Glyceride-Glycerol Precursors in the Intestinal Mucosa*“f

It has been shown by several workers that fatty acids liberated by the hydrolyses of their esters, such as amyl (I), ethyl (2), or the optically active mannite (3), appear in the thoracic duct chyle as triglycerides. In comparing the absorption of free palmitic acid and some of its esters, Lyman (4) found that in each case the fat deposited in the tissues was tripalmitin. It was assumed, therefore, that the absorbed fatty acids combine with glycerol in the intestinal mucosa to form triglycerides (5). Recent studies, however, with deutero-glycerol (6, 7), Ci4labeled glycerol (S), and conjugated trilinolein in which the glycerol moiety was labeled (9) showed that free glycerol, ingested with free fatty acid or hydrolyzed from glycerides during digestion, does not appear in lymph triglycerides. Reiser and Williams (10) found that ingested 1-palmitoxy-3-hydroxyacetone, labeled in the ketone and fatty acid moieties, appeared in rat lymph as triglycerides with little loss of glycerol activity. They suggested that dihydroxyacetone esterifies with fatty acids as the first step in the resynthesis of triglycerides by the intestinal mucosa. Bublitz and Kennedy (11) have demonstrated, however, that liver contains a glycerol kinase, enabling that tissue to incorporate free glycerol into L-ol-glycerophosphate, and thence into phosphatidic acid (12), triglycerides (13), and phospholipides (12). This subject has been reviewed by Kennedy (14). The studies in viva indicate that glycerol kinase is not present in the mucosa or that free glycerol is so rapidly diluted and absorbed that it is not available for glyceride resynthesis during fatty acid absorption. To test the possibility that there is no glycerol kinase in the intestinal mucosa, uniformly labeled fructose diphosphate was prepared from uniformly labeled sucrose. The labeled fructose diphosphate and aldolase were added to swine intestinal mucosa homogenates with the appropriate cofactors to produce the probable labeled precursors, dihydroxyacetone and n-cY-glycerophosphate, by enzymatic schism (15). Unlabeled dihydroxyacetone phosphate, r.-cY-glycerophosphate, and glycerol were added to similar reaction mixtures. The added unlabeled “precursors” which did not reduce the activity of the isolated glyceride are not glyceride-glycerol precursors. The unlabeled “precursors” which did dilute the activity of the resultant triglycerides are glyceride-glycerol precursors. Since, by this