Specific activities of uridine phosphorylase and uridine kinase in Ehrlich ascites carcinoma cells and 6-azauracil- and 6-azauridine-treated sublines in successive transplant generations.

THE development of resistance to 5-fluorouracil (FU) in Ehrlich ascites carcin-oma (EAC) cells has been accompanied by decreases in the activities of uridine kinase (Reichard et al., 1962) or of uridine phosphorylase and deoxyuridine phos-phorylase (Reichard, Skold and Klein, 1959). These enzymes of the " uracil pathway " of pyrimidine biosynthesis are involved in the conversion of FU to its active metabolite, 5-fluorodeoxyuridine 5'-monophosphate (Chaudhuri, Montag 1960b). Significant decreases in uridine kinase activity were observed by Reichard et al. (1962) in acetone powders of four sublines of the FU-sensitive tumour, ELD, after treatment with FU for 25 to 30 passages, although the range of activity in the acetone powders prepared from the parental (untreated) ELID cells was apparently 40-70 units (Reichard et al., 1962; Skold, Magnusson and Revesz, 1962). However, early resistance to FU occurred before significant decrease in uridine kinase activity, and the authors concluded that other factors were responsible for the development of such resistance. Several years ago, studies similar in design to those of Reichard, Skold and Klein (1959), were initiated in this laboratory on the relationship of uridine kinase and uridine phosphorylase activities of Ehrlich ascites carcinoma cells to the possible development of resistance to 6-azauracil (AzU) and 6-azauridine (AzUR) in vivo. Since uridine phosphorylase (Pontis, Degerstedt and Reichard, 1961) and uridine kinase (Skold, 1960c) from EAC cells catalyse the formation of AzUR from AzU, and the formation of 6-azauridine 5'-monophosphate (AzURP) (the active metabolite (Pasternak and Handschumacher, 1959; Handschumacher, 1960)) from AzUR, decreased activity of one or both enzymes might be associated with resistance to AzU. Handschumacher et al. (1962) stated that in resistant bacteria and certain strains of experimental tumours (unidentified) a common feature was the lack of formation of the 5'-phosphate from AzUR. Thus, decreased phosphorylation of uridine and AzUR to the respective nucleotides was associated with the development of resistance to AzUR in L-5178-Y leukaemia cells (Pasternak et al., 1961). However, a mutant of Streptococcus faecalis resistant to AzU was unable to convert AzU or uracil to the respective nucleoside, but could phosphorylate uridine (Handschumacher, 1957). In comparison, development of resistance to AzUR during therapy of human neoplas-tic disease has been associated with an adaptive increase in the synthesis of pyrimi-dines via the de novo pathway (Fallon et al., 1962; Handschumacher et al., 1962).