Mammalian Galactose Dehydrogenase

A nicotinamide adenine dinucleotide-requiring galactose dehydrogenase (d-galactose:NAD oxidoreductase, EC 1.1.1.48) has been identified in rat liver. The conditions for extraction of the enzyme from liver were studied and it was purified 100-fold. Liver galactose dehydrogenase activity is 7 times greater than glucose dehydrogenase, and, in contrast with the latter enzyme, 85% of the activity is present in the soluble cellular fraction. In crude preparations and under optimum conditions, its specific activity is similar to that of galactokinase (7 mµmoles per min per mg of protein). Galactose dehydrogenase catalyzes the oxidation of the aldehyde group of galactose to produce galactonolactone which under the alkaline conditions of assay is spontaneously converted to galactonic acid. There is evidence that liver contains an enzyme which rapidly catalyzes the hydrolysis of the lactone to yield the free acid; this reaction appears to be irreversible. Liver is the only rat tissue with substantial galactose dehydrogenase activity, although some activity is detectable in preparations of testis, kidney, intestine, and mammary gland. Human red and white blood cells have no detectable activity. Galactose dehydrogenase activity is present in liver preparations of eight mammalian species, including man. Pigeon liver has very weak activity, and none is found in frog or fish liver.