In the enzymatic method for non-protein nitrogen, most components are measured by a decrease in absorbance at which wavelength?

• A. 340 nm
• B. 450 nm
• C. 600 nm
• D. 293 nm

Answer: (A)

The test relies on oxidation of NADH in an enzymatic sequence. After urease breaks urea into ammonia, the ammonia participates in a reaction that uses NADH and converts it to NAD+. NADH has a strong absorbance peak at 340 nm, so as NADH is consumed the absorbance drops. That decrease tracks how much non-protein nitrogen is present. Other wavelengths wouldn’t reflect this NADH loss: 450 nm is typically for dye-based products, 600 nm is used for other colorimetric indophenol-like assays, and 293 nm is in the UV region with more interference. So monitoring at 340 nm accurately reflects the NADH change and thus the NPN level.