Galactose-Metabolizing Enzymes in the Rat

Abstract

The induction of galactose-type cataracts in rats with a yogurt diet (1) resembles the natural cataractogenic action of the milk sugars in patients with galactosemia, a hereditary disease characterized by galactose-1-phosphate uridyl transferase deficiency. The role of the transferase and other galactose-metabolizing enzymes in the pathogenesis of cataracts in rats has not been assessed, however, although enzymes in the glycolytic (2-4) and oxidative shunt pathways (5, 6) are inhibited when rats are fed strict galactose diets. Indeed, the decreasing activities of the liver enzymes, galactokinase (7), transferase (8, 9), and uridine diphosphogalactose-4-epimerase (10) in the aging rat suggest that one or all of the enzymes of the sugar nucleotide pathway limit carbohydrate metabolism in rats fed on galactose. There appears to be a similar age-dependence in the respective red cell enzymes.

Materials and Methods. Male Wistar rats were bled from the heart at 1, 2, 3, 8, and 16 weeks of age in groups of three or more. Galactose-1-phosphate uridyl transferase was assayed in the red cells by a fluorometric method (11, 12) and galactokinase, by an isotopic method (13). Red cells from five healthy men and women, ranging in ages from 21 to 50 years, served as controls for the conditions of the assays, which had been selected for enzymes from human tissues. The production of reduced triphosphopyridine nucleotide (TPNH) at constant rates (Fig. 1) by the transferases from the two species assured us, for example, that the substrate conditions during the 30 min incubation period were adequate for the rat transferase. Nor was differential absorbance of rat hemoglobin a source of error in calculating transferase activities, as the hemoglobins of both species approached their maxima at 410 nm.