You Are Trying to Isolate an Enzyme That Catalyzes the Conversion

You are trying to isolate an enzyme that catalyzes the conversion of A $\rightarrow$ B, and you have a sensitive assay for this enzyme. After lysing open the cells, you find that the activity is associated with the membrane fraction, not the soluble fraction (which you throw away). You then follow the procedure shown below. Explain the sudden re-occurrence of activity at the end of your protocol.