Lactate dehydrogenase (LD) catalyzes the following reaction: Lactate+ NAD+ (in the presence of LD goes to) -> pyruvate + NADH. As the reaction is written, which of the following techniques can be used to assess LD activity?

• A. Measure the colorimetric product pyruvate.
• B. Measure the colorimetric product NADH.
• C. Measure the increase in absorbance at 340 nm as NADH is produced.
• D. Measure the decrease in absorbance at 340 nm as NADH is produced.

Answer: (C)

The key idea is that lactate dehydrogenase activity creates NADH, and NADH has a distinct absorbance at 340 nm. As the reaction proceeds, NAD+ is reduced to NADH, so the rate of increase in absorbance at 340 nm directly reflects how fast LD is working. This makes monitoring the rise in NADH’s absorbance at 340 nm the most straightforward and quantitative way to assess LD activity.

Other approaches are less direct. Measuring pyruvate colorimetrically would require an additional reaction to detect pyruvate, adding steps and potential variability. Measuring a hypothetical colorimetric product of NADH isn’t the standard approach—NADH is best tracked by its absorbance at 340 nm rather than a separate color change. Measuring a decrease in absorbance at 340 nm would imply NADH is being consumed, which isn’t the case here—the product is NADH, not its disappearance.