Clinical Chemistry - Liver and GI Function, Lipid Metabolism, and Cardiovascular Disease Flashcards
Liver Vessel Anatomy
Hepatic Artery - Blood in from Aorta
Portal Vein - blood in from GI tract (to detoxify blood prior to entering systemic circulation)
Common Bile Duct - transports bile to Gall Bladder
Kupffer Cell
Stationary liver macrophages that “filter” portal (GI) blood of bacteria, debris, foreign proteins, toxins, etc.
Hemoglobin Processing
RBCs broken down in reticuloendothelial system (spleen)
- Fe transported by transferrin to marrow/storage
- Globin proteins recycled to amino acids
- Heme oxidized to biliverdin -> biliverdin reduced to bilirubin -> bilirubin bound to albumin and transported to liver
Bilirubin conjugated to glucoronide and excreted into canaliculi to gb/duodenum
GI Metabolism of Bilirubin
Conjugated bilirubin is metabolized by intestinal flora to urobilinogen (colorless). Urobilinogen is then oxidized (80%) to urobilin which gives stool brown color
Block of bile duct
Blockage causes back up of bilirubin, not allowing formation of urobilin. Patients present with pale stool. Can be sign of pancreatic tumor.
Bile Acids
Formed by metabolism of cholesterol - can be administered to lower cholesterol
Emulsifies dietary fat for lipase hydrolysis.
Increased in liver dz, but test is inefficient (dz can be indicated by other tests and bile acids offer no other dx info)
First Pass Metabolism
Portal vein brings absorbed molecules to liver prior to general circulation. Liver is able to metabolize toxins before being released to rest of the body.
Liver Function Test (LFT)
"Hepatic function panel" Total Protein Albumin AST ALT ALP Total bilirubin Direct bilirubin
Total Protein
Biuret reaction: Cu reacts with peptide backbone in alkaline medium
RR: 6.0-8.0 mg/dL
Increased: dehydration, increased synthesis (MM), extended tourniquet time
Decreased: low synthesis (liver dz, malnutrition), increased loss (kidney dz, burns,)
Albumin
Bromcresol green or purple rxn: BCG overestimates in hemolysis, BCP underestimates in renal dz&bilirubinemia
RR: 3-5-5.0 gm/dL
Increased: dehydration, tourniquet time
Decreased: liver dz, renal, GI, burns, nutritional deficit
Aminotransferases
ALT & AST
Enzymes found within hepatocyte involved in amino acid metabolism.
Not specific for liver function but a marker of cellular integrity.
Measured by kinetic coupled enzyme rxn
ALT
Alanine aminotransferase
Kinetic assay measuring decreased absorbance at 340 nm (NADH -> NAD+)
Somewhat liver specific
AST
Aspartate aminotransferase
Kinetic assay measuring decreased absorbace at 340 nm (NADH -> NAD+)
Poor tissue specificity
ALP
Alkaline Phosphatase
Associated with biliary canaliculi - not tissue specific, liver/bone/kidney isoforms (post-trans glycosylation) plus intestinal and placental isoenzymes -> may need source determination if elevated
Biliary obstruction induces ALP synthesis
Not a marker of cellular integrity
Measured by hydrolysis of p-nitrophenyl phosphate at pH 10.3 (increasing rate rxn)
Activity increases 2%/day in fridge
GGT (Gamma-glutamyltransferase)
Not tested for in LFT
GGT in plasma is primarily liver-related - more specific marker of hepatobiliary dz than ALP
Forms of bilirubin
Unconjugated: Circulates tightly bound to albumin to protect against uptake by fatty tissues (brain), poorly soluble in plasma
Conjugated: Circulates freely in plasma, very soluble, mono/di-glucaronyl
Delta: monoglucoronide bound to albumin rather than second glucoronide; results from prolonged bili elevations, cannot be easily measured
Measurement of bilirubin
Jendrassik-Grof method:
Sulfanilic acid reacts with soluble bili to form colored product –> Direct Bili (conj + delta)
Adding caffeine-benzoate enhances solubility of unconj bili –> Total Bili
Assay photosensitive and affected by hemolysis
Pre-Hepatic Jaundice
Increased bili load to liver:
- hemolytic processes/ineffective erythropoiesis (transfusion rx, B12 def.), internal bleeding
- primary unconjugated bilirubin
Hepatic Jaundice
Primary liver defect in conjugating bilirubin due to congenital defects or active liver disease
GILBERTS SYNDROME: benign, defect in ugt1a1 (conj. enzyme), increase in u-c bili
CRIGLER-NAJJAR SYNDROME: conjugation defect (high u-c bili), type 1 fatal (total absence) type 2 not (partial absence)
DUBIN JOHNSON/ROTOR SYNDROMES: defect in transport of conj bili to bile; increase in conj bili, benign
Neonatal Jaundice
Normal for short time after birth due to high RBC turnover and glucoronyl enz immaturity
- KERNICTERUS: increase of u-c bili can cross into brain when levels exceed albumin binding capacity, death/brain injury
- measured by spec in babies (vitA/carotene interferes in adults) or POC device through skin (skin color interference)
- Bhutani Nomogram shows risk zones of bili vs. age
Post-Hepatic Jaundice
Decreased excretion of conj bili to bile
- increase in direct bili, pale stool (low urobilinogen), bilirubinuria (filtered by kidney)
- intrahepatic due to liver dz
- extrahepatic due to physical block of biliary ducts (gall stones,tumors) –> CHOLESTASIS “Obstructive Jaundice”
Ammonia
Liver responsible for getting NH3 from protein metabolism and GI bacteria
- converts to urea in urea cycle
- impaired by hepatitis & cirrhosis, increased by GI bleed or severe liver failure
- measurement: glutamate dehydrogenase converts nh3, a-KG, NADPH to glutamate and NADH (dec. abs at 340nm)
- perform stat due to instability, sample iced, beware of airborne contamination (cleaner)
- very small amounts, measured in mcmol
Acute Hepatitis
Inflammation of liver affecting hepatocytes
- elevated ALT, AST levels -> ALP, GGT elevations mild, bili high in prolonged dz
- ALCOHOLIC: lower AST, ALT than in viral (peaks 50-125, 100-300)
- VIRAL: AST:ALT less than 1, levels high (peaks 300-800, 400-1200)
- ISCHEMIC/TOXIC: hypoxic tissues causes very high AST:ALT (>1) peaks 100-10k, 800-6000, prolonged coag tests
Obstructive Jaundice (Cholestasis)
Lower levels of AST, ALT with progressive increase in ALP & GGT
Direct bilirubin increase parallels ALP/GGT rise
Caused by bile duct obstruction (stones), intrahepatic hepatitis, tumors/metastases, infiltration (amyloid, leukemia)
Chronic Hepatitis
Viral (hep B/C)
NASH (Non-Alcoholic Fatty Liver dz): common in patients with metabolic syndrome
Hemochromatosis: increased iron uptake
Wilson’s Ds: copper accumulation
Cirrhosis
Liver fibrosis due to chronic hepatits or cholestasis –> portal hypertension (fluid buildup, GI bleeds), edema, decreased LFT
Scarring -> fibrous tissue -> contracts and increases resistance
Pancreas - Endocrine function
“secrete into bloodstream” - Islets of Langerhans
- Insulin: lowers glucose
- Glucagon: raises glucose
- Somatostatin: down-regulates others
- Pancreatic polypeptide: multiple functions
- Gastrin: pathologically (normally from stomach)
Pancreas - Exocrine function
“secrete into space” - Acinar cells secrete into ducts
- Bicarbonate-rich fluid to neutralize gastric acidity (secretin)
- Digestive enzymes as proenzymes (CKK- cholecystokinin)
Pancreatitis
Inflammation of the pancreas due to autodigestion by enzymes
- reflux of bile or duodenal contents (biliary obstruction, alcoholism, hyperlipid/calcemia/pth
- signs & symptoms: abdom pain, hypovolemia (intra-abdominal fluid), hypocalcemia (lipolysis of peri-pancreatic fat & soap precipitation)
soap: fat + lye to form calcium salts
Amylase
- hydrolyzes 1,4-glycoside bonds in starch
- secreted by pancreatic acinar cells and salivary glands (1/3:2/3)
- Calcium cofactor, Chloride enhances activity
- high in pancreatitis; rises in 5-8 hrs, peaks 12-72 hrs, elevated 3-5 days
Amylase assay
Use artificial substrate, measure kinetically (405-415nm), RR method specific
- P-amylase measured by Ab inhibition of S-amylase (substrate will only be able to bind to p-amy)
- Values 3-5x ULN more specific for pancreatitis
- elevations also from: panc trauma, obstruction/peptic ulcer/gastrities (high p-amy), GU dz (high s-amy), opiates, renal dz
Lipase
- hydrolyzes trigs to 2-monoglycerides
- present in pancreas, stomach, intestine (pancreas 2000x other tissues)
- w/pancreatitis: increases 4-8hrs, peaks 24 hrs, decreases 8-14 days
- peaks first, followed by amylase
Lipase Assays
- hydrolysis of glycerides to glycerol (detected)
- hydrolysis of triolein emulsion with decreasing turbidity
- measured kinetically
- specificity to pancreatic lipase enhanced by addition of colipase and bile salts -> more specific for pancreatic origin if >5x ULN
- greater sensitivity/specificity than amylase so preferred
Cystic Fibrosis
- Autosomal recessive disorder resulting in altered mucus secretion in lungs and pancreas
- CTFR mutation: abnormal Na/Cl transport across membranes
- May present as: intestinal obstruction (newborns), recurrent pulmonary infections (child), malabsorption (adults)
Pancreatic tumors
- Insulinoma: severe hypoglycemia (endocrine)
- Glucagonoma: hyperglycemia, lipolysis
- VIPoma: vasoactive intestinal polypeptide, severe diarrhea (pancreatic cholera)
- Zollinger-Ellison syndrome (gastrinoma): HCL hypersecretion, recurring peptic ulcer
- Adenocarcinoma (panc. cancer): CA 19-9 non specific marker, 5yr survival 6%, high fatality
GI Function: Stomach
Secretion of:
- HCL: vagus nerve stimulated by sight/smell of food, gastrin secreted when full (gastric distension)
- Pepsinogen: inactive zymogen converted to pepsin when pH
GI Function: Intestine
Small Intestine:
- Duodenum: addition of digestive fluid
- site of most digestion and nutrient absorption
- carbs as monosaccs, proteins as AAs and dipeptides, lipids as glycerol/fatty acids/cholesterol
Large Intestine:
- Primary site of water absorption
Stomach Disorders
- Gastric ulcers: NSAIDS (decrease mucus production), H. pylori attaches to mucosal cells
- Zollinger-Ellison: gastrinoma leads to excess acid and diarrhea
- Stomach cancer: 6x incidence w/ H. pylori (Ab tested in serum, Ag in stool, Urea breath test w/labeled CO2 pill to indicate active urea hydrolysis in stomach)
Intestinal Disease
Maldigestion: decreased food breakdown, lack of HCl, cholestasis, pancreatic insufficiency
Malapsorption: decreased absorption in small intestine, can arise from maldigestion, loose/greasy stools
- Xylose absorption test: non-metabolizable carb given orally, tested in urine and blood to determine malabsorption
Celiac Disease
- Auto-immune ds of small intestine: Ab against gliadin (gluten protein)
- diagnosed by biopsy or serology*: anti-tissue transglutaminase (TTG-IgA) -> 10% patients have IgA deficiency that can give false neg (total IgA done first)
Inflammatory Bowel Disease (IBD)
autoimmune w/ genetic component
Ulcerative Colitis:
- limited to colon (lg bowel), mucosal lining
- present as pain & diarrhea
- measured Atypical pANCA -> ind. immunofluorescence
Crohn’s Disease
- commonly sm. bowel but can affect all of GI tract, entire tract wall
- pain & diarrhea plus malabsorption
- measured ASCA -> EIA for IgG/IgA
Colorectal (Colon) Cancer
- 2nd leading cause of cancer death
- Fecal occult blood: Guaiac method detects peroxidase activity of Hgb, Immunochem method detects hgb with antibodys
- Carcinoembryonic Antigen (CEA): tumor marker used to monitor disease not used for dx
Lipoproteins: Chylomicrons
Exogenous Pathway
- produced in the intestine from absorbed lipids (gotten from diet)
- trigs hydrolyzed by pancreatic lipase into glycerol & fatty acids -> intestinal cells re-form trig plus apo B-48 (truncated B-100 VLDL protein)
- transferred to blood via sup vena cava & lymphatic system, hydrolized by endothelial lipoprotein lipase in muscle & fat tissues, remnants taken up by liver
- large and scatter light: post-prandial turbidity
- LIGHTEST OF LPs -> CREAM LAYER
Lipoproteins: VLDL
Endogenous Pathway
- Trig-rich lipoprotein with B-100
- Produced in liver from hepatic synthesis of trigs following excess intake of carbs/satfat/transfat
- Trigs hydrolyzed by ELL in muscle/fat, become IDL before being converted to VLDL by hepatic lipase
- large and scatter light like chylomicrons but DO NOT float in stored plasma
Lipoproteins: LDL
Bad Cholesterol
- considered atherogenic (hardening of arteries)
- formed from the removal of trigs from VLDL
- LDL receptors for uptake by liver and peripheral cells
- small size allows penetration into EC space of blood vessel walls -> become oxidized, phagocytized by macrophages turn into FOAM CELLS
LDL Cholesterol: Lipoprotein (a)
- LDL particle with apo(a) bound to B-100
- significant homology with plasminogen a fibrinolytic enzyme precursor -> can compete with plasminogin, preventing clot lysis
- genetically determined, responds poorly to therapy
Lipoproteins: HDL
Good Cholesterol
- Liver and intestine synthesize nascent HDL -> accumulates cholesterol esters internally “trapped”, reverse cholesterol transport
- Liver selectively removes cholesterol esters, metabolized to bile acids and secreted to bile
- CETP transfers cholesterol from HDL to LDL, from tissues to liver for metabolism
- higher in women due to high estrogen (lowers post-menopause)
Lipoprotein Metabolism Summary
- Chylomicrons transport dietary trigs from intestine to tissues (lymph, blood, muscle, fat)
- VLDL transports endogenous trigs from liver to tissues for energy or storage (becomes LDL from IDL)
- Cholesterol synthesized in liver is transported to tissues via VLDL -> IDL -> LDL
- HDL picks up cholesterol from peripheral cells/lipoproteins and transports to liver for excretion in bile
Lipid Panel
First line of testing for cv risk
- Trigs
- Total Chol
- HDL Chol
- Calculated LDL Chol
requires 9-12 hr fasting to calculate LDL to eliminate presence of chylomicrons
Trig measurement
Most assays are glycerokinase based w/ varying secondary rxn
Interference from bili, hemolysis, ascorbate (peroxidase rxn)
Serum glycerol induces error (in diabetes, stress, iv, etc) assays where glycerol is blanked available
Cholesterol measurement (total, HDL, LDL)
All assays enzymatic
HDL: precipitation of apo(b) containing lipoproteins with selective blocking reagents, reagents will react w/HDL only
LDL: calculated estimate with Friedewald equation
LDL = Chol - HDL Chol - Trig/5
- trig/5 represents VLDL lps
Direct LDL: selective blocking reagents, no fasting required, expensive
Development of Atherosclerosis
- Endothelial lining of arteries is “invisible” to coag factors
- Hyperlipidemia allows LDL into subendothelium, LDL oxidizes (foam cells) and induces inflammatory process**
- Fibrous cap develops over the plaque (continued proliferation of sm muscle cells) hides endothelial injury
- long standing process, clot forms when endothelial lining ruptures and exposes foci/injury markers
C-Reactive Protein
- APR produced by the liver in response to injury, non-specific but sensitive marker of coronary inflammation
- Prognostic value for coronary artery dz and subsequent coronary syndrome
- measured by nephelometry or turbidimetry
- Independent of cholesterol as risk factor, but related
