Guanosine metabolism in novikoff hepatoma cells: Isolation and characterization of guanosine resistant variants

Clones resistant to 0.15% guanosine were isolated from rat hepatoma cells. Analysis of cell extracts from these clones revealed the presence of normal levels of purine nucleoside phosphorylase activity but less than 2% of the parental level of hypoxanthine-guanine phosphoribosyltransferase activity. In addition, the resistant cells transported guanosine and inosine at less than 2% of the rate of sensitive cells. Despite this low rate of transport, the resistant cells were still capable of metabolizing extracellular guanosine and inosine. The ability of the resistant cells to metabolize guanosine and inosine without requiring their direct transport lends support to the existence of a membrane localized form of purine nucleoside phosphorylase which metabolizes extracellular purine nucleosides.