Multiple genetic changes determine ribose utilization by Novikoff hepatoma cell variants

In variants of the Novikoff hepatoma cell line, the ability to use D-ribose as a carbon source appeared to be due to changes in the expression of ribokinase. Examination of ribokinase activity was prompted by the finding that uptake of radiolabeled ribose was linear for 30 min in six variants but became saturated within 2 min in nine other variants. The linear uptake of ribose was due to a high rate of phosphorylation by ribokinase. Variants which showed linear uptake kinetics had ribokinase levels of 6.8 +/- 1.7 nm/min per mg protein as compared to the parental levels of 0.90 +/- 0.25 nm/min per mg protein. The nine variants which showed saturable uptake kinetics had low parenteal levels of ribokinase. However, these variants showed a change in the subcellular location of that activity. The enzyme was predominantly membrane-associated in both parental cells and high ribokinase variants. In contrast, the low ribokinase variants had a cytoplasmic form of the enzyme. A more general membrane change probably occurred in these variants, since they showed an increased sensitivity to the unrelated membrane reactive compounds, phytohemagglutinin and ouabain.