1. 1. The addition of substances which induce ATP degradation in Ehrlich ascites tumor cells leads to the extracellular appearance of inosine and hypoxanthine. The accumulation of these metabolites is generally progressive, but with 5 mM glucose the phenomenon is a transient one, the 260 mμ absorbing substances subsequently disappearing from the medium. In experiments with whole cells, [8-14C]-inosine disappearance from the extracellular medium was found to be accompanied by incorporation of the isotopic label into nucleic acids. 2. 2. The utilization of inosine and hypoxanthine, as well as other purines and their ribonucleosides, was investigated. Isotopic experiments with cell-free extracts containing 5-phosphoribosyl 1-pyrophosphate indicated the formation of IMP from hypoxanthine and 5-phosphoribosyl pyrophosphate, AMP from adenine and 5-phosphoribosyl pyrophosphate, GMP from guanine and 5-phosphoribosyl pyrophosphate, but no XMP from xanthine and 5-phosphoribosyl pyrophosphate. Cell-free extracts containing ATP catalyzed the formation of adenine nucleotides from adenine but no purine ribonucleotides from inosine or guanosine. These two nucleosides must first be cleaved by purine nucleoside phosphorylase(s) in order to yield ribonucleotides in reactions involving 5-phosphoribosyl pyrophosphate.
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