Abstract
A rapidly recording stopped-flow apparatus has been used to measure the rate of reactions catalysed by carbonic anhydrase. The method allows high concentrations of enzyme to be used so that the contributions to the observed rate by the non-catalysed and buffer-catalysed rates become unimportant. The rates observed were proportional to the enzyme concentration up to the highest concentrations studied, about one tenth of that found in the red blood cell. It was shown that under physiological conditions the enzyme is largely in the form uncombined with substrate. The activity of human carbonic anhydrase at physiological temperature, pH, ionic strength and substrate concentration was measured directly. From this it can be estimated that the enzyme in the red cell increases the rate of CO 2 output from bicarbonate by about 13 000 fold. This figure may be compared with the 700-fold increase necessary for CO 2 evolution during the transit of the blood through the lung capillaries.
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