A Kinetic Analysis of the Uptake of Cytosine‐β‐d‐Arabinoside by Rat‐B77 Cells: Differentiation between Transport and Phosphorylation

We present here a differentiation by kinetic methods of the tandem processes of transport and metabolic trapping during uptake of cytosine‐β‐d‐arabinoside by intact rat fibroblasts. Transport across the cell membrane occurs by a carrier‐mediated mechanism displaying a K_m of ∼ 500 μM and a V of ∼ 300 pmol × min^‐1× (10⁶ cells)^‐1. The subsequent metabolic trapping (phosphorylation) has a K_m of ∼ 15 μM and V of ∼ 0.25 pmol × min^‐1× (10⁶ cells)^‐1. In this system, transport is rate‐limiting for the first phase of the uptake process whereas phosphorylation becomes rate‐limiting when internal concentration of radioactive labeled substrate exceeds that in the extracellular medium. The duration of the first phase depends on the substrate concentration.