6. Determination of Enzyme Activity. Clinical Importance of Enzymes. Examples of enzymes in diagnosis of certain Diseases: Myocardial infarction, Hepatitis, Acute Pancreatitis, Parotitis.
Covers how enzyme activity is measured, including spectrophotometric, manometric, titrimetric and radioisotopic methods, the international unit and specific activity, and the factors that change activity. It then explains why tissue-specific enzymes and isoenzymes are useful in diagnosis, and walks through myocardial infarction, hepatitis, acute pancreatitis and parotitis. A table compares liver enzyme patterns across several liver conditions, and lists of tests for pancreatitis diagnosis, aetiology and severity are included.
Topic 6 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 12 sections, about 909 words (7 min of reading), with 16 flashcards.
In this note
Determination of Enzyme Activity.
Determination of Enzyme Activity
Principle
Measurement Methods
Units of Enzyme Activity
Factors Affecting Enzyme Activity
Clinical Importance of Enzymes
Key Concepts
Examples of Diagnostic Use of Enzymes in Specific Diseases
Hepatitis (Liver Inflammation)
Acute Pancreatitis
Parotitis (Inflammation of the Parotid Gland)
Determination of Enzyme Activity
The activity of an enzyme refers to the rate at which it catalyzes a reaction — that is, how fast substrate is converted into product under specific conditions. Measuring this activity is one of the most important tools in both biochemistry and clinical diagnostics.
Principle
Enzyme activity is determined by measuring either:
The decrease in substrate concentration, or
The increase in product concentration over time.
The rate of the enzymatic reaction is directly proportional to the enzyme concentration — as long as substrate concentration is sufficiently high (saturating conditions).
Measurement Methods
Spectrophotometric Methods
These are the most common methods in clinical biochemistry.
The reaction mixture is placed in a cuvette, and changes in absorbance are recorded by a spectrophotometer.
Example: Measurement of NADH oxidation or NAD⁺ reduction at 340 nm — since NADH absorbs light strongly at this wavelength.
Manometric Methods
Used when a reaction produces or consumes a gas (e.g., O₂ or CO₂).
The volume or pressure change of the gas is measured using a manometer.
Titrimetric Methods
Used when the reaction produces or consumes acids or bases.
The change in pH or the amount of titrant required to maintain a constant pH is
recorded.
Radioisotopic Methods
A radioactive isotope is incorporated into the substrate or product, and the rate of isotope appearance/disappearance is measured.
Used mainly in research, not in routine clinical labs.
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