Inhibition of cultured cell growth by tungstate and molybdate

The growth of suspension cultured cells of Nicotiana tabacum (tobacco) was inhibited completely by 100 μM tungstate. Even though molybdate reversed the tungstate inactivation of nitrate reductase activity, the growth inhibition was not reversed. The growth inhibition of N. tabacum, Daucus carota, Glycine max and Solanum tuberosum suspension cultured cells by tungstate was similar in media with or without amino acids as a source of reduced nitrogen. Only in the case of G. max was a slight reversal caused by the amino acids. Tungstate was slightly less inhibitory to the growth of a nitrate reductase-lacking mutant N. tabacum line (nia-63) than to the line with nitrate reductase. These results indicate that tungstate must inhibit the cell growth of the four species used, predominantly, in some way other than by inhibiting nitrate reductase activity. Similar studies with molybdate, a sulfate analog which apparently competes with sulfate at the ATP sulfury-lase enzyme, showed that 1 mM concentrations were completely inhibitory to cell growth. The addition of sulfate or cysteine, as a source of reduced sulfur, and amino acids, as a source of reduced nitrogen, in most cases did not reverse the molybdate inhibition appreciably. Some reversal was seen only by sulfate with D. carota cells and by cysteine plus amino acids with D. carota and G. max. These results indicate that selection for tungstate or molybdate resistance will in general not select for higher levels or other alterations in the activity of nitrate reductase or ATP sulfurylase, respectively, since these ions do not inhibit growth by primarily affecting these enzymatic steps in cultured cells of the four species studied.