In Situ Determination of Sialic Acid on Cell Surface with a pH-Regulated Polymer Enzyme Nanoreactor

Sialic acid (SA) is an important monosaccharide that is involved in incurable cancer immunotherapy. However, it is difficult to detect SA in situ using the existing strategy based on the SA-terminated glycopeptide extraction from the cell lysate. The countermeasures of the bottleneck caused by cell disruption and peptide extraction should be designed based on a "cell-surface attachment and controlled enzymolysis" protocol. Herein, a poly(styrene-co-maleic anhydride-acrylic acid-concanavalin A) (PSM-PAA-ConA) was synthesized and developed as a pH-regulated enzyme nanoreactor after being loaded with sialidase and myoglobin. The nanoreactor showed controllable biocatalysis induced by a cascade enzyme reaction and applied for the in situ detection of SA on a living cell surface. The addition of an acidic solution resulted in a decrease in the size of the nanoreactor and enhancement of its permeability, triggering an "on" state of the SA catalysis. Subsequent pH increase led to increased hydrophilicity of the nanoreactor, increasing its size and resulting in the catalytic "off" state. ConA assisted the cell-surface attachment of the enzyme reactor. Furthermore, SA on the surface of living cancer cells was successfully monitored by the pH-regulated enzyme nanoreactor, demonstrating the feasibility of high specificity in situ analysis for SA. This pH-induced catalytic efficiency control by the enzyme nanoreactor provides a potential platform for functional stimuli-responsive catalytic systems as well as a strategy for in situ analysis of biomolecules on the cell surface.