Comprehensive Notes on Cardiac Enzymes and Creatine Kinase

Cardiac Enzymes

Enzymes of Clinical Significance

• Acid phosphatase (ACP): Prostatic carcinoma

• Alanine aminotransferase (ALT): Hepatic disorder

• Aldolase (ALD): Skeletal muscle disorder

• Alkaline phosphatase (ALP): Hepatic disorder and Bone disorder

• Amylase (AMY): Acute pancreatitis

• Angiotensin-converting enzyme (ACE): Blood pressure regulation

• Aspartate aminotransferase (AST): Myocardial infarction, Hepatic disorder, and Skeletal muscle disorder

• Chymotrypsin (CHY): Chronic pancreatitis insufficiency

• Creatine kinase (CK): Myocardial infarction and Skeletal muscle disorder

• Elastase-1 (E1): Chronic pancreatitis insufficiency

• Glucose-6-phosphate dehydrogenase (G-6-PD): Drug-induced hemolytic anemia

• Glutamate dehydrogenase (GLD): Hepatic disorder

• γ-Glutamyltransferase (GGT): Hepatic disorder

• Glutathione-S-transferase (GST): Hepatic disorder

• Glycogen phosphorylase (GP): Acute myocardial infarction

• Lactate dehydrogenase (LD): Myocardial infarction, Hepatic disorder, Hemolysis, and Carcinoma

• Lipase (LPS): Acute pancreatitis

• 5’-Nucleotidase: Hepatic disorder

• Pseudocholinesterase (PChE): Organophosphate poisoning, Genetic variants, Hepatic disorder, and Suxamethonium sensitivity

• Pyruvate kinase (PK): Hemolytic anemia

• Trypsin (TRY): Acute pancreatitis

Enzymes and Organ Function

Liver Enzymes

• Alanine aminotransferase (ALT)

• Aspartate aminotransferases (AST)

• γ-glutamyltransferase (GGT)

• Alkaline phosphatase (ALP)

• Lactate dehydrogenase (LD)

• 5-nucleotidase (NTP)

• Glutamate dehydrogenase (GLD)

• Glutathione-s-transferase (GST)

• Serum cholinesterase (CHE) or Pseudocholinesterase (PChE)

Skeletal Muscle Enzymes

• Creatine kinase (CK)

• Aldolase (ALD)

• Aspartate aminotransferases (AST)

Cardiac Enzymes

• Aspartate aminotransferase (AST)

• Creatine kinase (CK)

• Glycogen phosphorylase (GP)

• Lactate dehydrogenase (LD)

Pancreatic Enzymes

• Amylase (AMY)

• Lipase (LPS)

• Trypsin (TRY)

• Chymotrypsin (CHY)

• Elastase 1 (E1)

Creatine Kinase (CK or CPK)

• Molecular Weight: Approximately $82,000$ Da.

• Function: Associated with ATP regeneration in contractile or transport systems.

  • In muscle cells, it's involved in storing high-energy creatine phosphate.

  • During muscle contraction, creatine phosphate is used, producing ATP.

  • This reversible reaction is catalyzed by CK:
    $Creatine + ATP \rightleftharpoons Creatine\ Phosphate + ADP$

Tissue Source

• Highest activities: Skeletal muscle, heart muscle, and brain tissue.

• Smaller quantities: Bladder, placenta, GIT, thyroid, uterus, kidney, lung, prostate, spleen, liver, and pancreas.

Diagnostic Significance

• Elevated CK Level:

  • Cardiac and skeletal muscle disorders: Rhabdomyolysis, acute myocardial infarction (AMI), and muscular dystrophy (Duchenne type).

    • Total CK levels are sensitive but not specific indicators, as CK elevation is found in various other abnormal cardiac and skeletal muscle conditions.

  • Central nervous system disorders: Strokes, seizures, nerve degeneration, and central nervous system shock. Damaged blood-brain barrier.

  • Hypothyroidism, malignant hyperpyrexia, and Reye's syndrome.

  • Serum CK levels and CK/progesterone ratio for diagnosis of ectopic pregnancies.

  • Total serum CK levels as an early diagnostic tool to identify Vibrio vulnificus infection

• CK isoenzyme fractions are more specific indicators of various disorders than total levels.

CK Isoenzymes

• CK isoenzymes are dimers with two subunits: M (muscle) and B (brain).

• Electrophoretic separation:

  • CK-BB (brain type) → CK-1 (migrates fastest to anode)

  • CK-MB (hybrid type) → CK-2

  • CK-MM (muscle type) → CK-3 (slowest mobility)

• In serum, healthy individuals primarily have CK-MM with a bit of CK-MB (less than 6% of total CK). CK-BB is not normally detectable.

• Separation of CK isoforms via high-voltage electrophoretic separation.

  • Isoforms occur following cleavage of the carboxyl-terminal amino acid from the M subunit by serum carboxypeptidase N.

  • Three isoforms for CK-MM and two for CK-MB have been described, but the clinical significance is not well established.

Creatine Kinase Isoenzymes - Tissue Localization and Sources of Elevation

• CK-MM

  • Tissue: Heart, Skeletal Muscle

  • Condition: Myocardial infarction, Skeletal muscle disorder, Muscular dystrophy, Rhabdomyolysis, Polymyositis, Hypothyroidism, Malignant hyperthermia, Physical activity, Intramuscular injection

• CK-MB

  • Tissue: Heart, Skeletal muscle

  • Condition: Myocardial infarction, Myocardial injury, Ischemia, Angina, Inflammatory heart disease, Cardiac surgery, Duchenne-type muscular dystrophy, Polymyositis, Malignant hyperthermia, Reye’s syndrome, Rocky Mountain spotted fever, Carbon monoxide poisoning

• CK-BB

  • Tissue: Brain, Bladder, Lung, Prostate, Uterus, Colon, Stomach, Thyroid

  • Condition: Central nervous system shock, Anoxic encephalopathy, Cerebrovascular accident, Seizure, Placental or uterine trauma, Carcinoma, Reye’s syndrome, Carbon monoxide poisoning, Malignant hyperthermia, Acute and chronic renal failure

CK-MM

• Major isoenzyme found primarily in striated muscle.

• Normal serum levels: 94% - 100%.

• Skeletal muscle: Almost entirely CK-MM, with a small amount of CK-MB.

• Heart muscle activity: Predominantly CK-MM, with some CK-MB (20%).

• Injury to both cardiac and skeletal muscle leads to CK-MM elevations.

• CK-MM may be elevated for up to 48 hours post-exercise.

CK-BB

• Not usually measurable in tissue damage due to small quantity and short half-life (1-5 hours).

• Highest concentrations in the CNS, GIT, and the uterus during pregnancy.

• CK-BB in serum indicates extensive damage to the brain or carcinoma of various organs, such as prostatic carcinoma and other adenocarcinomas.

• Elevated in CNS damage, tumors, childbirth, and the presence of macro-CK (an enzyme-immunoglobulin complex).

CK-MB

• Values greater than 6% of total CK are suggestive of AMI (Acute Myocardial Infarction).

• Suspected AMI:

  • Troponin + CK-MB + myoglobin (sometimes)

• Troponins (I & T):

  • Non-enzyme proteins, more sensitive and specific markers for myocardial damage.

  • Elevate even in the absence of CK-MB elevations.

• Following AMI, CK-MB levels start to rise within 4-8 hours, peak at 12-24 hours, and return to normal within 2-3 days.

• Specificity of CK-MB levels is increased in conjunction with LD isoenzymes and/or troponins.

• Time-activity curves of enzymes in myocardial infarction show that CK, especially CK-MB, increases initially, followed by AST and LD. LD is elevated the longest.

• All enzymes usually return to normal within 10 days. ULN = Upper Limit of Normal.

Unusual CK Isoenzyme: CK-Mi

• Bound to the exterior surface of the inner mitochondrial membranes of muscle, brain, and liver.

• Migrates to a point cathodal to CK-MM as a dimeric molecule with two identical subunits.

• Present in serum in dimeric state and oligomeric aggregates with a high molecular weight ($350,000$ Da).

• Not present in normal serum and not present following MI.

• Incidence: 0.8% to 1.7%.

• Presence does not correlate with any specific disease state but appears to be an indicator of severe illness.

• Detected in cases of malignant tumor and cardiac abnormalities.

Macro-CK

• Migrates to a position midway between CK-MM and CK-MB.

• Largely comprises CK-BB complexed with IgG or IgA.

• Can also describe complexes of lipoproteins with CK-MM.

• Incidence in sera: 0.8% to 1.6%.

• No specific disorder is associated, but it is age- and sex-related, most frequent in women older than 50.

• These atypical forms may be measured as CK-MB, leading to erroneously high CK-MB levels.

Creatine Kinase: Test Methodology

CK Isoenzymes

• Measured via electrophoresis, ion-exchange chromatography, and several types of immunoassays (radioimmunoassay RIA and immune-inhibition methods).

• Immunoassays that measure enzyme mass are more sensitive than activity-based assays.

• Two primary assay methods for enzyme activity:

Assay Enzyme Activity

• Forward Reaction (Tanzer-Gilvarg):

  • Decrease in absorbance at 340 nm is determined.

  • Optimum pH: 9.0.

  • Mechanism:
    $Creatine\ Phosphate + ADP \rightarrow Creatine + ATP$

  • The forward reaction is coupled with the pyruvate kinase-LD-NADH system.

• Reverse Reaction (Oliver-Rosalki):

  • Most commonly performed method.

  • Optimum pH: 6.8.

  • Increase in absorbance at 340 nm is determined.
    $Creatine + ATP \rightarrow Creatine\ Phosphate + ADP$

  • The reverse reaction is coupled with the hexokinase-G-6-PD-NADPH system.

  • The reaction proceeds two to six times faster than the forward reaction, depending on the assay conditions, with less interference from side reactions.

Creatine Kinase: Source of Error

• Hemolyzed serum leads to elevated CK activity.

  • Erythrocytes are devoid of CK but rich in AK activity. Hemolysis greater than 320 mg/L interferes.

  • AK reacts with ADP to produce ATP, which then participates in the assay reaction, causing falsely elevated CK levels.

  • Trace hemolysis causes little, if any, CK elevation.

• Serum should be stored in a dark place because CK is inactivated by light.

  • Activity can be restored after storage in the dark at 4°C for 7 days or at -20°C for 1 month when the assay is conducted using a sulfhydryl activator.

• People who are physically well-trained have elevated CK baseline levels.

• Bedridden patients have decreased CK activity.

Creatine Kinase: Reference Range

• Total CK:

  • Males: 46 to 171 U/L (37°C) (0.8 to 2.9 ukat/L)

  • Females: 34 to 145 U/L (37°C) (0.6 to 2.4 ukat/L)

    • Higher values in males are attributed to increased muscle mass.

  • Enzyme reference ranges are subject to variation depending on the method used and the assay conditions.

• CK-MB: <5% of total CK

• CK relative index (CKI):
  $CKI ($