Lactose synthesis. III. The distribution of C 14 in lactose of milk after intra-arterial injection of acetate-1-C 14

Abstract

Acetate-1-C 14 was injected over a half-hour period into the left pudendic artery sinus of a cow. The left and right sides of the udder were milked separately and the lactose was isolated from the milk samples. The glucose and galactose moieties were degraded by Leuconostoc mesenteroides fermentation. High C 14 activity appeared in the lactose of the milk collected from the left side 57 min. after the start of the experiment but reached a high activity on the right side only after 105 min. The distribution of C 14 was determined in the milk sample at 100–105 and 220–225 min. There was a striking difference in the C 14 distribution patterns of the lactose from the injected and noninjected sides. The galactose on the left (injected side) from the 100-min. milk sample contained 90 % of the C 14 of the lactose; 89 % of this C 14 was in the C-4 position and only 6.7 % in C-3. Eighty per cent of the C 14 of the entire lactose molecule was in a single carbon. The glucose moiety had a much more symmetrical distribution of C 14. Eighty-five per cent of the C 14 was in carbons 3 and 4, and C-3 contained 65 % of the activity of C-4. The lactose of the milk from the noninjected side had equal total activities in the glucose and galactose, and the isotope distribution patterns were symmetrical in both the galactose and glucose and comparable to that of the glucose of the lactose from the injected side. Similar results were obtained in the degradation of the milk samples drawn at 220–225 min., except the C-4 of the galactose on the injected side now contained 73 % of the C 14 of the galactose, and C-3 had 21 %. The results clearly demonstrate that the precursors of the glucose and galactose moieties are not identical. It is proposed that free glucose, in equilibrium with the blood glucose, is the precursor of the glucose moiety and the hexose phosphate is the precursor of the galactose moiety. The glucose and galactose moieties of the noninjected side and the glucose moiety on the injected side are believed to have similar C 14 patterns because a major part of their C 14 is derived from blood glucose. The blood glucose-C 14 is formed from acetate-C 14 via transformations in the liver and thus has a symmetrical C 14 distribution and low activity. The galactose on the injected side reflects the metabolism of acetate by the udder per se. The C 14 of acetate is converted in the udder via the Krebs cycle to hexose phosphate (not to free glucose); thus this C 14 is converted in a direct route to galactose. The acetate-C 14 on this side is received prior to its major dilution and prior to the time that the blood glucose becomes highly labeled by liver metabolism. Thus the galactose moiety on the injected side becomes highly active while the glucose moiety has very little activity. The almost exclusive labeling of C-4 in the galactose may occur because of a slow triose isomerase reaction or perhaps because of a unique and unknown synthetic pathway for galactose.