LDH in enzymodiagnostics
Enzymodiagnostics is a method for detecting diseases and assessing their severity based on measuring enzyme activity in biological fluids. The main goal of this approach is to establish the precise localization of the pathological process, its stage, and treatment efficacy.
All blood plasma enzymes by origin are divided into three large groups:
- Secretory. Synthesized by cells and purposefully secreted into plasma, where they perform their primary function (e.g., blood coagulation factors).
- Excretory. Produced in exocrine glands and secreted into the GI lumen to aid digestion. They enter the blood only when glands are damaged (e.g., amylase and lipase in pancreatitis).
- Cellular (indicator). Lactate dehydrogenase belongs to this group. These proteins function exclusively inside cells. Normally, their plasma activity is consistently low and results solely from natural cell death and tissue turnover. However, upon cytolysis (membrane destruction) or deep necrosis, a significant release of enzymes occurs, leading to a manifold increase in their blood activity.
LDH isoenzymes and their tissue distribution
The appearance of the cytosolic enzyme LDH in plasma indicates increased cell membrane permeability. However, total enzyme activity does not accurately identify which specific organ has been damaged. To solve this problem, isoenzyme spectrum analysis is used.
Isoenzymes (isozymes) are multiple molecular forms of the same enzyme. They catalyze the exact same reaction but differ in structure, physicochemical properties (specifically, electrophoretic mobility), and, most importantly, tissue representation.
The lactate dehydrogenase molecule consists of two types of subunits: H (heart) and M (muscle). Their various combinations form five isoforms, which are separated by plasma electrophoresis:
- LDH-1 and LDH-2 — most active in the heart muscle and kidneys.
- LDH-4 and LDH-5 — predominate in skeletal muscles and the liver.
- Other variants are present in remaining tissues in various proportions.
Diagnostic interpretation of the LDH spectrum
Determining the isoenzyme spectrum allows not only to establish the fact of tissue damage but also to localize the affected organ. This is a key advantage of isoenzyme analysis over measuring total nonspecific enzyme activity. The normal range of total LDH activity in serum is $55\text{--}140$ IU/L.
A healthy individual exhibits a physiological distribution of isoforms without a pronounced shift in either direction. When pathology develops, the electrophoretic pattern changes:
| Condition | Change in LDH spectrum | Shift characteristics |
|---|---|---|
| Myocardial infarction | Sharp increase in LDH-1 and LDH-2 levels | "Left shift of the spectrum" |
| Hepatitis | Increased LDH-5 level | "Right shift of the spectrum" |
| Skeletal muscle damage | Increased LDH-4 and LDH-5 levels | "Right shift of the spectrum" |
Thus, plasma electrophoresis is performed for the differential diagnosis of heart, liver, and skeletal muscle diseases. For example, in myocardial infarction, LDH activity (due to the LDH-1 fraction) begins to rise $24\text{--}48$ hours after the attack. In poisonings with organochlorine compounds or viral hepatitis, an increase in "hepatic" fractions is observed.
Remembering the distribution of LDH isoenzymes is facilitated by a numerical association: "one" (LDH-1) is the body's main motor (the heart), while "five" (LDH-5) corresponds to the five lobes of the liver. Accordingly, a shift of the spectrum toward the beginning (left) indicates an infarction, while a shift toward the end (right) indicates hepatitis.
Class and exam questions
Why is it important to determine LDH isoenzymes rather than just total activity in myocardial infarction?
An increase in total LDH activity is nonspecific and may be associated with damage to various organs. Isolating the LDH-1 and LDH-2 fractions allows for precise localization of necrosis in the heart muscle, ruling out liver or skeletal muscle pathology.
How soon after a myocardial infarction does LDH appear in the blood?
LDH reacts relatively late: its level begins to rise 24–48 hours after myocardial damage. For earlier diagnosis, other indicator enzymes are used, such as creatine phosphokinase (CK), which rises as early as 3–6 hours.
What does the appearance of cytosolic and mitochondrial enzymes in plasma indicate?
The release of cytosolic enzymes (which include LDH) into the blood indicates reversible increased permeability of cell membranes. The appearance of mitochondrial enzymes (e.g., m-AST), on the other hand, indicates deep necrosis and organelle destruction.
What else to study on this topic
- Creatine phosphokinase (CK) and the diagnostic significance of the CK-MB fraction
- Aminotransferases (ALT and AST) as markers of liver and heart pathologies
- De Ritis ratio: calculation and clinical interpretation
- Excretory blood plasma enzymes (amylase, lipase, trypsin)
- Differences between organ-specific and organelle-specific enzymes
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