Lecture 10: Biochemical Markers of Cardiac Injury Flashcards
What are the cardiac injury markers?
Lactate Dehydrogenase
Creatinine Kinase
Myoglobin
B-type Natriuretic Peptide (BNP)
What is lactate dehydrogenase and what does it do?
Cytosolic enzyme that maintains glycolysis in anaerobic conditions
It converts pyruvic acid into lactic acid
What kind of test is used to measure lactate dehydrogenase?
Endpoint
Kinetic spectrophotometric assay
Measures rate at 340 nm
What are the organs that produce lactate dehydrogenase?
Heart
Skeletal
Muscle
Liver
Kidney
RBCs
How fast does LDH rise? When does it peak? When does it vanish?
8-12 hrs to rise
72-144 hrs to peak
8-14 days to vanish
What is LDH used for in a clinical laboratory setting?
Used as a cardiac marker - increased when tissues are damaged
However, different tissues contain structurally different versions of LDH
What is the structure of LDH?
The structure of LDH is a tetramer, 4 subunits bounded non-covalently
Subunits designated M (muscle) and H (heart)
Different combination of M and H make the isoenzymes
What are the different enzymes of lactase dehydrogenase? Which isoenzyme is closest to the anode and which isoenzyme is closest to the cathode in electrophoresis?
LD-1 = H4
LD-2 = H3M
LD-3 = H2M2
LD-4 = HM3
LD-5 = M4
H4 is closest to anode on electrophoresis
M4 is closest to cathode on electrophoresis
Where are hybrid LDH isoenzymes found?
Heart
Kidney
RBCs
What are the quantities of LDH isoenzymes from highest to lowest?
LD5 > LD2 > LD3 = LD1 > LD4
How do LD-isoenzymes help you determine the origin of an elevated LDH?
Myocardial infarction: LD1 > LD2 concentration
Liver damage: Only LD5 is high in conc.
Liver + heart damage: LD1 + LD5 are high
What is creatine kinase and what does it do?
A cytosolic & mitochondrial enzyme that catalyzes the formation of phosphocreatine from creatine using ATP
What are the tests used to measure CK?
Measures enzyme activity
1. Creatine phosphate + ADP ➔ (CK) ➔ creatine + ATP
2. ATP + glucose ➔ (hexokinase) ➔ G6P + ADP
3. G6P + NADP ➔(G6PD)➔ phosphogluconate + NADPH + H+
Rate of increase in absorbance at 340 nm is measured
What are the organs producing CK?
All muscles (skeletal, cardiac, smooth)
Placenta
Brain
Kidney
Lung
How fast does CK rise? Peak? Vanish?
Rise: 3-8 hr
Peak: 10-24 hr
Vanish: 3-4 days
What are the pros and cons of CK?
Pros:
1. Easy & widely available
2. Rises rapidly
3. Falls fast enough to use as re-infarction marker
Cons:
1. Because CK is found in skeletal muscle, muscle injury/bruise/contusion can cause an increase in levels, which may lead to false positive
2. Many medications cause a rise
3. After workout, elevated
What is the structure of CK?
Dimer
2 subunits bounded non-covalently
Subunits are M (muscle) and B (brain)
What are the isoenzymes of CK?
CK1 = MM
CK2 = MB
CK3 = BB
What can CK isoenzymes be used for?
Elevated CK-MB can help diagnose cardiac injury
(Replaced by troponin)
What is myoglobin and what does it do?
It is a monomeric protein
Function:
1. Intracellular oxygen carrier for muscle cells
2. During physiologic stress, oxymyoglobin releases oxygen for use in metabolic processes
What are the tests used to measure myoglobin?
Measured by immunoassay that uses monoclonal antibodies and non-radioactive detection techniques
How fast does myoglobin rise? Peak? Vanish?
Rise: 1-3 hr
Peak: 6-9 hr
Vanish: 1 day
What is B-type Natriuretic Peptide (BNP) and what does it do?
Marker of congestive heart failure
Function:
1. Natriuresis - sodium excretion through kidneys
2. Diuresis - increased flow of urine due to increased fluid intake
3. Inhibition of RAAS
4. Inhibition of sympathetic nervous system
When there is stress on the cardiac ventricular wall, BNP is released to counter the fluid overload that ultimately precipitates CHF, hence acts as biochemical marker for CHF
What are some tests for BNP?
BNP (active form):
Chemiluminescent assay + Ab-acridinium ester + Ab-paramagnetic latex particles
NT-proBNP (NT fragment of immediate precursor):
Electrochemiluminescent sandwich immunoassay
What are troponins?
Regulatory proteins that are one of the contractile proteins found in all myofibrils
What are the function(s) of troponin? Mention their subunits.
Troponin-C (TnC) binds Ca
Troponin-I (TnI) inhibits component
Troponin-T (TnT) is a tropomyosin binding component
What are some of the cons that come with a high sensitive troponin assay?
hs-Troponin Assay?
1. Can be elevated due to excessive physical activity which may cause confusion for diagnosis
2. High sensitivity so report in ng/L
What organ produces troponin? Which subunits are different or the same in the body?
Fast and slow twitch skeletal
Cardiac
Cardiac has specific forms of TnI and TnT named cTnI and cTnT
Cardiac and slow twitch TnC are the same
How fast does troponin rise? Peak? Vanish?
Rise: 3-8 hrs
Peak: 24-28 hrs
Vanish:
TnI = 3-4 days
TnT = 5-10 days
What are the pros of using troponin as a biochemical marker?
- Organ specific
- In healthy people, undetectable unlike other markers which means any detection of troponin is pathological
What are the cons of using troponin as a biochemical marker for cardiac injury?
- Cardiac troponin is 100% specific for myocardial cell death but not for ischemic injury
- Other cardiac conditions can increase troponin such as heart surgery, pacemaker insertion, severe tachychardia/brachycardia - Chronic renal failure patients have slightly increased TnT troponin because:
a. Kidney cannot clear TnT
b. Cardiomyopathy occurs due to renal failure - Multiple manufacturers for TnI hence results cannot be compared as RRs are different
