Validity of the continuous spectrophotometric assay of Kalckar for adenosine deaminase activity

Both adenosine and inosine obey Beer's law to 1.0 mM at 265 nm and pH 7.4 at 25 degrees C. Murphy et al. (1) claimed serious deviation from Beer's law above 200 microM for both substances, and concluded that the assay of adenosine deaminase activity based on recording spectrophotometric change at 265 nm as originally suggested by Kalckar produces anomalous results. The data herein presented show that this is not so, and that the large number of published studies of adenosine deaminase activity assayed by this method are indeed valid and should not be dismissed as artifactual as suggested by Murphy et al.