A Kinetic Analysis of the Uptake of Cytosine‐β‐d‐Arabinoside by Rat‐B77 Cells

Abstract

Cytosine-.beta.-D-arabinoside is a synthetic nucleoside which has been useful in the treatment of human leukemias. Kinetic methods were used to differentiate the tandem processes of transport and metabolic trapping during uptake of cytosine-.beta.-D-arabinoside by intact rat fibroblasts [B77 cells]. Transport across the cell membrane occurs by a carrier-mediated mechanism displaying a Km of .apprxeq. 500 .mu.M and Vmax of .apprx. 300 pmol/min per (106 cells). The subsequent metabolic trapping (phosphorylation) has a Km of .apprxeq. 15 .mu.M and Vmax of .apprxeq. 0.25 pmol/min per (106 cells). In this system, transport is rate-limiting for the 1st phase of the uptake process and phorphorylation becomes rate-limiting when intracellular concentration of labeled substrate exceeds that in the surrounding medium. The duration of the 1st phase depends on the substrate concentration.