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.