Blood samples sent for glucose estimation are collected in tubes containing sodium fluoride. The anticoagulant works because fluoride:

• A Inhibits hexokinase, blocking the first phosphorylation step of glycolysis
• B Activates glucokinase so that glucose is locked inside leucocytes rather than metabolised
• C Chelates calcium and also denatures all glycolytic enzymes irreversibly
• D Inhibits enolase, preventing glycolytic consumption of glucose by red cells in the tube ✓

Explanation

Fluoride ions inhibit enolase by complexing with its magnesium cofactor, blocking conversion of 2-phosphoglycerate to phosphoenolpyruvate. Glycolysis halts in the erythrocytes inside the tube, so glucose is not consumed before analysis and the measured level reflects the true in vivo value. Hexokinase inhibition would prevent measurement entirely if performed enzymatically, since most glucose assays rely on hexokinase.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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