Lactose synthesis. VI. Labeling of lactose precursors by glycerol-1,3-C-14 and glucose-2-C-14.

It is generally agreed that blood glucose is the major source of carbon for the synthesis of both hexose portions of lactose by the mammary gland (1, 2), but the exact mechanism of lactose synthesis remains in doubt. An abridged outline of some possible pathways of labeled carbon in the biosynthesis of lactose is presented in Fig. 1. In Reaction A of Fig. 1, uridine diphosphate galactose is the galactosyl donor and glucose l-phosphate the galactosyl acceptor. The enzyme catalyzing this reaction has been described (3), but so far a confirmation of this work has not been reported. Hexokinase (4, 5)) phosphoglucomutase (6), uridine triphosphate glucose l-phosphate pyrophosphorylase (7)) and uridine diphosphate galactose 4-epimerase have all been found in mammary gland (8, 9). A second postulated pathway involves the condensation of free glucose and UDP-galactose (Fig. 1, Reaction B). This mechanism was proposed to account for the results obtained in tracer experiments with the perfused isolated cow udder (10) and in uivo when the labeled substrate was injected into the pudic artery of cows (11, 12). In both types of experiments, it was found that the glucose moiety of the lactose differed from the galactose. With glycerol-l ,3-Cl4 (la), the galactose contained much more Cl* than the glucose and the carbon atoms in positions 4 and 6 of the galactose were much more heavily labeled than the carbon atoms in positions 1 and 3. The glucose was approximately equally labeled in carbon atoms 1, 3, 4, and 6 and had a Cl4 pattern similar to the blood glucose. The excess labeling in carbon atoms 4 and 6 of galactose has been tentatively explained by proposing (12) that a transaldolase type of exchange of triose phosphate-Cl* occurs with fructose-B-P, which introduces a high degree of labeling into carbon atoms 4 and 6 (Fig. 1, Reaction C). Ljungdahl et al. (13) have shown such an exchange using purified transaldolase from yeast. The difference in labeling of the glucose and galactose is readily explained by Mechanism B because the free glucose would not be subject to the exchange reaction and thus would not acquire heavy labeling in carbon 4 and carbon 6. An alternative proposal was also made (la), namely that there may be a particular architecture within the cells that permits compartmentalization of certain enzymes. Thus, substrate pools may arise which are not in equilibrium. Therefore, glu-