Physical and catalytic properties of the isozymes of adenosine deaminase from human red blood cells.

Adenosine deaminase (adenosine aminohydrolase, EC3.5.4.4) has been purified from human erythrocytes using a simple chromatographic procedure. Purified enzyme was obtained from individuals who were homozygous for the principal isozyme (ADA 1) as well as from individuals who were heterogyzous for the major variant (ADA 2-1). Although ADA 1 and ADA 2-1 are electrophoretically distinguishable, they have many common physical and catalytic properties. No significant differences between the two isozymic forms were found in measurements of molecular weight, catalytic activity in the presence of various substrates and inhibitors, pH optimum, turnover number, and stability in conditions of both high and low pH. ADA 2-1 was, however, substantially less stable than ADA 1 with respect to thermal denaturation. These studies support the idea that adenosine deaminase activity in erythrocytes is lower in those individuals who possess the variant form of the enzyme.