Catabolism of Ap 4 A and Ap 3 A in human serum

A hydrolase splitting adenosine (5′)triphospho(5′)adenosine (Ap 3 A) and adenosine(5′)tetraphospho(5′)adenosine (Ap 4 A) has recently been highly purified from human plasma [Lüthje, J. and Ogilvie, A. (1985) Eur. J. Biochem. 149 , 119–127]. This enzyme has been shown to have 5′‐nucleotide phosphodiesterase activity (5′‐NPD). Three isoenzymes splitting Ap 4 A and Ap 3 A were found in human serum by means of native polyacrylamide gel electrophoresis. They exactly comigrated with the 5′‐NPD isoenzymes I, III and IV according to published nomenclature, and were designated Ap 4 Aase isozymes I, III and IV. Their K m values with Ap 4 A as a substrate were 3 μM, 2 μM and 10 μM, respectively. No Ap 4 A splitting activity corresponding to 5′‐NDP‐II was found. Further experiments were designed to prove the identity of Ap 4 Aases with 5′‐NPD isoenzymes. Corresponding isozymes of both activities showed identical behaviour upon delipidation of serum with n ‐butanol: activities I and III were inactivated, whereas IV remained unaffected. Addition of phosphate stimulated Ap 4 Aase and 5′‐NPD isoenzymes I and III, whereas both activities of isozyme IV were inhibited. Further evidence for the identity was obtained when investigating a series of normal and pathological sera showing decreased as well as increased activities of the single isoenzymes. In all cases Ap 4 Aase and 5′‐NPD isoenzymes showed a linear correlation.