When running an activity assay for LDH we measure

Question

BlackTom AI · Accepted Answer

In an LDH activity assay, the enzymatic reaction converts lactate to pyruvate with the concomitant oxidation of NADH to NAD+. This means that NADH is consumed over time, leading to a decrease in NADH levels that can be monitored spectrophotometrically (often at 340 nm).

Option 1: The disappearance of NADH. This is the correct concept for typical LDH activity assays, because the rate of NADH decrease reflects LDH activity as lactate is oxidized to pyruvate.

Option 2: The appearance of Pyruvate. While pyruvate is indeed produced by the LDH-catalyzed reaction, the assay most commonly tracked is the decline in NADH, not the appearance of pyruvate. Therefore this option describes a product change rather than the measurable signal used.

Option 3: The appearance of NADH. This would imply NADH is being formed, which is opposite to the actual chemistry of the LDH-catalyzed reaction in a standard forward assay (lactate to pyruvate) where NADH is consumed.

Option 4: The disappearance of Pyruvate. Pyruvate is formed, not disappeared, in the LDH-catalyzed reaction under typical assay conditions; observing a loss of pyruvate would not reflect LDH activity in this setup.

Overall, the key point is that LDH activity assays monitor the consumption of NADH rather than the appearance or loss of pyruvate, making the disappearance of NADH the relevant signal.

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