Biosynthesis of lactose by mammary gland slices from the lactating rat.

The incorporation of the isotope from D-glucose-lJ4C, D-glucose-l-3H, D-glucose-Z J4C, D-ghcose-3-3H, D-glucose3 ,4-14C, D-glucose-6-14C, D-glucose-6-3H, uniformly labeled D-glucoseJ4C, uniformly labeled D-galactoseJ4C, and D-mannose-lJ4C into lactose by slices of mammary glands of lactating rats (14 to 20 days post partum) was studied. The lactose formed was hydrolyzed either with acid or with /3-galactosidase, and the component hexoses were isolated and assayed for isotopic content. Three findings indicate that D-glucose, and not a-D-glucose l-phosphate, acts as the galactosyl acceptor during lactose biosynthesis. (a) The extent of conversion of the label of the variously labeled glucoses to lactose was the same regardless of the site of the label and regardless of whether it was 14C or 3H. (b) The addition of unlabeled glucose to the incubation medium was required for incorporation of 14C from D-mannose-lJ4C into lactose. (c) Lactose formed in the experiments with D-mannose-1-14C and unlabeled glucose as substrates was labeled almost exclusively in the galactose portion. The results of periodate degradation of the hexose moieties of lactose formed in experiments in which D-glucose-6-14C served as substrate revealed that little carbon from the triose stage enters the phosphohexose pool via reversal of the Embden-Meyerhof pathway. In the presence of unlabeled D-glucose (1 X 1OF M), the incorporation of the label of uniformly labeled D-galactose14C into lactose was at least 10 times that into COZ, indicating that galactose probably enters the metabolic scheme in mammary gland as D-galactose l-phosphate.