Adenine aminohydrolase. An investigation of specificity.

In addition to their action upon adenine, the adenine aminohydrolases of Azofobacfer vinelandii and of Candida ufilis have been found capable of acting upon 6-chloropurine, 6-iodopurine, 6-hydrazinopurine, 2-amino-6-chloropurine, 2,6-diaminopurine, and 7-aminothiazolo[5,4-dlpyrimidine. In each case the enzyme catalyzes the replacement by a hydroxyl group of the substituent on position 6 or on the position analogous to position 6 of the purine ring system. Kinetic constants for each of these substrates are reported. Km appears to reflect affinity for the enzyme rather than turnover number, and some of these compounds gave higher turnover numbers than the natural substrate, i.e. adenine. In addition, several purine analogues have been found to act as competitive inhibitors of adenine aminohydrolase. Several structurally related acyclic compounds, including guanylurea, biuret, and biguanide, have also been found to inhibit adenine aminohydrolase competitively. Milk xanthine oxidase also was found to be competitively inhibited by these acyclic compounds. The adenine aminohydrolases purified from Azofobacfer vinelandii and from Candida ufilis do not differ strikingly in those catalytic properties investigated.