Administration of tungsten to rats at levels of 1 to 100 ppm in the drinking water resulted in proportionate decreases in xanthine oxidase and sulfite oxidase activities and the native molybdenum electron paramagnetic resonance signal of the latter as well as total liver molybdenum. The time course of loss of the two enzyme activities fitted first order kinetics in all tissues studied. The t_1/2 for sulfite oxidase activity was longer than that for xanthine oxidase in both liver and kidney, while both activities disappeared rapidly from intestinal tissue. The quantitative aspects of the biphasic loss of hepatic molybdenum revealed the existence of an unidentified and relatively unstable pool of molybdenum. This pool appeared to be replaced by tungsten early in the course of administration of the metal at 100 ppm. The simultaneous presence of 1 ppm of molybdenum almost completely superseded the effect of 100 ppm of tungsten. The results have been interpreted to indicate inhibition of molybdenum utilization by dietary tungsten. The tungsten effect was also observed when the metal was administered intraperitoneally.

Injection of molybdenum into rats depleted of hepatic sulfite oxidase and xanthine oxidase activities by tungsten treatment resulted in significant restoration of both activities in 5½ hours. The use of cycloheximide and puromycin showed that the reactivation was independent of new protein synthesis. The inability to effect such reactivation in vitro points to the requirement for activation or modification of molybdate as a prerequisite for prosthetic attachment to the enzymes.

Unlike tungsten, vanadium was incapable of generating experimental molybdenum deficiency in the rat.