NITRATE REDUCTASE-ACTIVITY OF PLASMA-MEMBRANES FROM CULTURED CARROT CELLS

Cultured carrot cells (Daucus carota L.) reduced nitrate to nitrite at a slow rate (0.4 mu moles/g dry wt . h) without any additions to the reaction medium. This rate was doubled or tripled in pre sence of 100 mu M NADH. Ethanol and other alcohols stimulated the basal rate 8-10-f old. Isolated carrot plasma membranes also reduced nitrate to nitrite at a rate of 80 nmoles/mg protein . h. This plasma membrane-bound nitr ate reductase activity was estimated to be 1.7% of the total activity. Nitrate reduction by carrot cells was inhibited 56% by sodium tungsta te, 57% by potassium cyanide, and 87% by gold chloride. It was stimula ted by plasma membrane electron transport inhibitors (retinoic acid an d chloroquine) and ATPase inhibitors (diethylstilbestrol). From differ ential effects of some stimulators or inhibitors in the presence or ab sence of NADH, it can be implied that the nitrate reductase activity o f cultured carrot cells was due to a transmembrane enzyme exhibiting a n exogenous nitrate reductase activity when NADH was added.