Kinetic constant changes by hydrogen and phosphate ions in the clam adenosine deaminase

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Abstract 1. 1. Adenosine deaminase in the clam midgut gland was stable at pH 4.0 and 40°C. The V max / K m was varied with temperature but was not in the presence of phosphate at pH 4.5. 2. 2. Phosphate lowered the apparent activation energy from 20 to 15 kcal/mol. The V max was temperature dependent and pH independent, whereas the K m was pH dependent. 3. 3. A significant decrease in K m was observed at pH 6.0 by adding phosphate which had little effect on the value at pH 4.5. 4. 4. The enzyme is competitively inhibited by PCMB. The K i at pH 4.5 (20 nM) was not altered and at pH 6.0 (60 nM) markedly increased by phosphate.

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