Publisher Summary This chapter discusses the chemical significance of adenosine aminohydrolase, 5′-adenylic acid aminohydrolase, adenine nucleoside, nucleotide aminohydrolase, guanine aminohydrolase, and guanosine aminohydrolase. The partially purified adenine aminohydrolase from Azotobacter vinelandii catalyzes the anaerobic conversion of adenine to hypoxanthine without addition of dialyzable cofactors (76). The kinetic behavior of enzyme from Candida utilis and Saccharomyces cerevisiae was similar to the A. vinelandii preparation. Homogeneous preparations of adenosine aminohydrolase have been obtained from mucosa of calf duodena duodena, calf serum, and calf spleen. Further, several preparations for 5′- activated protein kinase (AMP) aminohydrolase have been described, which yield homogeneous enzyme. Of these the method utilizing the direct absorption of AMP aminohydrolase from a crude rabbit muscle extract with cellulose phosphate and its consequent elution provides a one-step purification of ultracentrifugal and electrophoretically homogeneous enzyme in high yields. Partially purified preparations of guanine aminohydrolase have been reported from rabbit liver rat liver, Clostridium acidurici , rat brain, and lingcod muscle. The rat brain enzyme occurs in both the mitochondrial and supernatant fraction; the latter fraction yielded two forms, A and B from diethylaminoethyl-cellulose, which were subsequently purified 70- and 600-fold, respectively. Kinetic, immunochemical, and electrophoretic studies revealed that the mitochondrial enzyme was distinct from supernatant enzyme B. A distinction between the supernatant A and B forms was less certain.