L4 Clinical Chemistry

🧠 Concepts Introduced

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Learning outcome: To describe the clinically important enzymes and isoenzymes To explain the enzyme level changes in certain disorders

Cellular localisation of circulating enzymes

Membrane-bound enzymes: ALP and GGT

Cytoplasmic enzymes: LDH and ALT

Mitochondrial enzymes: AST isoenzymes, CK isoenzymes

isoenzyme - structurally different but perform the same functions

Secreted enzymes: amylase & lipase Plasma-specific enzymes: thrombin

ALT, alanine aminotransferase AST, aspartate aminotransferase ALP, alkaline phosphatase CK, creatine kinase GGT, gamma-glutamyltransferase LDH, lactate dehydrogenase

Key Definitions & Concepts

Isoenzymes: These are structurally different forms of an enzyme (encoded by different genes or modified post-translation) that catalyze the same biochemical reaction. However, they may differ in physical properties, tissue distribution, or kinetic parameters, which helps doctors pinpoint which specific organ is damaged.

Glossary of Key Enzymes Listed

ALT (Alanine Aminotransferase): A cytoplasmic enzyme found predominantly in the liver; highly specific for liver injury.
AST (Aspartate Aminotransferase): Found in both the cytoplasm and mitochondria of various tissues, notably the liver, heart, and skeletal muscle.
ALP (Alkaline Phosphatase): A membrane-bound enzyme found largely in the liver, bones, placenta, and intestines.
CK (Creatine Kinase): Found in tissues that consume ATP rapidly, especially skeletal muscle, heart muscle, and the brain.
GGT (Gamma-Glutamyltransferase): A membrane-bound enzyme heavily concentrated in the liver and biliary system; sensitive to hepatobiliary disease and alcohol consumption.
LDH (Lactate Dehydrogenase): A cytoplasmic enzyme found in almost all major body tissues, serving as a general marker of cell damage or hemolysis.

Regulation of Serum Enzyme levels

In a healthy individual, enzymes primarily function inside cells (intracellularly) or in specific tissues. Small amounts of these enzymes enter the blood plasma as a result of normal cell turnover and maintenance. Consequently, normal serum enzyme levels reflect a steady state between tissue release and reticuloendothelial clearance. When tissue damage occurs or cellular processes are altered, this equilibrium is disrupted, leading to measurable changes in serum enzyme activity.

Factors affect tissue enzyme levels

rate of synthesis = Higher gene expression or protein synthesis within target cells increases intracellular enzyme concentration, providing a larger pool that can potentially leak into the plasma.
rate of cell turnover = Tissues undergo constant physiological cell death (apoptosis) and renewal. Tissues with high turnover rates release more baseline intracellular enzymes compared to tissues with low turnover rates
rate of release = Dependent on cell membrane permeability. Intact membranes keep enzymes contained; compromised membranes allow them to diffuse out.
status of activation = Some enzymes are synthesized as inactive precursors (zymogens) or require cofactors/post-translational modifications to become active. Measured serum enzyme levels depend on active enzyme activity rather than just physical presence.

Mechanisms of Tissue Damage & Induced Release

DAMAGE & INDUCED RELEASE

Hypoxia (ischemia) = Lack of oxygen deprives cells of ATP, leading to failure of membrane transport pumps (Na+/K+ -ATPase), cell swelling, membrane disruption, and enzyme leakage.
Chemical/drug toxicity (heavy metals, alcohol, paracetamol overdose) = Substrates like heavy metals, alcohol, or paracetamol (acetaminophen) metabolites directly damage cellular membranes or cause oxidative stress.
Trauma (extreme cold, heat, radiation) = damages structural integrity
Infection (bacteria, parasites) = produce toxins or directly lyse cells.
Immune-mediated (allergic reactions, autoimmune) = trigger complement activation and cytotoxic cell lysis.
Genetic defects (muscular dystrophy) = lead to chronic membrane fragility and continuous enzyme leakage into serum.
Nutritional deficiencies (vitamins, minerals) = impairs metabolic processes, undermining cell membrane structural integrity.
Chemical-induced (alcohol stimulates GGT synthesis in liver) = induce hepatic microsomal enzyme synthesis, notably increasing Gamma-Glutamyltransferase (GGT) levels without requiring cell necrosis
Cancer-induced (leukemic cells produce more LDH)

Enzymes Clearance

Bone marrow, spleen, and liver (reticuloendothelial system, RES) involves receptor-mediated endocytosis and lysosomal degradation by tissue macrophages clears most enzymes in serum

Malfunction/damage in RES results in higher levels of circulating enzymes because they are not cleared efficiently

Unlike hormonal systems, there is no direct negative feedback loop that suppresses tissue enzyme production or release when clearance is impaired.

Kidney Excretion Exception: Amylase (and to a lesser extent, Lipase) is small enough to pass through the renal glomerulus into the urine. Renal impairment can therefore cause elevated serum amylase levels even without pancreatic damage.

Parenchymal vs. Non-Parenchymal Influences

Increased Enzyme Activity Parenchymal influences: Cell death: Hypoxic/ischemic, injury/trauma Increased turnover of cells (liver cell damage, leukemia) Enzyme induction (due to drug effects)

Non-parenchymal influences: Slowed or prevented clearance (due to splenectomy or kidney disease)

Decreased Enzyme Activity Parenchymal influences: Loss of functional parenchyma (end-stage cirrhosis) Genetic defects Synthesis issues (nutritional deficiencies)

Non-parenchymal influences: Clearance issues (lymphatic obstruction) Dilution (after saline infusion) Incorrect collection (wrong tube used in blood collection)

Quantification & Laboratory Analysis

Clinical Utility of Measuring Enzymes

Non-invasive measure of changes to tissue health and function

Small amounts of enzymes in circulation are normal and reflect tissue turnover (normal physiological processes)

Larger amounts of enzymes in circulation indicate tissue damage or stimulation

Units of Enzyme Activity

International Union of Biochemistry (IUB)

Units per Liter (U/L) - common in laboratories
1 U (unit) is defined as the amount of enzyme that catalyzes the conversion of 1 micromole of substrate per minute

International System of Units (SI unit)

The SI unit for enzyme activity is the katal
1 katal refers to the amount of enzyme that converts 1 mole of substrate per second

Comparison of Diagnostic Methodology

Mass Assays (Immunoassays)

Measure enzyme concentration (total amount of enzyme present) Use antibodies and secondary detection systems to bind and detect enzymes in the specimen Measure both active and inactive forms of the enzyme Output is in mass units (ng/mL)

Advantage / Disadvantage:

Can measure specific isoenzymes (CK-MB for myocardial infarction)
Relatively slow (hours)
Relatively expensive (due to antibodies and reagents used)
Used selectively in diagnostics
Non-functional Detection: A elevated reading may include denatured or inactive enzymes that no longer perform biological functions

Direct ELISA: The target antigen (enzyme) attaches directly to the surface plate, and a single labeled primary antibody binds to it to yield a signal upon substrate addition.

Indirect ELISA: The target antigen attaches to the plate, an unlabeled primary antibody binds to it, and a labeled secondary antibody binds to the primary antibody.

Sandwich ELISA: A specific capture antibody immobilized on the surface binds the target enzyme, and a second detection antibody binds a different epitope on the same enzyme. This format offers exceptionally high specificity.

Competitive ELISA: Unlabeled target enzyme in the patient sample competes with a labeled target enzyme for a limited number of antibody binding sites.

Activity Assays

Measure enzyme activity (how much of the enzyme is functional / how fast an enzyme works) Use substrates provided in commercial reagent, with product formation dependent on the active enzyme in the specimen measuring its catalytic reaction rate under controlled laboratory conditions (pH, temperature, optimal substrate concentrations). Measure only active enzyme Output is in activity units (U/L)

can differentiate: design inhibitor to block other isoenzymes eg. block CK-MM & CK-BB to find CK-MB

Advantage / Disadvantage:

Do not differentiate isoenzymes (measure total amylase without distinguishing between pancreatic or salivary isoforms)
Relatively fast (minutes)
Relatively inexpensive (enzymes are usually produced by E. coli etc)
Widely used in clinical settings

Criteria Preferred for Diagnostic Enzymes

Measurable in clinically useful specimens = Must be easily detected in routinely collected, non-invasive or minimally invasive fluids, primarily blood serum or plasma (and occasionally urine).
Adequate concentrations = Present in sufficient baseline or post-injury quantities so that modern clinical chemistry analyzers can accurately measure it without requiring hyper-sensitive, cost-prohibitive technology.
Possesses activity = Retains measurable functional catalytic activity (or recognizable structural epitopes) once released into the bloodstream.
Simple steps to measure = Can be assayed using simple, rapid, and cost-effective procedures suitable for high-throughput automated laboratory analyzer workflows.
Tissue specificity: Specific enough to reflect the change in disease status. Highly concentrated in specific target organs/tissues so that elevated serum levels clearly point to a specific organ's disease or damage (e.g., ALT for liver vs. CK for muscle).
Clearance rate: Slow enough to allow for measurement over a useful time frame, but fast enough to reflect changes in tissue status promptly

Complications in Testing: Macroenzymes

Antibody-antigen complex formation and diagnostic misinterpretation

Tissue damage may release abnormally high concentrations of enzymes from cells into the bloodstream

The surge in enzyme levels may trigger immune response

Anti-enzyme antibodies are produced, potentially leading to autoimmune or other immune-related responses

Form antibody-antigen complex that retains enzyme activity

May cause misdiagnosis in serum enzymes

in detail: Massive Antigen Release: Severe or chronic tissue damage releases abnormally high concentrations of intracellular enzymes into the circulation.

Immune Response: The surge of enzymes acts as an antigenic stimulus, triggering the immune system to produce anti-enzyme autoantibodies.

Complex Formation: The autoantibody binds the enzyme, forming a large antibody-antigen complex (macroenzyme).

Altered Clearance: Because of their large physical size, macroenzymes cannot be efficiently cleared by the Reticuloendothelial System (RES) or filtered by the kidneys.

Diagnostic Misinterpretation: The macroenzyme complex retains its catalytic activity. Consequently, routine activity assays report persistently, unexplained elevated serum enzyme levels (e.g., Macro-Amylase, Macro-AST, Macro-CK) in an otherwise healthy patient, leading to potential misdiagnosis, unnecessary invasive testing, or over-treatment.

Isoenzymes & Clinical Applications

Clinical Profiles of Common Serum Enzymes

Transaminases, ALP, GGT, CK, LDH, Lipase/Amylase, Cholinesterases

Transaminases: ALT (alanine aminotransferase), AST (aspartate aminotransferase): Hepatocellular damage

ALP (Alkaline Phosphatase) and GGT (Gamma-Glutamyl Transferase): Biliary obstruction, excessive alcohol intake (GGT)

CK (Creatine Kinase): Muscle injury

LDH (Lactate Dehydrogenase): Cell turnover (in leukemia)

Lipase & Amylase: Pancreatic damage (exocrine function)

Cholinesterases: Organophosphate poisoning happens when pesticides, herbicides, or nerve agents inhibit the enzyme acetylcholinesterase (AChE). This causes a toxic buildup of the neurotransmitter acetylcholine at nerve synapses, leading to severe overstimulation of the nervous system

Fundamental Properties of Isoenzymes

React on same substrate
Different kinetics
Different Half-lives

Laboratory Techniques for Differentiating Isoenzymes

Immunoassay: Differentiate isoenzymes based on their immunological properties

High-resolution electrophoresis: This method, exemplified by ALP (Alkaline Phosphatase), separates isoenzymes based on their migration rates in an electric field, which is influenced by their size and charge

Activity-measuring methods: These methods use enzyme-specific inhibitors, like pancreatic Amylase, to measure the activity of isoenzymes and distinguish between them

Deep Dive: Clinically Significant Isoenzymes

Aspartate Aminotransferase (AST): Cytoplasmic vs. Mitochondrial distinction Relevant Isoenzymes: Mitochondrial and Cytoplasmic Used to differentiate liver disease etiologies

Cytoplasmic AST is released during mild liver damage

Mitochondrial AST is associated with more severe liver injury, such as in cases of alcoholic hepatitis or cirrhosis, where the mitochondria are damaged

Alkaline Phosphatase (ALP): Tissue sources (Bone, liver, placental, intestinal, Regan) Relevant Isoenzymes: Placenta, Bone, Liver, Intestine, "Regan"/Oncofetal Differentiation of the source of elevated ALP levels

Bone ALP: Elevated in bone disorders such as Paget's disease, bone cancer Liver ALP: Elevated in cholestatic liver disease, biliary obstruction, or hepatocellular injury Placental ALP: Appears during pregnancy, and elevated in ovarian tumour Intestinal ALP: Elevated in intestinal diseases, rarely measured Regan/oncofetal ALP: Germ cell tumours and lung cancer

Creatine Kinase (CK): Molecular forms (MM, MB, BB) and cardiac/skeletal diagnostics Relevant Isoenzymes: Muscle (MM), Heart (MB), Brain (BB) Historically significant. CK-MB is still in use

CK-MB: Previously used to diagnose myocardial infarction (heart attack). CK-MB is released into the bloodstream during heart muscle damage, but troponins are now the 'preferred' marker due to their higher specificity and sensitivity

CK-MM: Used to assess muscle injury or disorders, such as rhabdomyolysis or muscular dystrophy, where muscle breakdown occurs

CK-BB: Mainly found in the brain. Its elevation can indicate brain injury or certain types of cancers like prostate cancer, lung cancer & colorectal cancer. Rare use

Lactate Dehydrogenase (LDH): Relevant Isoenzymes: Many forms (numbered 1-5) with moderate tissue specificity Historically significant, but limited modern use