Which physical parameter is observed on the phosphatase inactivation curve?

• A. pH
• B. Turbidity
• C. Viscosity
• D. Color - more yellow is bad

Answer: (D)

The main idea is that phosphatase inactivation is tracked using a colorimetric readout of enzyme activity. In many phosphatase assays, the reaction produces a colored product (for example, p-nitrophenol turns yellow when released by phosphatase activity). The inactivation curve is drawn from measuring how this color changes over time or with heat—higher residual activity shows more color, while effective inactivation yields less color. Therefore, color is the observable parameter because it directly reflects how much enzyme activity remains. If more yellow appears, it signals more product formation and thus more enzyme activity, which means the inactivation isn’t complete (hence “more yellow is bad”). pH, turbidity, and viscosity influence or describe conditions or sample properties, but they are not the readout used to quantify remaining phosphatase activity on the curve.