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

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.

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.