Interpreting an Enzyme Assay with a Suspect Blank
Students assay catalase from potato extract by measuring O2 bubbles or absorbance of a dye that changes as H2O2 is consumed. They run: (A) extract + H2O2, (B) boiled extract + H2O2, (C) buffer + H2O2, and (D) extract + buffer (no H2O2). Results after 5 min: A shows strong activity, B shows none, C shows a slight signal, and D shows a slight signal that grows over time. The substrate stock was made the previous week and stored at room temperature. One student concludes that boiling denatures the enzyme and that the reaction is enzymatic.
1. Evaluate whether the data support the conclusion that the activity is enzymatic, using the controls.
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Partial support. The boiled-extract control (B) showing no activity is consistent with protein denaturation, and the loss of activity after boiling argues against a purely chemical reaction. However, control C (buffer + H2O2) shows a slight signal, indicating some non-enzymatic H2O2 breakdown or dye instability, so the baseline is not zero. Control D (extract without substrate) showing a rising signal suggests the extract itself contributes to the signal (e.g., turbidity or dye interaction). Therefore the observed activity in A must be corrected by subtracting the C and D backgrounds before attributing it to catalase. The conclusion is supported only after background correction and replication.
2. The substrate was a week old and stored at room temperature. Explain how this could distort results and what should be done.
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H2O2 decomposes slowly, especially with light and warmth, so its effective concentration at assay time is uncertain and lower than labeled. Lower substrate concentration can reduce reaction rate even with unchanged enzyme, mimicking reduced enzyme activity; it also makes the C control's background more variable. Remedy: prepare fresh H2O2, standardize its concentration (e.g., by titration or by measuring absorbance against a known standard), store protected from light and heat, and run a substrate-only control with each batch. Report enzyme activity relative to measured substrate concentration, not nominal concentration. This is a judgment about validity of measurement, not recall of a definition.