The molecular specificity in the biosynthesis of diacylglycerols by rat intestinal mucosa was examined by means of radioactive markers, thin-layer chromatography with silver nitrate and gas-liquid chromatography with radioactivity monitoring. Bile salt micelles of alternately labeled monoacylglycerols and free fatty acids were incubated with everted sacs of intestinal mucosa for various periods of time and the diacylglycerols were isolated by solvent extraction and thin-layer chromatography. Stereospecific analyses of the X-1,2-diacylglycerols labeled from 2-monoacylgiycerols showed that the sn-1,2-isomers (45–55%) were slightly in excess of the sn-2,3-isomers (34–45%) with the X-1,3-diacylglycerols accounting for the rest of the radioactivity (5–10%). This suggests that racemic diacylglycerols may be intermediates in the resynthesis of dietary fat in rat intestinal mucosa. Detailed analyses of the molecular species of the sn-1,2-diacylglycerols labeled from free fatty acids revealed that 10–45% of the total did not contain the acid present in the 2-monoacylglycerol supplied, and therefore had originated from the phosphatidic acid pathway. These findings are at variance with those obtained in isolated microsomes, which have suggested an inhibition of the phosphatidic acid pathway by monoacylglycerols as well as have given evidence of an exclusive synthesis of sn-1,2-diacylglycerols from 2-monoacylglycerols.
Quantitative determination of plasma free fatty acids and triglycerides by thin-layer chromatography