Lactose synthesis. II. The distribution of C14 in lactose of milk from the perfused isolated cow udder.

The distribution of Cl4 in milk lactose after intravenous injection of various labeled compounds has been determined by Schambye, Wood, and Kleiber (l), who found that the Cl4 pattern of the glucose differed from that of the galactose particularly when acetate-l-Cl4 or NaHC403 was administered. They proposed that free glucose (in equilibrium with blood glucose) was the precursor of the glucose moiety, whereas the hexose phosphate esters synthesized in the mammary gland directly from glucose and from smaller molecules were the sources of the galactose residue. The interpretation of these experiments in vivo is complicated by the difficulty of differentiating changes which originated in the udder’s own metabolism from those that occurred in other organs. It therefore was desirable to conduct experiments in which the contribution of CL4 from the liver to the blood glucose was eliminated, and all synthesis was confined to the mammary gland. The synthesis of lactose in slices or homogenates of mammary gland has been accomplished with carbohydrates as substrates (2-8) but has not been reported with acetate. However, Cowie et al. (9) found that acetate-l-Cl4 is incorporated into lactose by the isolated perfused cow udder; this system therefore appeared to be the most suitable for the test. The results obtained with cow udder perfused with acetate-l-Cl4 or propionate-l-Cl4 confirm previous proposals (1) for the synthesis of lactose. The Cl4 was found almost exclusively in the galactose moiety of the lac-