Specific activity of purified Carbonic Anhydrases. Specific activity of purified, His-tagged carbonic anhydrase enzymes. Substrates used for this assay were bicarbonate (50 mM, dark bars) and CO2 (saturated solution, grey bars). CA activity was measured by comparing the rapid change of pH in the presence of CA to the slower pH change in the absence of CA. The pH change was monitored using phenol red and the absorbance A557nm was measured for 60 sec. Average values from three experiments were plotted with error bars representing the standard deviation.