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Formation of an Adenyl Xanthosine Monophosphate Intermediate by Xanthosine 5'-Phosphate Aminase and Its Inhibition by Psicofuranine

From the ¹ From the Department of Microbiology, University of Southern California School of Medicine, and the Los Angeles County General Hospital, Los Angeles, California 90033

The amination of xanthosine 5'-phosphate and its inhibition by psicofuranine was examined with substrate amounts of purified xanthosine 5'-phosphate aminase from Escherichia coli. In the absence of NH₃, incubation of the enzyme with adenosine triphosphate-8-¹⁴C and XMP-8-¹⁴C for 10 min results in the conversion of ATP to AMP without concomitant formation of guanosine monophosphate. Shorter periods of incubation permit the accumulation of an electrophoretically distinct intermediate which contains radioactivity derived equally from ATP-8-¹⁴C and XMP-8-¹⁴C. The formation of the intermediate is accompanied by the formation of an equivalent amount of inorganic pyrophosphate. The intermediate is cleaved in the presence of NH₃ to yield AMP and GMP or in the absence of NH₃ to AMP and XMP. Psicofuranine does not inhibit the hydrolytic cleavage of the intermediate to AMP and XMP. In contrast, the psicofuranine-inhibited aminase cannot catalyze the aminolysis of the preformed intermediate to AMP and GMP, nor can it condense ATP and XMP to form the intermediate despite its undiminished ability to bind both of these substrates.

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