Brain adenosine aminohydrolase

AMP aminohydrolase, an enzyme relatively specific for AMP, has been observed in reptiles (44), erythrocytes (38), snail (45), unfertilized fish eggs (46), invertebrates (47), a variety of mammalian tissues (20), and a particulate fraction of pea seeds (48). Evidence suggests that the frog muscle AMP aminohydrolase is located within or just beneath the sarcolemma (49). The rabbit skeletal and heart muscle enzymes were found in the cytoplasm and mitochondria (20, Jfi, 50, 51), while the enzyme of kidneys and gills of freshwater fish was located in the cytoplasmic fraction (52). The enzyme occurs in most areas of the rat (53) and rabbit brain (54). The nonspecific enzyme from several microbial sources deaminates adenosine triphosphate (ATP) and adenosine diphosphate (ADP) as well as AMP (see Section V). 

In general, AMP aminohydrolase specificities have not been thoroughly defined perhaps because of difficulties until recently in obtaining pure enzyme. In addition to AMP and dAMP, the muscle enzyme catalyzes the deamination of V -methyl AMP, iV -ethyl AMP, for-mycin-5 -monophosphate, adenosine-5 -monosulfate, adenosine-5 -phos-phoramidate, adenosine, ADP (133), adenosine-5 -phosphorothioate, and 6-chloropurine 5 -ribonucleotide (124) ATP, GMP, CMP, 2 -AMP, 3 -AMP, 3, 5 -cyclic AMP, 3-iso-AMP, V-methyl AMP, toyocamycin-5 -monophosphate, tubercidin-5 -monophosphate, and 6-mercaptopurine-5 -ribonucleotide are not deaminated (133). The elasmobranch fish muscle, carp muscle, and avian brain enzymes appear to be specific for AMP and dAMP (123, 125, 126). Extracts from pea seed and erythrocytes and the purified calf brain enzyme are specific for AMP (48, 131, 134). 

Muscular work is accompanied by the production of ammonia, the immediate source of which is adenosine 5 -phosphafe (AMP). This facf led to the recognition of another substrate cycle (Ghapter 11) that fimctions by virtue of the presence of a biosynthetic pathway and of a degradafive enzyme in the same cells (cycle A, Fig. 25-17). This purine nucleotide cycle operates in the brain as well as in muscle. The key enzyme 5 -AMP aminohydrolase (AMP deaminase step a, Fig. 25-17) also occurs in erythrocytes and many other tissues. Persons having normal erythrocyte levels but an absence of this enzyme in muscles suffer from muscular weakness and cramping after exercise. ... 

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