Purpose: To develop a tool to measure the pH at the surfaces of individual cells.
Procedures: The SNARF pH-sensitive dye was conjugated to a pHLIP® peptide (pH-Low Insertion Peptide) that binds cellular membranes in tumor spheroids. A beam splitter allows simultaneous recording of two images (580 and 640 nm) by a CCD camera. The ratio of the two images is converted into a pH map resolving single spheroid cells. An average pH for each cell is calculated and a pH histogram is derived.
Results: Surface pH depends on cellular glycolytic activity, which was varied by adding glucose or deoxy-glucose. Glucose was found to decrease the surface pH relative to the pH of the bulk solution. The surface pH of metastatic cancer cells was lower than that of non-metastatic cells indicating a higher glycolytic activity.
Conclusions: Our method allows cell surface pH measurement and its correlation with cellular glycolytic activity.
Keywords: SNARF fluorescence; Tumor acidity; Warburg effect; pH measurements; pHLIP.