Unit 3: Quantitative Tests

I. Orientation — Colorimetric and Enzymatic Quantitation

Quantitative biochemical tests determine the concentration of a substance by comparing a measurable signal from an unknown sample with signals produced by standards of known concentration. In colorimetric assays, the colored product is measured spectrophotometrically; in enzymatic assays, substrate conversion is coupled to formation of a colored compound.

• Beer–Lambert law: Absorbance is proportional to concentration and path length under suitable conditions.
  
  TEXT

    A = εlc

  
  Here, A is absorbance, ε is molar absorptivity, l is the optical path length in centimeters, and c is concentration.
• Standard curve: A series of known standards is treated like the unknown, and their absorbances are plotted against concentration.
• Blank correction: The reagent blank contains all assay components except the analyte and provides the background absorbance.
• Linear range: Unknown concentrations should fall within the standard-curve range; samples above it require dilution.
• Replicate measurement: Repeated readings help identify pipetting errors, bubbles, precipitates, and instrument instability.
• Basic calculation: For a linear response passing through the origin, concentration may be estimated from the ratio of sample and standard absorbances.
  
  TEXT

    Csample = (Asample / Astandard) × Cstandard

  
  Csample and Cstandard are analyte concentrations, while Asample and Astandard are blank-corrected absorbances.

II. Determination of carbohydrates by Dubois method — Phenol–sulfuric acid assay

A. Purpose and principle

The Dubois method is a broad colorimetric assay for total carbohydrates. Concentrated sulfuric acid hydrolyzes polysaccharides and dehydrates monosaccharides to furfural derivatives, which react with phenol to produce a yellow-orange complex measured near 490 nm.

• Analyte measured: The assay estimates total carbohydrate rather than identifying individual sugars.
• Chemical conversion: Hexoses form 5-hydroxymethylfurfural, whereas pentoses form furfural under strong-acid conditions.
• Color development: Phenol reacts with these dehydration products, producing an intensity approximately proportional to carbohydrate concentration in the working range.
• Typical wavelength: Absorbance is commonly read at 490 nm for hexose-rich samples.
• Reference standard: Glucose is frequently used; results are therefore commonly expressed as glucose equivalents.

B. Determination of carbohydrates by Dubois method

This determination involves preparing carbohydrate standards and unknowns, adding phenol and concentrated sulfuric acid, and measuring the developed color.

• Reagents: Use a carbohydrate standard such as glucose, approximately 5% phenol, concentrated sulfuric acid, and distilled water.
• Tube preparation: Prepare a reagent blank, several glucose standards, and the unknown sample in labeled tubes.
• Reaction sequence: Add the sample or standard, add phenol, then carefully add sulfuric acid down the tube wall or directly according to the laboratory protocol.
• Mixing and heating: Mix consistently; the exothermic acid reaction supplies heat for color development. Allow all tubes to stand for the same time before reading.
• Spectrophotometry: Zero the instrument with the blank and read standards and unknown at approximately 490 nm using the same cuvette orientation.
• Calibration: Plot absorbance on the vertical axis against glucose concentration on the horizontal axis. Use the line equation for the unknown rather than relying only on visual comparison.
  
  TEXT

    A = mC + b
    Cunknown = (Aunknown − b) / m

  
  m is the slope, b is the intercept, and C is carbohydrate concentration.
• Worked example: If the standard curve is A = 0.010C + 0.020 and the unknown absorbance is 0.420, then C = (0.420 − 0.020)/0.010 = 40 concentration units, before applying any dilution factor.

C. Applications and limitations

The Dubois method is useful for total carbohydrate estimation but must be controlled carefully because acid and phenol reactions are hazardous and chemically nonspecific.

• Applications: It is used for sugars in serum, plant extracts, food samples, glycogen preparations, and chromatographic fractions.
• Interference: Other substances that generate furfural derivatives or absorb near 490 nm may contribute to the signal.
• Standard dependence: Glucose, fructose, sucrose, and polysaccharides can produce different color yields; the reported value is therefore standard-equivalent.
• Safety: Concentrated sulfuric acid causes severe burns and must be handled with eye protection, gloves, and appropriate ventilation.
• Quality control: Keep acid volume, phenol concentration, reaction time, and reading wavelength constant because each affects color intensity.

III. Determination of total proteins by Lowry’s method — Copper–Folin colorimetry

A. Purpose and principle

Lowry’s method estimates total protein through two linked reactions: peptide bonds reduce alkaline cupric ions to cuprous ions, and the copper-treated protein reduces the Folin–Ciocalteu reagent, producing a blue color.

• Biuret component: In alkaline medium, peptide bonds coordinate with Cu²⁺; the resulting copper–protein complex contributes to color formation.
• Folin component: Phosphomolybdotungstate in Folin reagent is reduced to a blue-colored product, strongly increasing sensitivity.
• Protein standard: Bovine serum albumin (BSA) is commonly used to construct the calibration curve.
• Measurement: Absorbance is often read around 660 nm, although the exact wavelength depends on the specified Lowry protocol.
• Chemical dependence: Color yield depends on protein composition because tyrosine, tryptophan, cysteine, and peptide bonds contribute unequally.

B. Determination of total proteins by Lowry's method

The test requires alkaline copper reagent followed by Folin reagent, controlled incubation, and comparison with BSA standards.

• Reagents: Typical components include alkaline sodium carbonate, copper sulfate, potassium sodium tartrate, and diluted Folin–Ciocalteu reagent.
• Standard series: Prepare known BSA concentrations, for example 0, 20, 40, 60, 80, and 100 µg/mL, if compatible with the laboratory protocol.
• Alkaline copper reaction: Add the copper reagent to each tube, mix, and allow sufficient time for the copper–protein complex to form.
• Folin addition: Add Folin reagent rapidly and mix immediately because the reagent is sensitive to timing and local concentration.
• Incubation: Incubate all tubes under identical conditions, commonly at room temperature, until the blue color develops.
• Reading: Measure absorbance near 660 nm against the reagent blank and plot absorbance against BSA concentration.
• Sample calculation: Determine the concentration from the calibration equation and multiply by the dilution factor.
  
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    Protein concentration in original sample
    = concentration read from curve × dilution factor

• Worked example: If a diluted sample gives 35 µg/mL from the curve and was diluted 1:10, the original concentration is 35 × 10 = 350 µg/mL, or 0.35 mg/mL.

C. Applications and limitations

Lowry’s method is sensitive and suitable for many protein preparations, but its chemistry is affected by protein composition and interfering reagents.

• Applications: It is used for serum proteins, cell extracts, enzyme preparations, and fractions collected during protein purification.
• Interfering substances: Detergents, reducing agents, chelators, ammonium salts, and strongly buffered solutions can alter copper or Folin reactions.
• Protein-to-protein variation: A BSA calibration curve does not guarantee identical response for every protein; values are best interpreted as BSA equivalents.
• Timing sensitivity: The interval between Folin addition and absorbance reading should be uniform across standards and samples.
• Precipitation control: Protein precipitation or incomplete mixing causes turbidity and falsely high absorbance.
• Improved accuracy: Dilute samples into the standard range and use matrix-matched standards when the sample contains unusual buffers or additives.

IV. Determination of blood glucose by glucose oxidase method — Enzyme-specific glucose assay

A. Purpose and principle

The glucose oxidase method measures blood glucose by enzymatically oxidizing β-D-glucose to gluconic acid and hydrogen peroxide, followed by detection of the peroxide through a peroxidase-coupled color reaction.

• Primary enzyme reaction:
  
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    β-D-glucose + O₂ + H₂O
    → gluconic acid + H₂O₂

  
  Glucose oxidase catalyzes oxidation of glucose, and H₂O₂ is the hydrogen peroxide produced.
• Color-forming reaction:
  
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    H₂O₂ + chromogen(reduced)
    --peroxidase-->
    colored chromogen(oxidized) + H₂O

  
  Peroxidase uses hydrogen peroxide to oxidize a chromogen such as 4-aminoantipyrine with a phenolic compound.
• Specificity: Glucose oxidase is relatively selective for β-D-glucose, making the method more specific than general reducing-sugar tests.
• Measurement: The colored product is commonly read near 505 nm, depending on the chromogen system.
• Result convention: Blood glucose may be reported in mg/dL or mmol/L.
  
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    Glucose (mmol/L) = Glucose (mg/dL) / 18

B. Determination of blood glucose by glucose oxidase method

This determination uses a blank, glucose standard, and blood sample treated with an enzyme reagent and measured photometrically.

• Specimen: Serum or plasma is commonly used. Whole blood must be processed according to the validated procedure.
• Glycolysis prevention: Separate serum or plasma promptly because cells continue consuming glucose after collection. Fluoride-containing tubes may inhibit glycolysis.
• Reaction setup: Add the specified volume of sample, standard, and blank to separate tubes, then add glucose oxidase–peroxidase reagent.
• Incubation: Incubate at the stated temperature and time, such as 37°C for a fixed interval, because enzyme activity and color development are temperature-dependent.
• Absorbance: Read the colored product against the reagent blank at the recommended wavelength, commonly about 505 nm.
• Calculation:
  
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    Glucose concentration
    = (Asample / Astandard) × Cstandard

  
  Asample and Astandard are blank-corrected absorbances, and Cstandard is the concentration of the glucose standard.
• Worked example: If the sample absorbance is 0.36, the standard absorbance is 0.45, and the standard is 100 mg/dL, then glucose is (0.36/0.45) × 100 = 80 mg/dL, equivalent to about 4.44 mmol/L.

C. Applications and limitations

The glucose oxidase assay is widely applied to clinical blood-glucose testing, but specimen handling and chemical interference directly affect accuracy.

• Applications: It supports estimation of fasting or postprandial glucose and monitoring of glucose metabolism in clinical and research samples.
• Oxygen dependence: Because oxygen participates in the glucose oxidase reaction, unusual oxygen conditions can affect results, particularly in highly controlled or miniaturized systems.
• Peroxide interference: Substances that consume hydrogen peroxide or react with the chromogen may produce falsely low or high values.
• Hemolysis and lipemia: Hemoglobin, turbidity, and sample color can alter photometric readings if not corrected by the method.
• Temperature and timing: Enzyme reactions must use identical incubation conditions for blank, standard, and samples.
• Clinical reporting: State the unit clearly; 100 mg/dL is approximately 5.56 mmol/L, and reference interpretation depends on fasting status and the laboratory’s validated range.