JOSEPHINE SEE SALSER AND M. EARL BALlS

2 Abbreviations used are: IMP, inosine monophosphate; AMP, adenosine monophosphate; XMP, xanthosine monophosphate; 6-MP, 6-mercaptopurine; ATP, adenosine triphosphate; 6-thio IMP, 6-thioinosinate; GTP, guanosine triphosphate; NAD, nicotinamide adenine dinucleotide; DPN-diphosphopyridine nucleotide ; SAICAR, succinoaminoimidazolecarboxamide ribo nucleotide; CMC-carboxymethyl cellulose; Tris, tris(hydroxy methyl) aminomethane; TCA, trichioroacetic acid. Received for publication September 10, 1964; revised December 23, 1964. tive and/or quantitative differences in these nucleotide conversions in tumor tissues compared to host tissues. To test the tenability of such a proposal, more detailed studies of the enzymatic reactions involved in purine ribonucleo tide interconversions have been undertaken with S-180 and host liver as the experimental system. . Studies with bacterial (10) and ascites tumor cell (5) systems indicate that some purine and pyrimidine analogs might owe their inhibitory effects to their ability to func tion as nepressors of enzyme synthesis. This possibility has also been investigated by assaying for the enzymes of nucleotide interconversion (Chart 5) in tumors and livers of animals which have been treated with 6-MP. The results of these studies indicate that differences in enzymatic capacity between 5-180 and host liver do exist and suggest that these differences may be of sufficient magnitude to play an important part in the anti-tumor specificity of 6-MP in 5-180.