Influence of Excess Dietary Molybdenum on Rat and Calf Liver and Heart Enzymes

Abstract

Experiments were designed to study the effect of excess dietary molybdenum on various enzymes of the liver and cytochrome oxidase of the heart of a monogastric and a ruminant animal in an attempt to gain information as to the mechanism of molybdenosis in animals. Rats fed diets containing added molybdenum at 500 and 800 ppm showed molybdenum toxicity symptoms, including diarrhea and decreased rate of growth. These diets did not affect tyrosine oxidase activity or concentration of copper in the liver, but caused an increase in the concentration of molybdenum in this organ and a reduction in liver respiration and liver xanthine oxidase. A second group of rats was fed highmolybdenum diets which raised the concentration of molybdenum in the liver but did not cause diarrhea. These rats also showed a reduction in liver xanthine oxidase activity. Thus, it appears that possible decreased utilization of protein, resulting from diarrhea as exhibited by the first group of rats, was not solely responsible for the reduced xanthine oxidase activity noted. Also, increased liver molybdenum concentration per se can influence the level of the enzyme. A slight lowering in blood uric acid concentration was observed in these rats. The cytochrome oxidase activity in the heart was not affected. The concentration of molybdenum found in the liver upon feeding 500 ppm molybdenum in the diet was not exceeded when 800 ppm was fed. It appears that there may be a liver “threshold” for molybdenum of 500 ppm or less; above this dietary level, liver storage is not increased. Calves maintained on diets containing 200 and 400 ppm molybdenum were observed to have increased concentrations of molybdenum in the livers, but no decrease in liver respiration or xanthine oxidase or in heart cytochrome oxidase.