The plasma membrane NADH oxidase of soybean has vitamin K1 hydroquinone oxidase activity

Isolated plasma membrane vesicles and the plasma membrane NADH oxidase partially purified from soybean plasma membrane vesicles exhibited a cyanide-insensitive vitamin K-1 hydroquinone oxidase activity with isolated plasma membrane vesicles. Reduced vitamin K-1 (phylloquinol) was oxidized at a rate of about 10 nmol/min/mg protein as determined by reduced vitamin K-1 reduction or oxygen consumption. The K-m for reduced K-1 was 350 mu M. With the partially purified enzyme, reduced vitamin K-1 was oxidized at a rate of about 600 nmol/min/mg protein and the K-m, was 400 mu M. When assayed in the presence of 1 mM KCN, activities of both plasma membrane vesicles and of the purified protein were stimulated (0.1 mu M) or inhibited (0.1 mM) by the synthetic auxin growth factor 2,4-dichlorophenoxyacetic acid. The findings suggest the potential participation of the plasma membrane NADH oxidase as a terminal oxidase of plasma membrane electron transport from cytosolic NAD(P)H via reduced vitamin K-1 to accepters (molecular oxygen or protein disulfides) at the cell surface. (C) 2000 Elsevier Science B.V. All rights reserved.