An Immunochemical Study of the Changes in Chicken Liver Xanthine Dehydrogenase Activity during Dietary Adaptation

Abstract

A specific rabbit antiserum was prepared against chicken liver xanthine dehydrogenase [EC 1.2.1.37] and used to investigate changes in the amount of the enzyme protein in the liver during dietary adaptation. When the enzyme activity was decreased by the administration of a low protein diet, quantitative immunoprecipitation and Laurell electroimmunoassay showed not only a decrease in the amount of the enzyme antigen but also a reduction of the apparent molecular activity of the enzyme. This was confirmed by the finding that the activity-flavin ratio and the activity-molybdenum ratio of the enzyme were lowered in chickens adapted to the low protein diet. The relative rate of synthesis of xanthine dehydrogenase, expressed as radioactivity incorporated into the enzyme protein divided by radioactivity incorporated into total cell homog-enate protein, was measured by pulse-labeling the liver protein with L-[4,5−³3H]leucine. Xanthine dehydrogenase was isolated by quantitative immunoprecipitation followed by electrophoresis of the dissolved immunoprecipitate on sodium dodecyl sulfate-polyacrylamide gel. The relative rate of synthesis increased 6– to 9–fold, while the enzyme activity increased 10– to 30–fold in vivo following the administration of a high protein diet. The rate of degradation of xanthine dehydrogenase was higher in chickens adapted to the low protein diet than in those adapted to the high protein diet, the apparent half lives being 1 day and 3 days, respectively. Although some limitations and assumptions were present in the techniques used in these experiments, these results suggest that the increase in the hepatic xanthine dehydrogenase content in the chickens adapted to the high protein diet was mainly due to a rise in the rate of enzyme synthesis and, to a minor degree, to a decrease in the rate of enzyme degradation.