AbstractTreatment of BALB/c‐3T3 mouse fibroblasts with 3′‐led to a rapid accumulation of 3′‐phosphates and the kinetics of this process has been determined. Concomitant with accumulation of these compounds, the adenine ribonucleotide pool was reduced. The kinetics of the two processes suggested that they were tightly coupled. The inhibitory effect of relatively high concentrations of coformycin indicated that IMP was an intermediate in the catabolic pathway. Similar experiments with Ehrlich ascites tumor cells were performed in Ringer‐Hepes solution at pH 6.5 or 7.5 and with varying concentrations of orthophosphate. The experiments were performed with cells where ATP was [3H]‐. This allowed the determination of the catabolism of adenine ribonucleotides to labeled nucleosides under conditions where added adenosine was phosphorylated. The results showed that at low phosphate concentration (5.8 mM) at pH 6.5 adenosine may be phosphorylated at a rate that was completely balanced to the concomitant catabolism of adenine ribonucleotides; that is, there was apparently a tight kinetic coupling between anabolism of adenosine and catabolism of adenine ribonucleotides. With 3′‐a corresponding effect was obtained although the apparent coupling between phosphorylation of 3′‐and catabolism of adenine ribonucleotides was not complete. When experiments were performed at the same pH but at high concentration of phosphate (45 mM) there was in contrast no coupling between the two processes; that is, ATP was present in constant amounts while 3′‐phosphates accumulated at a high rate. In experiments with adenosine under these conditions there was still some although a relatively limited degree of apparent coupling between phosphorylation of adenosine and catabolism of adenine ribonucleotides. In both lines of cells used and with both adenosine and 3′‐, the main products of the catabolism of adenine ribonucleotides were inosine and hypoxanthine. With 3′‐there was in addition (about 20%) formation of xanthosine, suggesting that IMP dehydrogenase had also been activated. These results lead to the suggestion that adenosine (or 3′‐) may be phosphorylated in two ways. 1) Phosphorylation may depend on an adenosine kinase unrelated to catabolism of adenine ribonucleotides. 2) Phosphorylation may be tightly coupled to catabolism of adenine ribonucleotides. A nucleoside phosphotransferase may catalyze the transfer of a phosphoryl group from IMP to adenosine (or 3′‐) to form AMP (or 3′‐) and inosine, a process that may be tightly coupled to an AMP deaminase reaction. The IMP formed in the latter reaction may not be released but transferred to the phosphotransferase. In contrast, the AMP formed in the phosphotransferase reaction should be in equilibrium with soluble AMP. It is assumed that a physical complex may exist, possibly in a membrane bound form, between AMP deaminase and the nucleosi