Xanthosine-5 -phosphate aminase

The second step in the conversion of inosinic acid to guanylic acid is the aminolysis of xanthylic acid with glutamine by xanthosine-5 -phosphate aminase [65]. This aminase, isolated fromf. coli B, is inhibited allosterically by adenosine and adenylic acid [320], and it is also inhibited by psicofuranine (9-0-D-psicofuranosyladenine) (LXXIV) [284, 321-326], which apparently is not... 

The related antibiotic decoyinine (angustmycin A, LXXIII) [285] also inhibits xanthosine-5 -phosphate aminase [329], presumably in the same manner as psicofuranine (LXXIV), but less effectively. There is evidence that psicofuranine (LXXIV) and decoyinine (LXXIII) may be interconvertible in cells [329]. 

The aminating enzyme, xanthosine 5 -phosphate aminase, has been purified partially from Aerobader 168, 169), pigeon liver 170), and calf thymus 171). The liver and thymus enzymes required glutamine, but utilized ammonia to a small extent as an aminating agent. Glutamic acid and aspart ne were ineffective. 

The aminase could not be detected in the guanine-less mutant of Aerobacter aerogenes that accumulates xanthosine. The accumulation of xanthosine, instead of xanthosine 5-phosphate, is most probably caused by the action of phosphatases. 

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