Expression of key enzymes of purine and pyrimidine metabolism in a hepatocyte-derived cell line at different phases of the growth cycle.

The effect of growth phase on enzymatic activities of the de novo and salvage pathways for purine and pyrimidine nucleotide synthesis was studied in a hepatocyte-derived cell line from the rat. The cells were in lag phase after plating for 36 h; log phase started at 48 h and persisted up to 120 h of culture. Then the cells stopped growing and entered into plateau phase (144 h). In non-proliferating cells (144 h of culture) the basal activities of the enzymes of purine de novo biosynthesis were 1.7- to 6.8-fold higher than in normal rat liver, those of pyrimidine de novo synthesis showed 0.6- to 30-fold increase in activity. The purine salvage enzymes were unchanged, and the pyrimidine salvage enzymes were 3.1- to 7.4-fold higher compared to normal liver. During the growth cycle all enzymes except the purine salvage enzymes, which did not change, showed a peak in activity at 72 h of culture (log phase). The increase in activity in log phase compared to plateau phase was 1.3- to 2.4-fold for purine de novo synthetic enzymes, 1.1- to 2.4-fold for pyrimidine de novo enzymes, and 1.4- to 4.7-fold for pyrimidine salvage enzymes. The specific activities of the enzymes in exponentially growing cells were comparable either to that in 24-h regenerating liver, or to that in hepatomas of low or medium growth rate. It was concluded that the enzymatic pattern and metabolic state of the cells shared some features with regenerating liver, others with tumors, although they were not tumorigenic after transplantation into athymic nude mice.