BIOCHEMISTRY

Biochemical markers of myocardial injury - by K D Mehta

 

Goals of Diagnostic tests:
Differentiate patients with and without AMI.
Laboratory diagnosis of AMI.
Estimate extent of myocardial damage.
Identify coronary reperfusion.
Characteristics of Markers:
High concentration in myocardium.
Not found in other tissues.
Released rapidly and completely.
Released in direct proportion to the damage.
Provide a convenient diagnostic time window.

 

Determining Factors:
- Size
- Solubility
- Specificity for myocardium
- Specificity for irreversible injury 		- Cellular localization
- Release ratio
- Detectability
- Clearance
Biochemical markers of MI:
Enzymatic:
-Creatine Kinase (CK)
-Lactate Dehydrogenase (LD)
-Asparatate Amino Transferase (AST)
Non-enzymatic:
-Cytoplasmic:
-Myoglobin
-Heart Fatty Acid Binding proteins
Non-cytoplasmic:
-Myosin Fragments
-Troponins
Creatine Kinase: Structure
Dimer of two subunits B (Brain) and M (Muscle)
(MW  40,000).
Synthesized in cytosol of myocytes.
Single subunits are enzymatically inactive.

Isoenzymes:
CK-BB /CK-1 – brain, prostate, gut and lung.
CK-MB /CK-2 – heart (25-40% of CK), skeletal muscle.
CK-MM /CK-3 – skeletal and cardiac muscle.

Isoforms: having different isoelectric points
CK-33 – gene product
CK-32 – one lysine residue removed
CK-31 – both lysine removed
CK-22 – gene products
CK-21 – lysine residue removed

 

Other forms:
CK-mt- in heart 15% of total CK.

Macro-CK-in 6% of hospitalized patients (Heat stable).
*Type 1
-complex of CK-1 and IgG or CK-3 and IgA.
-causes false positive in AMI.

*Type 2
-oligomeric mitochondrial CK.
-found in severely ill adults or children with myocardial disease.
-interfere with the assay of CK-2.

Creatine Kinase in MI:
-An initial lag phase.
-Rises within 4 to 8 h, peak at 24 h, and return to normal by 36 to 72 hrs.
-First CK-2 rises, usually 10-25 times.
-Usually CK-2 is < 3-6% of total CK, rises 10-30% after infarction.
-CK-2 mass/ CK activity ratio >2.5 differentiate from skeletal injury.
-12 hourly CK-2 adequate and cost effective.
-Early detection of reperfusion is not possible with total CK.
-CK-2 helps in reperfusion diagnosis.
-CK-2 and CK-3 isoforms are early markers of MI.
-CK-22/ CK21 ratio peaks 1.5 h after injury.
-CK-33/ CK31 ratio peaks within 3 h, > 1 abnormal.
-CK-33/ CK31 ratio peaks within 4-6 h in successful reperfusion, late peaking – unsuccessful/ thrombolysis.

 

Lactate Dehydrogenase (LD):
-A tetramer composed of M (muscle) and H (heart) subunits, MW-34,000.
-LD1 (H4) Heart, erythrocytes, brain, pancreas and kidney.
-LD2 (H3M) Heart.
-LD3 (H2M2) Erythrocyte.
-LD4 (HM3) Liver.
-LD5 (M4) Skeletal muscle.

Other forms of LD:
-LD -X (LDc) – sixth LD isoenzyme composed of 4 X (or C) subunits present in postpubertal human testes.

-LD-6- seventh LD isoenzyme present in severely ill patients.

 

Lactate Dehydrogenase in MI:
-Rises within 12-24 h, peak at 48-72 h, and remain elevated for >7 days.
-Usually 3-4 fold rise as high as 10 fold 86% patients of AMI  LD activity.
-LD-1 has an increased tissue specificity.
-Normally LD-2 is > LD-1 (LD-1 / LD-2 is < 0.76).
-Ratio > 0.76 suggestive of AMI, value >1.0 improves specificity (Flipped ratio).
-Persistent abnormality in LD-1 / LD-2 ratio indicates reinfarction.
-Intravascular or extravascular haemolysis  LD-1 and LD-2 levels.

 

Aspartate Amino Transferase:
-Highest concentration in heart, also present in liver, skeletal muscle, kidneys and brain.
-Elevated in 90% patients of MI, nonspecific.
-Rises within 6-8 h, peak at 24-48 h, and return to normal by 4-6th day.
-Extent of myocardial damage appears to parallel peak values reached from release from myocardium after infarction.

Myoglobin:
-A heme protein abundant in heart and skeletal muscle (MW17,800).
-Sensitive but nonspecific marker of AMI.
-Detectable 2 h after AMI, peaks at 3-15 h, return to normal by 24 h.
- levels found in 65% patients of AMI.

Staccato phenomena – a pattern of discontinuous release of myoglobin consisting of multiple peaks which reflects cyclical spontaneous coronary reocclusion and reperfusion of ischemic myocardium.

 

Heart Fatty Acid Binding Proteins:
-Low molecular mass protein (MW 15,000) abundant in the cytoplasm of myocardial cells.
-An important intracellular fattyacid carrier protein.
-Initial and peak elevation earlier than CK-2.
-Sensitivity, specificity and predictive accuracy of H-FABP are higher than myoglobin for AMI within 6 h of onset of chest pain.

Myosin fragments:
-Myosin consists six proteins – 2 heavy chains and 2 pairs of myosin light chain (MLC) I and II.
-2 forms of MLC exist in ventricles and atria and 3 forms in skeletal muscle.
-MLC I and II dissociate from contractile apparatus after cell necrosis.
-MLC detected within 6 h, remains elevated for 7 days or more.
-Prolonged elevations permit retrospective detection up to 2 weeks.

 

Troponins:

-Complex of 3 Troponins TrI, TrT and TrC.
-Isoforms of TrI and TrT in slow-twitch, fast-twitch and cardiac muscle.
-TrC in fast-twitch/ Cardiac.
-Isoforms are product of different genes.
-TrI and TrT are highly specific for myocardial injury.
-Both  in 6 h and remain high for 7 days with peak times 10 - 24 h (biphasic).
-Half-life of TrT is ~ 2 h in blood, remain elevated due to continuous release of the bound form.

AFTER 48 hours of AMI NOT MUCH SIGNIFICANCE

 

Cardiac enzyme release pattern. (A = myoglobin, B = troponin after STEMI, C = CK-MB, D = troponin after NSTEMI.) (from Hollander, et. al, An Emergency Physicians Guide to Cardiac Troponins)

 

 

CHOICE OF MARKER FOR MI
-Molecular size
-Location
-Degradation
-Specificity
-Sensitivity and utility

CK-MB is gold standard

TrT

 

SUMMARY OF A SERUM MARKER AFTER AMI

 

CHARACTERISTICS OF SERUM MARKER


Early Appearance: - CK-MB, Troponin & Myoglobin

High specificity: - CK-MB, TrI, TrT

Wide diagnostic window: – Troponin LDI, Myosin L.C.

Predicts repurfusion: - Myoglobin, CK-MB

Indicator reinfarction: - CK-MB