Initial Reactions in the Metabolism of D- and L-Glyceraldehyde by Rat Liver* BERNARD R. LANDAut AND WILFRIED MERLEVEDE WITH THE TECHNICAL ASSISTANCE OF HOLLIS

There appear to be several pathways in liver by which Dand n-glyceraldehyde can be converted to dihydroxyacetone phosphate and n-glyceraldehyde phosphate. Thus, the following reactions have been described in liver preparations: phosphorylation of n-glyceraldehyde to n-glyceraldehyde phosphate (Fig. 1; Pathway A) (2); formation of n-glyceraldehyde phosphate from n-glyceraldehyde via n-glyceric acid (Pathway B) (3-6) ; and reduction of n-glyceraldehyde to glycerol with the latter’s conversion to dihydroxyacetone phosphate (Pathway C) (7-9). By either Pathway A or B, n-glyceraldehyde labeled in position 3 with Cl4 (indicated by an asterisk in Fig. 1) should yield via the Embden-Meyerhof pathway glycogen containing glucose units labeled in carbon atoms 1 and 6. However, via Pathway C, glycerol formed from n-glyceraldehyde-3-Cl4 (the label is now traced in Fig. 1 by a circle) should yield dihydroxyacetone-3phosphate-l-Cl4 (10) and therefore 3,4-labeled glucose. If isotopic equilibration of trioses via triose isomerase is slow relative to hexose formation (11-13) by either Pathway A or B, more activity should be in carbon 6 than in carbon 1, whereas via Pathway C, more activity should be in carbon 3 than in carbon 4. By reaction via transaldolase, Cl4 from n-glyceraldehyde-3-Cl4 may also be introduced into carbon 6 of fructose (14). n-Glyceraldehyde phosphate can be formed from L-glyceraldehyde (Fig. 2; Pathway A) (15), but no reaction is known for further metabolism via the Embden-Meyerhof pathway. There is evidence for the conversion of L-glyceraldehyde to n-glyceraldehyde phosphate via series of reactions (Pathway B) of which L-glyceric acid and hydroxypyruvic acid are intermediates (3, 6). L-Glyceraldehyde labeled in position 3 with Cl4 (indicated by an asterisk in Fig. 2) should then yield 1,6-labeled glucose with, if isotopic nonequilibration of trioses occurred, greater activity in carbon 6 than in carbon 1. n-Glyceraldehyde can be reduced to glycerol (Pathway C) (7, 16) which would yield, when labeled, dihydroxyacetone-3-phosphate-3-Cl4 (lo), and also, therefore, 1,6-labeled glucose. However, by this latter pathway, if nonequilibration occurred, activity in carbon 1 should be greater than in carbon 6. The relative contribution of these pathways to the metabolism of glyceraldehyde has not been assessed previously by tracings with labeled carbon. However, n-fructose-6-Cl4 and n-sorbose-