an increase in the pH. Excessive diuretic use can sometimes initially cause an increase in chloride, but most commonly results in hyponatremia and causing a contractile alkalosis.

Respiratory acidosis is most commonly caused by CO2 retention usually due to ventilation failure. Decreased cardiac output and hypotension also cause acidosis. Less blood is pumped to the heart so less CO2 is getting transported to the lungs to be expelled. Chronic lung conditions such as COPD result in an inability to ventilate properly and to expel CO2. Certain drugs cause depression of the respiratory center in the brain and can cause respiratory acidosis. Some of these drugs are barbiturates, opiates and ethanol (alcohol).

Respiratory alkalosis is primarily caused by hyperventilation (increased alveolar ventilation). This results in a decreased arterial PCO2. Any condition which decreases pulmonary compliance causes a sensation of dyspnea. Dyspnea is not a single sensation and there are at least three distinct sensations including air hunger, work/effort, and chest tightness. These sensations cause a state of hypoxia which is caused by the hyperventilation.

-Caleb

General Endocrinology

Hormones make up the endocrine system and act on almost every tissue in the body. Hormones are substances that are produced by a specialized cell that circulates in the blood. The best example of this is insulin which is secreted by the beta cells in the pancreas.

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Credit for the photo goes to Pearson Education, Inc.

There are multiple forms of chemical signaling that hormones utilize. The first is autocrine where the cell targets itself. Signaling across gap junctions occurs when a specialized cell targets another cell that is connected via a gap junction. Paracrine is when the targeted cell is nearby. Endocrine which will be the primary focus for today is when the cell produces hormones or chemical signals that have to travel through the blood stream to act on distant cells. Depending on the receptor type to these hormones distinguishes the action it has on the recipient tissue or cell. Receptors can by cytoplasmic, ion channels, tyrosine kinase receptors, or a G-protein coupled receptor. There can also be different types of hormones. Protein hormones utilize calcium as a secondary messenger. The action potential of protein hormones is quick as opposed to steroid hormones. The action of steroid hormones is slow as steroids are not as membrane permeable as protein hormones. Its important to note that hormones are released in pulses. Each pulse has an amplitude and period.

The endocrine system needs feedback control loops to function properly. Negative control loops maintain hormonal balance. Positive control loops are actually what causes physiological changes in the tissues involved.

The endocrine system starts in the hypothalamus. The hypothalamus releases releasing hormones to stimulate the anterior and posterior pituitary to secrete effector hormones that act on various sites of the body.

The anterior pituitary otherwise known as the adenohypophysis secretes the majority of the hormones. Releasing hormones are secreted by the hypothalamic neurons into the hypothalamopituitary portal system. These hormones are then carried down the pituitary stalk by this portal system into the adenohypophysis. The anterior pituitary secretes adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), growth hormone (GH), prolactin (PRL), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). These all act on their respectable tissues/cells to secrete specific hormones. ACTH acts on the adrenal gland, which sits on top of the kidneys. The adrenal gland is responsible for secretion of catecholamines (epinephrine/norepinephrine) that influences the flight or fight response as well as glucocorticoids such as cortisol which have physiological effects throughout the entire body. TSH acts on the thyroid gland to secrete the thyroid hormones T3 and T4. These hormones also have wide-spread physiological function throughout the body. GH acts on the liver and influences bone, muscle, and tissue growth. PRL acts on the mammary glands such as the breast glands to stimulate growth and to start lactation. FSH and LH act on the testes of males to secrete inhibin and testosterone as well on the ovaries in females to secrete estrogen, progesterone, and inhibin. Decreased or elevated levels of any of these hormones can have detrimental effects on normal physiological processes. These discrepant levels can either be from primary disease (In the organ where the hormones are produced) or it can be secondary disease, i.e. from the hypothalamus, or pituitary.

Oxytocin and vasopressin (ADH) are the hormones secreted by the posterior or neurohypophysis pituitary. These are synthesized in the paraventricular supraoptic nuclei of the hypothalamus and are carried down the pituitary stalk by axonal transport. These hormones are then released into the general circulation in the neurohypophysis. Oxytocin works in females and males. It effects the uterine smooth muscle and mammary glands in females and in males it effects the smooth muscle in the ductus deferens and the prostate gland. Vasopressin or ADH promotes water retention in the distal tubules and collecting ducts of the kidneys. SIADH is excess ADH secretion and results in concentrated urine, and a low serum concentration. In other words there is low serum sodium which is bad! Diabetes insipidus on the other hand is deficiency in ADH.

-Caleb

Overview of the Immune System; Part One

The overall function of the immune system is to prevent or limit infection. It is essential for survival. Multiple organ systems, cells, and proteins are involved in the immune response. It is the most complex system that the human body has. The immune system is differentiated into two directions. Innate or non-specific immunity or Acquired (specific) immunity.

The Innate immune system consists of many components. The skin acts as a mechanical barrier and is typically the first line of defense against foreign substances. Mucous membranes consist of the bodies normal microbiota which compete with invading microbes. The mucous membranes are also lined with mucous and cilia which act in an elevator type motion to push foreign substances away. Physiological barriers such as temperature, pH and the complement system. The more acidic environment that a lower pH offers disrupts bacterial growth. Antimicrobial proteins and peptides are present in different epithelial locations in the body. Lysozymes are present in the tears and saliva and cleave the peptidoglycan cell wall present in bacteria. Secretory phospholipase A2 is present in the gut and can enter the bacterial cell and hydrolyze lipids in the cell membrane. Lectins target gram positive bacteria and forms pores in the membranes. Defensins integrate into the lipid and form pores which causes loss of membrane integrity. These defensins are present in PMNs (neutrophils) and lamellar bodies in the gut. Cathelicidins are present in neutrophils and macrophages in the lungs and intestines and distrupt membranes. Histatins are constitutively produced by the glands in the oral cavity and are active against pathogenic fungi.  Inflammation plays a huge role in the Innate immune system. Inflammation induces vasodilation and increase in capillary permeability causing an influx of immune cells like PMNs and macrophages. Inflammation can be observed by the four cardinal signs; rumor (redness), tumor (swelling), color (heat), and dolor (pain). The innate immune response is a rapid response.

Innate Immunity

The complement system recognizes features of microbial surfaces and marks them for destruction by coating them with C3b. There are three distinct pathways; the classical pathway, the lectin pathway, and the alternative pathway. All pathways generate a C3 convertase which cleaves C3, leaving C3b bound to the microbial surface and releasing C3a. In the classical pathway the activated C1s cleaves C4 to C4a and C4b which binds to the microbial surface. C4b then binds C2, which is cleaved by C1s to C2a and C2b forming the C4b2b complex. C4b2b on the microbial surface is an active C3 convertase which cleaves C3 to C3a and C3b. This results in opsonization of the bacterial surface by C3b. The C4b2b3b complex is an active C5 convertase leading to the development of the membrane-attack complex. Each complement component (C4a/b, C2a/b, C3a/b) have different functions, but that is another discussion for another time. The lectin pathway of complement activation is when mannose-binding lectin (MBL) and ficolins recognize and bind to carbohydrates on the pathogen surface. Ficolins are similar to MBLs, but have a different carbohydrate binding domain. MBLs bind with high affinity to mannose and fucose residues. Conversely ficolins bind oligosaccharides containing acetylated sugars. When MBL binds to a pathogen surface MBL-associated serine protease (MASP)-2 is activated and cleaves C4 and C2 similar to the classical pathway. The alternative pathway is an amplification loop for C3b formation that is accelerated by properdin (factor P) in the presence of pathogens. Properdin stabilizes the C3bBb complex. C3 undergoes spontaneous hydrolysis to C3(H20) which binds to factor B, allowing it to be cleaved by factor D into Ba and Bb. The C3(H20)Bb complex is essentially a C3 convertase which cleaves more C3 into C3a and C3b. C3b molecules result in opsonization of bacterial surfaces. Its important to recognize that all pathways lead to generation of a C5 convertase. C4b2a4b in the classical pathway, C4b2a3b in the lectin pathway, and C3b2Bb in the alternative pathway. C5 is cleaved into C5a/b that initiates the assembly of the terminal complement components. These are the terminal complement components that form the membrane-attack complex.

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The membrane attack complex consists of an assembly of C6, C7, and C8. This complex undergoes a conformational change that results in polymerization of C9 which generates a large pore in the cell membrane. Host cells contain CD59 which prevents the assembly of the C9 molecules preventing the formation of the membrane-attack complex.

C3a, C4a, and C5a are unique in that these complement components are called anaphylatoxics. They initiate a local inflammatory response when systemic injection of these molecules occurs. They induce smooth muscle cell contraction and increased vascular permeability. They induce adhesion molecules and activate mast cells that invade and populate submucosal tissues to release inflammatory mediators such as histamine and TNF-a.

The Acquired or adaptive immune system is all about specificity. The Humoral branch of the acquired immune system is executed by the B lymphocytes that produce antibodies to specific antigens. The cell-mediated branch consists of antigen presenting cells (APC) such as the dendritic cells processing foreign substances and presenting proteins of those substances as antigens through the major histocompatibility complex (MHC) to CD8 T lymphocytes. These are cytotoxic T-cells that kill these foreign antigens. The acquired immune response is a slow response because it takes the body time to produce antibodies. An important aspect of the adaptive response is memory. Once antibodies have been produced to an antigen, these responses last and the time it takes to produce an antibody on subsequent exposures is rapidly decreased.

These two different systems work in conjunction to produce an adequate and sustained response. When foreign antigens are processed and expressed on the surface of APCs as MHC peptides, pro-inflammatory cytokines such as IL-12p70, IL-18, and IFN-a are secreted. These attract NK cells which primarily attack viruses as well as PMNs and macrophages that phagocytize these antigen peptides to destroy them. Adaptive immunity is also started with dendritic cells that also undergo antigen uptake and processing. This is also called the maturation signal. This signal is augmented by IFN-y and TNF-a secreted by macrophages and NK cells. These dendritic cells either present the antigen to B lymphocytes which are the antibody producers or they present the antigen to CD4/CD8 T-cell lymphocytes.

There are multiple classes of antibodies. IgD is typically expressed on B-cell lymphocytes during differentiation with IgM. IgD is also present in the serum in low concentrations. IgM is a pentamer and the largest immunoglobulin. It is the first antibody that is produced in the immune response. IgA is in high concentration in the mucosal linings, saliva, and tears. Typically part of first line defenses. IgG is present in high concentrations in the serum. IgG is unique in that it can cross the placenta. IgE is involved in allergic reactions. It binds to mast cells and basophils causing degranulation.

-Caleb