Utilization of host purines by transplanted tumors.

Tumors derive their nutrients from the host. These materials may come directly from the diet via the blood, from labile pools which may exist in host tissues, or from the substance of host tissues through their breakdown and transport to the tumor. This has been well studied in the case of tumor-host protein relationships and has resulted in the concept of the tumor as a nitrogen trap, since it is in positive nitrogen balance even when the host is starving (7, 9, 13). There are indications tha t this situation exists with respect to other essential dietary components also (11). A similar phenomenon may exist with respect to nucleic acid precursors, but this has received less study. Purines are not required in the diet, and it is well known that both normal tissues and tumors are able to synthesize them from glycine and other precursors via the so-called de novo pathway (5, 6, 13). Itowever, when presented with preformed purines, tumors and normal tissues utilize these and incorporate them into cellular nucleic acids (1, 3, 8). Information regarding the extent of utilization of preformed purines by tumors has great relevance to cancer chemotherapy, particularly that involving antimetabolites. The relative importance of the two modes of synthesis of purine nucleotides in tumors was measured in several mouse ascites tumors by LePage and Sartorelli (7). Dancis and Balis (4) investigated the utilization of host nucleic acid purines by, among other systems, transplanted solid tumors; and, while some labeled purine was found in the tumor, it was a small amount, and these investigators considered it insignificant. In the present paper the problem has been reinvestigated, and the utilization of host purines by tumors has been measured. The results indicate that significant utilization of host purines by tumors does occur.