RELATIONSHIP BETWEEN URIDINE KINASE ACTIVITY AND RATE OF INCORPORATION OF URIDINE INTO ACID-SOLUBLE POOL AND INTO RNA DURING GROWTH CYCLE OF RAT HEPATOMA CELLS

Uridine kinase activity measured in cell‐free extracts of Novikoff rat hepatoma cells grown in suspension culture fluctuates about 10 fold during the growth cycle of the cells. Maximum specific activity (units/106 cells) is observed early in the exponential phase and then decreases progressively until the stationary phase. The rate of incorporation of uridine into the acid‐soluble pool by intact cells fluctuates in a similar manner and both the rate of uridine incorporation by intact cells and the uridine kinase actvity of the cells increase several fold before cell division commences following dilution of stationary phase cultures with freshmedium. Regardless of the stage of growth, uridine is rapidly phosphorylated to the triphosphate level by the cells. The rates of incorporation of uridine into the nucleotide pool and into RNA by intact cells fluctuate in a similar manner during the growth cycle. However, evidence is presented that indicates that alterations in the rate of incorporation of uridine into RNA are not simply due to changes in the rate of phosphorylation of uridine, but are regulated independently. Inhibition of protein synthesis by treating cells with puromycin or actidione causes a marked inhibition of incorporation of uridine into RNA, but has little effect on the phosphorylation of uridine to UTP for several hours. Thus the depression of incorporation of uridine into RNA probably reflects a decrease in the rate of RNA synthesis as a result of inhibition of protein synthesis. Inhibition of RNA synthesis by treating cells with actinomycin D does not affect the rate of conversion of uridine to UTP and thus results in the accumulation of labeled UTP in treated cells. Copyright © 1969 Wiley‐Liss, Inc.