Azaserine resistance in a plasma-cell neoplasm without change in active transport of the inhibitor.

Summary An investigation was made of azaserine entry into cells of a sensitive and resistant line of the plasma-cell neoplasm 70429. Transport experiments determined ( a ) the initial velocity of concentrative uptake in the first 1 or 2 minutes of exposure to the compound, ( b ) the total kinetics of uptake over 30-minute to 90-minute periods, and ( c ) the dependence of steady-state uptake on the extracellular concentration of the inhibitor. Azaserine was rapidly taken up by these cells and concentrated intracellularly, even within 1–2 minutes, to a level tenfold that of the extracellular medium at low extracellular concentrations; this distribution gradient was less at higher extracellular concentrations. The uptake leveled off at 20–30 minutes and remained constant over longer time intervals. Older cells showed somewhat less active transport than did the 10-day-old cell harvests used routinely in these experiments. The sensitive and resistant lines were found to be nearly identical in their capacity for active transport of azaserine; the kinetics and extent of concentrative uptake were very nearly the same in both lines over all the time periods examined. Over longer time intervals the resistant line was able to attain actually a slightly higher intracellular concentration, especially at higher extracellular concentrations of the inhibitor, but this may have been due to some damage of the sensitive cells by the inhibitor rather than to a real difference in transport capacity. The resistant line showed slightly less transport compared with the sensitive line only at very short time intervals (1-to 2-minute uptake) and this only at high concentrations of the inhibitor; even under these conditions uptake in the resistant line was 80–90 per cent of that in the sensitive line. It is concluded that a difference in transport capacity cannot explain the biological resistance of the 70429/Az(la) line nor account for the differences previously observed with intact cells in azaserine inhibition of purine biosynthesis. The experiments also measured the intracellular breakdown of azaserine. Both the extent and kinetics of breakdown were nearly identical for the sensitive and resistant lines, and it is concluded that increased breakdown of azaserine in the resistant cells cannot account for the resistance.