๐ฏ Aim: To estimate the concentration of glucose in human serum/plasma using the enzymatic GOD-POD (Trinder’s) method, verify linearity against standard glucose calibrators, and correlate findings with clinical diagnostic reference ranges.
๐ฌ Theoretical Principle & Enzymatic Cascade
The GOD-POD method is the gold-standard enzymatic photometric assay for blood glucose quantification due to its absolute specificity for $beta$-D-glucose over other circulating aldohexoses or reducing sugars:
In the first step, GOD catalyzes the oxidation of glucose to D-glucono-$delta$-lactone with the equimolar generation of hydrogen peroxide ($text{H}_2text{O}_2$). In the second coupled step, POD uses $text{H}_2text{O}_2$ to oxidatively couple 4-aminoantipyrine with phenol, synthesizing a bright pink quinoneimine chromophore. The intensity of color measured at $lambda_{max} = 505,text{nm}$ is directly proportional to glucose concentration.
Mix gently and incubate at $37^circtext{C}$ for exactly 10 minutes (or 20 minutes at $25^circtext{C}$). Measure absorbance of Standard ($A_S$) and Test ($A_T$) against Blank at $505,text{nm}$.
Diagnostic Calculation:
$$text{Serum Glucose (mg/dL)} = frac{A_{text{Test}}}{A_{text{Standard}}} times text{Concentration of Standard (100 mg/dL)}$$
$$text{Example: If } A_T = 0.380 text{ and } A_S = 0.400: quad text{Serum Glucose} = frac{0.380}{0.400} times 100 = 95.0,text{mg/dL (Normal Fasting)}$$
๐ฌ Viva-Voce Questions & Model Answers
Q1: Why is fluoride-oxalate used as an anticoagulant for blood glucose collection?
Sodium fluoride inhibits the glycolytic enzyme enolase by chelating magnesium ions ($text{Mg}^{2+}$), preventing red blood cells from consuming glucose ex vivo (which degrades glucose by ~5–7% per hour at room temperature). Potassium oxalate acts as the anticoagulant by precipitating calcium.
Q2: Why does Glucose Oxidase require the presence of mutarotase in commercial reagents?
Glucose Oxidase is absolutely specific for $beta$-D-glucopyranose. In aqueous solution, glucose exists in mutarotation equilibrium: ~64% $beta$-anomer and ~36% $alpha$-anomer. Mutarotase accelerates the interconversion of $alpha$-D-glucose to $beta$-D-glucose so that 100% of the sugar is oxidized within the assay incubation period.
Q3: How does high-dose Vitamin C (ascorbic acid) cause falsely low readings in GOD-POD?
Ascorbic acid is a potent reducing agent that competes directly with 4-aminoantipyrine/phenol for hydrogen peroxide ($text{H}_2text{O}_2$), scavenging peroxide before peroxidase can generate the pink quinoneimine dye.
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