CLINICAL CHEMISTRY CARBOHYDRATES, METABOLISM, AND ASSOCIATED DISEASES CARBOHYDRATES • Commonly referred to as sugar or starches • Major food source and energy supply of the body • Aldehydes or ketones derivatives of polyhydroxy alcohols composedof carbon, hydrogen and oxygen • General formula: C n (H 2 O) n • “Hydrates of carbon” CLASSIFICATION OF CARBOHYDRATES • Classified according to number of saccharide units present in the carbohydrate molecule • Saccharide unit - simplest form of carbohydrate MONOSACCHARIDES • Simpliest carbohydrate • Generally have 3-6 carbon atoms. • May be aldose or ketose o Aldose – terminal aldehyde carbonyl group o Ketose – with ketone carbonyl group in between two other carbon atoms • Characterized by the number of atoms: o Triose – 3 carbons o Tetrose – 4 carbons o Pentose – 5 carbons o Hexose – 6 carbons • Nomenclature depend on carbonyl & number of atoms COMMON MONOSSACHARIDES • Most common and medically important monossacharides • Glucose o Most abundant monosaccharide o Referred to when blood sugar is measured o Also known as dextrose (D-glucose) ▪ Dextrorotary - the hydroxyl group is on right • Galactose o Stereoisomer of glucose ▪ Same component but differ in arrangement o Can form lactose with glucose o Galactosemia – inability to metabolize galactose • Fructose o Fruit sugar o Can form sucrose with glucose o Found in honey o A ketose o Found in seminal fluid DISACCHARIDES • Composed of two monosaccharides • Joined together by glycosidic linkage. o α-glycoside – glycosidic linkage oriented down o β-glycoside – glycosidic linkage oriented up • Sucrose o Glucose + Fructose o Table sugar o Bound by α-glycoside • Lactose o Glucose + Galactose o Principal disaccharide found in milk (human and cows) o Bound by Bound by β-glycoside • Maltose o Glucose + Glucose o Used in beer breweries o Came from malt POLYSACCHARIDES • Composed of 3 or more monosaccharides • Oligosaccharides – composed of 3-10 monosaccharides • Cellulose o Found in cells of nearly all plants o Cannot be digested by humans o Unbranched polymer composed of repeating glucose units joined by 1- 4 β glycosidic linkage
• Glycogen o Major form in which polysaccharides are stored in animals o Stored in liver and muscles o Branched polymer ▪ Straight chains, some glucose units branch ▪ Straight chain - 1-4 α-glycosidic linkage ▪ Branching - 1-6 β glycosidic linkage • Starch o Repeating glucose units joined in a α-glycosidic linkage o Main carbohydrate found in seed of roots and plants o Two major forms: ▪ Amylose ▪ Amylopectin o Amylose ▪ 20% of starch molecules ▪ Unbranched, helical in shape o Amylopectin ▪ 80% of starch molecules ▪ Branched GLUCOSE • Primary energy source for human. • The only carbohydrate to be directly used for energy or stored as glycogen o No need for conversion • Does not freely enters into the cell. • Brain is completely dependent on glucose for energy production o 1⁄2 to 2/3 of glucose utilization in resting adults occurs in the CNS. • Metabolism generates pyruvic acid, lactic acid and acetylcoenzyme A • Complete oxidation yields carbon dioxide, water and adenosine triphosphate (ATP) METABOLISM OF GLUCOSE PATHWAYS OF GLUCOSE METABOLISM • Glycolysis – metabolism of glucose to pyruvate or lactate for energy production; synthesizing ATP • Gluconeogenesis – formation of glucose-6-phosphate from non-carbohydrate sources • Glycogenolysis – breakdown of glycogen to glucose for use as energy • Glycogenesis – conversion of glucose to glycogen for storage • Lipogenesis – conversion of carbohydrates to fatty acids • Lipolysis – decomposition of fat PANCREAS • Both an exocrine and endocrine gland in the control of carbohydrate metabolism • Exocrine o Produce and secretes amylase responsible for the breakdown of ingested complex carbohydrates • Endocrine o Secretes hormones’ insulin, glucagon and somatostatin from different types of cells found in the islets of Langerhans.
INSULIN • The primary hormone responsible for the entry of glucose into the cell. • Promotes cellular uptake of glucose by enhancing membrane permeability to cell in the liver, muscle and adipose tissue • Synthesized by the β-cells of islets of Langerhans. • Normally released when glucose levels are high o High glucose in plasma is toxic • The only hypoglycemic agent o Most important hormone in controlling glucose o Only substance that can lower glucose concentration • It promotes glycogenesis, lipogenesis and glycolysis; decreases glycogenolysis • Serum Insulin Measurements o Hemolysis is an interfering factor o Falsely low in the presence of hemolysis because of the presence of insulin-degrading enzyme found in the red blood cell. • Insulin receptors - binding site insulin molecule • When insulin bind to insulin receptor, it will cause activation of insulin receptor by phosphorylation (addition of phosphate molecule) o When phosphorylated, IRS-1 is also activated • Activated IRS-1 will activate PI 3-kinase enzyme • PI 3-kinase will cause phosphorylation of PIP2 to PIP3 • Phosphorylation of PIP2 to PIP3 - reversible by axn of PTEN • PIP 3 will activate Akt (Protein kinase T) o Akt - activate glycogen synthase and insertion of GLUT4 transporter into the membrane • Glycogen synthase - responsible for glycogenesis; convert glucose to glycogen for storage • GLUT4 transporter - allow entry of glucose from outside the cell into inside via passive diffusion o Can only be inserted on the presence of insulin • Legend ; o IRS-1 - Insulin Receptor Substrate 1 o PI 3-kinase - phosphoinositide 3-kinase enzyme o PIP 2 - phosphatidyl inositol diphosphate o PIP 3 - phosphatidyl inositol triphosphate o PTEN - phosphatase and tensin homolog GLUCAGON • The primary hyperglycemic agent o Increase glucose concentration • It is synthesized by the α-cells of the islets of Langerhans. • It is released during stress and fasting states. o Stress can cause hyperglycemia • It enhances catabolic functions during fasting periods; promotes glycogenolysis. • Reference value : 20-50 pg/mL. OTHER HYPERGLYCEMIC HORMONES • Glucocorticoids (cortisol and corticosteroids) o Secreted by adrenal cortex (cells of zona fasciculata) o They decrease intestinal entry of glucose into the cell o Promotes gluconeogenesis and lipolysis • Cathecolamines o Secreted by adrenal medulla (chromaffin cells) o Inhibits insulin secretion. o Promotes glycogenolysis and lipolysis • Growth hormones (somatotropin) o Secreted by anterior pituitary gland (Adenohypophysis) o Decreased cellular entry of glucose o Promotes glycogenolysis and glycolysis • Thyroid hormones o T3 and T4 o Produced by thyroid gland o Promotes glycogenolysis, gluconeogenesis and intestinal absorption of glucose. • Adrenocorticotropic hormone (ACTH) o Produced by anterior pituitary gland. o It stimulates release of cortisol from the adrenal cortex o It promotes glycogenolysis and gluconeogenesis • Somatostatin o Produced by the delta cells of the islets of Langerhans o Also synthesized by the paraventricular and arcuate nuclei of the hypothalamus o Inhibits the action of insulin, growth hormones and glucagon CLINICAL CONDITIONS ASSOCIATED WITH CARBOHYDRATE METABOLISM HYPOGLYCEMIA • Decreased serum glucose concentration, a life-threatening condition. • It results from an imbalance between glucose utilization and production. • Warning signs and symptoms are related to CNS. o Neurogenic - tremors, palpitations, anxiety, diaphoresis o Neuroglycopenic – dizziness, tingling, blurred vision, confusion, behavioral changes • Diagnosed based on Whipple’s Triad : o Low blood glucose level (≤50 or 60 mg/dL) o Hypoglycemia-associated signs and symptoms o Resolution of symptoms after glucose administration • Causes o Alcohol consumption can inhibit hepatic gluconeogenesis and increase glycogen phosphorylase activity, depleting hepatic glycogen stores and resulting in hypoglycemia. o Severe sepsis – decrease glycogen with failure of gluconeogenesis and enhanced glucose utilization o ESRD (End-Stage Renal Disease) – related to defective gluconeogenesis and impaired hepatic glycogenolysis due to poor nutritional status o Children with cortisol and GH deficiency o Alimentary (reactive) hypoglycemia – occurs within 4 hours after eating meal
• Hypoglycemic values : o 65 to 70 mg/dL – glucagon and other hyperglycemic hormones are released into the circulation o ≤ 60 mg/dL – strongly suggests hypoglycemia (series of RBS) o 55 mg/dL – observable symptoms of hypoglycemia appears o 50 mg/dL – impairment of cerebral function begins. HYPERGLYCEMIA • Increased serum glucose concentration (FBS: ≥ 126 mg/dL) • Toxic to beta cell function impairing insulin production. o Beta cell produces insulin o Prolonged/chronic condition cause toxicity to β-cells • Amylase determination - evaluate pancreatic function o STAT test o Pancreas - only organ produces hypoglycemic agent o Damaged pancreas has no beta cell causing lack of insulin leading no hormone decreasing glucose level • Laboratory Findings : o Increased urine and serum glucose ▪ Urine glucose - observed in reaching renal threshold for glucose reabsorption ▪ Renal threshold (glucose) - 180 mg/dL; excess will be released with urine o Increased urine specific gravity (concentration of substances in the urine) o Ketones in urine and serum (ketones = toxic) ▪ Ketones due to lipolysis and gluconeogenesis generating glucose from non-carbohydrate source o Decreased blood and urine pH (acidosis) o Electrolyte imbalance (↓Na + , ↑K + , ↓HCO 3 ) DIABETES MELLITUS • Most common disorder associated with hyperglycemia • A group of metabolic disorders characterized by hyperglycemia resulting from defects in insulin secretion, insulin receptors or both. • FBS of ≥ 126 mg/dL on more than one testing • Glucosuria – occurs when plasma glucose levels exceed 180 mg/dL with normal renal function. • Ketosis – occurs from excessive synthesis of Acetyl-CoA as the body attempts to obtains energy from non-carbohydrate source in the absence of adequate supply of carbohydrate metabolites. o Severe DM: ratio of β-hydroxybutyrate to acetoacetate is 6:1 (normal = 1:1) o Can be reversed by insulin administration o Increased ketones are toxic TYPE 1 DIABETES MELLITUS • Formerly known as: o Insulin Dependent Diabetes Mellitus (IDDM) o Juvenile Onset Diabetes Mellitus o Brittle Diabetes o Ketosis-Prone Diabetes • A result of cellular-mediated autoimmune destruction of the β-cells of pancreas. o Presence of insulin autoantibodies (attack insulin) o 80- 90% reduction in the volume of β-cells is required to induce symptomatic type 1 DM o Destruction cause insulin synthesis deficiency • Individuals with Type 1 DM have insulinopenia (absolute insulin deficiency) • Genetically associated with HLA DR3 and DR4 (located in Chromosome 6 in major Histocompatibility Complex) • Sign and symptoms : polyuria, polydipsia, polyphagia, rapid weight loss, hyperventilation, mental confusion and possible loss of confusion. o Three P (polyuria, polydipsia, polyphagia) o Polyuria - increase in urination; due to polydypsia o Polydipsia - excessive thirst o Polyphagia - excessive eating; due to inability to metabolize glucose for energy o Increased water intake - diluting plasma causing the body to release liquid to maintain homeostasis o Glucose is needed for brain function • More likely to produce ketosis than type 2 DM. o Increased glucose is not metabolized due inability to absorb glucose from insulin deficiency o Ketones are produced from gluconeogenesis forming glucose from non-carbohydrate sources • Complications : microvascular disorders (nephropathy, neuropathy, retinopathy) • High-risk individuals - high titers of multiple autoantibodies: o Glutamic acid decarboxylase (GAD65) – common in adults o Insulin autoantibodies (IAA) – common among children o Insulinoma-associated protein 2 autoantibodies (IA-2) o Zinc transporter 8 autoantibodies (ZnT8) IDIOPATHIC TYPE 1 DM • A form of type 1 diabetes mellitus that has no known etiology (idiopathic) • Strongly inherited • It does not have β-cell autoantibodies and have episodic requirements for insulin replacement (insulin-dependent) TYPE 2 DIABETES MELLITUS • Formerly known as: o Non-insulin Dependent Diabetes Mellitus (NIDDM) o Adult type/Maturity Onset Diabetes Mellitus o Stable Diabetes o Ketosis-Resistant Diabetes o Receptor-Deficient Diabetes Mellitus • Characterized by hyperglycemia due to an individual’s resistance to insulin. o Insulin receptors may be absent or altered; not match with insulin (lock-and-key) • Associated with strong genetic predisposition and not related to an autoimmune disease. o Geneticist’s nightmare o Negative β-cell autoantibodies o Type 2 DM is easily inherited • At risk of developing microvascular and macrovascular complications. • Has milder symptoms that type 1 DM but may result to: o Nonketotis hyperosmolar coma o Severe dehydration o Electrolyte imbalance (↓Na + , ↑K + ) o Increased BUN and Creatinine • If uncontrolled for prolonged time, Type 2 DM may be converted to Type 1 DM ( Glucose destroys β-cell)
• Risk factors : obesity, family history, advanced age, hypertension, lack of exercise, GDM, impaired glucose metabolism. TYPE 1 DM VS TYPE 2 DM TYPE 1 DM TYPE 2 DM Pathogenesis β-cells destruction Insulin resistance Incidence Rate 5-10% 90-95% Onset Any; most common to childhood/teens Any;most common with advancing age, race/ethnicity, hypertension, dyslipidemia, polycystic ovarian syndrome Risk Factors Genetic, autoimmune Genetic, obesity, sedentary lifestyle, polycystic ovarian syndrome, dyslipidemia, and hypertension C-peptide Levels Decreased or undetectable (destroyed β-cell) Detectable Pre-diabetes Autoantibodies (+) Autoantibodies (-) Symptomatology Symptoms develop abruptly Symptoms develop gradually (some may be asymptomatic) Ketosis Common, poorly controlled Rare Medication Insulin absolute Oral Agents • C-peptide levels - substances produced in insulin synthesis • Oral agents - mimics action of insulin to promote entry of glucose inside the cell GESTATIONAL DIABETES MELLITUS (GDM) • Disorder characterized by impaired ability to metabolize carbohydrate usually caused by a deficiency of insulin, metabolic or hormonal changes. • Occurs during pregnancy and disappears after o 30-40% of GDM cases converts to DM within 10 years • A type of glucose intolerance with onset or first recognition during pregnancy o Diabetic women who become pregnant are not included in this category. • Screening should be performed between 24 to 28 weeks of gestation. o 75 grams 2-hour OGTT • Women with GDM should be evaluated 6 to 12 weeks postpartum OTHER TYPES OF DIABETES • Pancreatic disorders/Pancreatectomy o Pancreatectomy - removal of pancreas o Cause inability to produce insulin • Endocrine disorders – Cushing’s syndrome, pheochromocytoma, acromegaly and hyperthyroidism o Cushing’s syndrome - increased cortisol o Pheochromocytoma - increased catecholamines o Acromegaly - increased growth hormone o Hyperthyroidism - increased thyroid hormone o Hormones affected are hyperglycemic • Drugs or chemical inducers of β-cell dysfunction (Dilantin and pentamidine) and impaired insulin action (thiazides, glucocorticoids) • Genetic disorders – Down syndrome (Trisomy 21), Klinefelter’s syndrome (XXY), Rabson-Mendengall syndrome, Leprechaunism, Huntington’s chorea and Turner syndrome o Down syndrome & Klinefelter’s - immune defects o Rabson-Mendengall syndrome & Leprechaunism - severe insulin resistance o Huntington’s chorea - death of brain cells o Turner syndrome - in females, partial or complete deletion of X chromosome • Exocrine disorders – cystic fibrosis, neoplasia, and hemochromatosis. IMPAIRED FASTING GLUCOSE • It is characterized by fasting blood glucose concentration between normal and diabetic value (FBS: 100-125 mg/dL) o Normal : 70 - 99 mg/dL IMPAIRED GLUCOSE TOLERANCE • Characterized by FBS of less than those required for diagnosis of DM, but the OGTT is between normal and diabetic value (2nd Hour OGTT: 140-199 mg/dL) INBORN ERRORS OF CARBOHYDRATE METABOLISM GALACTOSEMIA • Congenital deficiency of one of three enzymes involved in galactose metabolism o Galactose-1-phosphate uridyl transferase (most common deficiency) ▪ Converts galactose-4-epimerase to glucose o Galactokinase (GALK) o Uridine diphosphate galactose-4-epimerase (GALE) • Caused by failure to thrive syndrome in infants do n’t meet recognized standard of growth due to undernourishment • Clinical features : jaundice, hepatomegaly, easy bruisability, galactosuria, E. coli sepsis, cataract, hypotonia, and sensory neural deafness • Detected with newborn screening • Laboratory features : elevated blood and urine galactose • Diagnostic test : erythrocyte galactose-1-PO4 uridyl transferase activity ESSENTIAL FRUCTOSURIA • Autosomal recessive disorder characterized by fructokinase deficiency • Fructokinase catalyzes the conversion of fructose to fructose-1-phosphate • Diagnostic feature: Fructosuria HEREDITARY FRUCTOSE INTOLERANCE • It is a defect of fructose-1,6-biphosphate aldolase B activity in the liver, kidney and intestine o Fructose-1,6-biphosphate aldolase B - important in glycolysis pathway to metabolize fructose • It is also characterized by inability to convert fructose-1-phosphate and fructose-1,6-biphosphate into dihydroxyacetone phosphate, glyceraldehyde-3-phosphate, and glyceraldehydes. • Clinical features : irritability, lethargy, seizures, and hepatomegaly
FRUCTOSE-1,6-BIPHOSPHATE DEFICIENCY • It is a defect in fructose-1,6-biphosphate results in failure of hepatic glucose generation by gluconeogenic precursors such as lactate and glycerol. o fructose-1,6-biphosphate - product of glycolysis • Clinical features: hypoglycemia, lactic acidosis, convulsions, and coma. GLYCOGEN STORAGE DISEASE (GSD) • Autosomal recessive trait. • Result of inherited deficiencies of enzymes that control the synthesis or breakdown of glycogen. • Abnormality in quality or quantity of glycogen. • Cause liver damage : GSD types I, III, IV, VI, IX, 0 o Signs : hypoglycemia, and hepatomegaly • Cause muscular defect : GSD types V, VII o Signs : muscle cramps, exercise intolerance, fatigue and weakness • Von Gierke Disease – the most common GSD, associated with hyperlipidemia • Other GSDs – deficiencies of LDH, PK, phosphoglycerate kinase and mutase TYPE SYNONYM ENZYME DEFICIENT (Tissue affected) CLINICAL FEATURES Ia Von Gierke Glucose-6- phosphatase Hepatomegaly, retarded growth, seizures Ib Glucose-6-phosphate translocase Same as Ia, recurrent bacterial infections II Pompe 1,4-glucosidase Cardiomegaly, infantile death IIIa Cori Forbes Glycogen debrancher enzyme (liver and muscle) Hepatomegaly, muscle weakness, retarded growth, cardiomyopathy IIIb Glycogen debrancher enzyme (liver) Same as IIIa except muscle weakness IV Andersen Glycogen brancher enzyme Cirrhosis, esophageal varices, ascites V McArdle Muscle phosphorylase Myoglobinuria, muscle cramps VI Hers Liver phosphorylase Hepatomegaly, hypoglycemia VII Tarui Phosphofructokinase Pain and stiffness on exertion VIII Adenyl kinase Urinary excretion of cathecolamines IXa Phosphorylase kinase (liver) Hepatomegaly, hypoglycemia, delay in motor developments IXb) Phosphorylase (liver and muscle Hepatomegaly, retarded growth, muscle hypotonia X Cyclic AMP- dependent kinase Hepatomegaly XI Fanconi-Bickel Glucose-transporter 2 Hepatomegaly, rickets 0 Glycogen synthase No hepatomegaly, hypoglycemic symptoms in morning, growth delay CSF GLUCOSE • 40-60% of blood plasma glucose level • Any changes in blood sugar are reflected in CSF approximately one hour later due to lag is CSF glucose equilibrium time. • Blood glucose is collected 60 minutes before lumbar puncture for comparison. • Reference value : o Adult : 40-70 mg/dL o Child : 60-80 mg/dL • Increased : diabetes mellitus • Decreased : bacterial meningitis, tuberculosis, fungal and amoebic meningitis, subarachnoid hemorrhage, systemic hypoglycemia. • Normal CSF-to-glucose ratio : <0.5