Phosphorylation of 1-beta-D-arabinofuranosylcytosine by the cell-free extract of rat ascites hepatoma, in relation to the mechanism of natural resistance.
The enzyme that phosphorylates 1-beta-D-arabinofuranosylcytosine (ara-C) was examined in rat ascites hepatomas to clarify whether the capacity for the phosphorylation of ara-C would be related to natural resistance of the tumors to ara-C. Ara-C kinase activity in AH-66F cells, which were moderately sensitive to ara-C, was approximately the same as that in the bone marrow of tumor-bearing rats. L-1210 leukemia, which is sensitive to ara-C, had a higher activity of the enzyme than the mouse bone marrow. On the other hand, the naturally resistant hepatomas, AH-60C, AH-109A, and AH-66, were low in their ara-C kinase activity. The enzyme activity in the cytoplasmic extract (30,000 g supernatant) of the tumor and bone marrow cells was nearly proportional to the capacity for ara-C phosphorylation in intact cells. Thus, the mechanism of natural resistance to ara-C in rat ascites hepatomas was attributed mainly to the low levels of ara-C kinase activity itself. Apparent Km of the enzyme for ara-C was about 170 micron. However, probably due to the presence of the uncompetitive type of inhibitor(s), the level of apparent Km lowered close to 40 micron at higher concentration of the protein as the source of the enzyme, showing similarity to the value in intact cells.