Abstract
NADH-dependent vitamin K reductase activity in rat liver microsomes was measured by detecting the amount of the reduced form of vitamin K from the oxidized form of the vitamin. The enzyme activity was not detected when intact microsomes were employed as the enzyme source, but the solubilization of the microsomal enzyme with 1.5% Triton X-100 caused a development of the activity. Although the enzyme activity decreased gradually with time after the solubilization, the enzyme was stabilized by the addition of 20% glycerol and 2 mM vitamin C. Some optimal assay conditions for the vitamin K reductase were determined using the solubilized enzyme, and the standard assay method is described. Vitamin K reductase activity was not affected by warfarin and N-ethylmaleimide (NEM), but pyridoxal-5-phosphate (PAL-P) inhibited the activity, especially when microsomes were preincubated with PAL-P. The enzyme activity was not inhibited by N-methyltetrazolethiol (NMTT) and NMTT-containing antibiotics, suggesting that the hypoprothrombinemia caused by β-lactam antibiotics was not due to the inhibition of NADH-dependent vitamin K reductase.
