Table 1.

Effect of pH on ADA activity. Values are means ± SEM. Because the pH optimum for ADA1 and ADA2 are 7 and 6·5, respectively, ADA assays were carried out at different pH's in an effort to optimize the assay. A 4% decrease in activity was seen in duodenal samples when the pH was shifted from 7·0 to 6·5. In contrast there was an 11·7% increase in ADA activity in the macrophage lysates when pH was shifted from 7·0 to 6·5. It is important to keep in mind that ADA1 accounts for all of the ADA activity in the duodenal samples, but only 70–80% of the activity in the macrophage samples

	pH 7·0	pH 6·5
Duodenum	183·18 ± 0·48	176·33 ± 0·20
Macrophages	42·96 ± 0·03	48·0 ± 1·07