The color developed in this alkaline phosphatase assay occurs under which conditions?

• A. Acidic conditions
• B. Neutral conditions
• C. Alkaline conditions
• D. Variable pH

Answer: (C)

The color in this assay appears because the enzyme alkaline phosphatase works best in alkaline environments and produces a yellow color when the substrate is cleaved. In this test, alkaline phosphatase hydrolyzes p-nitrophenyl phosphate to p-nitrophenol, which becomes the intensely yellow nitrophenolate ion under alkaline conditions. This ion absorbs light in the visible range, giving the measurable color. If the pH were acidic or neutral, the enzyme activity would be reduced and the product would not be in its colored, ionized form, so the color development would be weak or absent. Therefore, color development occurs under alkaline conditions, not acidic, neutral, or variable pH.