Colorimetry (chemical Method) - Wikipedia

In enzymatic analysis (which is widely used in medical laboratories) the color reaction is preceded by a reaction catalyzed by an enzyme. As the enzyme is specific to a particular substrate, more accurate results can be obtained. Enzymatic analysis is always carried out in a buffer solution at a specified temperature (usually 37°C) to provide the optimum conditions for the enzymes to act. Examples follow.

Examples

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Cholesterol (CHOD-PAP method)

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1. Cholesterol + oxygen --(enzyme cholesterol oxidase)--> cholestenone + hydrogen peroxide
2. Hydrogen peroxide + 4-aminophenazone + phenol --(enzyme peroxidase)--> colored complex + water[11]

Glucose (GOD-Perid method)

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1. Glucose + oxygen + water --(enzyme glucose oxidase)--> gluconate + hydrogen peroxide
2. Hydrogen peroxide + ABTS --(enzyme peroxidase)--> colored complex[12]

In this case, both stages of the reaction are catalyzed by enzymes.

Triglycerides (GPO-PAP method)

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1. Triglycerides + water --(enzyme esterase)--> glycerol + carboxylic acid
2. Glycerol + ATP --(enzyme glycerol kinase)--> glycerol-3-phosphate + ADP
3. Glycerol-3-phosphate + oxygen --(enzyme glycerol-3-phosphate oxidase) --> dihydroxyacetone phosphate + hydrogen peroxide
4. Hydrogen peroxide + 4-aminophenazone + 4-chlorophenol --(enzyme peroxidase)--> colored complex[13]

Urea

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1. Urea + water --(enzyme urease)--> ammonium carbonate
2. Ammonium carbonate + phenol + hypochlorite ----> colored complex[14]

In this case, only the first stage of the reaction is catalyzed by an enzyme. The second stage is non-enzymatic.

Abbreviations

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• CHOD = cholesterol oxidase
• GOD = glucose oxidase
• GPO = glycerol-3-phosphate oxidase
• PAP = phenol + aminophenazone (in some methods the phenol is replaced by 4-chlorophenol, which is less toxic)
• POD = peroxidase