Lactose synthesis. V. C14 in lactose, glycerol, and serine as indicators of the triose phosphate isomerase reaction and pentose cycle.

Previous studies have indicated that blood glucose is the major precursor of both the glucose and galactose moieties of milk lactose. The Cl4 distribution patterns of the two moieties differ, however, and it appears that the glucose and the galactose moieties arise from the different precursors (l-4). It also has been suggested that the unsymmetrical distribution of Cl4 found in the galactose moiety may be caused by reactions of the pentose cycle and by a slow triose isomerase reaction (l-3). An investigation of these possibilities has been undertaken by unilateral injection of glucose-2,6-C’* and glucose-6-C14 into the pudic artery of a cow and by isolation and degradation of the glucose and galactose of the lactose, the glycerol of the neutral fat, and the serine of the casein. The results indicate that the pentose cycle does play a part in the labeling of the galactose, but the very marked asymmetry of the Cl4 apparently is not due to a deficiency of triose phosphate isomerase.