Participation of Phosphorylated Intermediates in in vivo Synthesis of Triglycerides from α-Monoglycerides in Rat Intestinal Mucosa

In a previous study (Clement and Paris, 1), we demonstrated the rapidity of absorption of doubly labeled (glycerol-C¹⁴ and palmitic acid-H³) α-monopalmitin and studied the biosynthesis of triglycerides by the intestinal mucosa in vivo.

Our technique involves a 30-minute incubation of the lipid constituents in a segment of intestine in situ in rats in which the pancreatic and biliary secretions have been directed exteriorly for 24 hours previously. The lipids (20 mg of doubly labeled α-monoglycerides and equimolecular amounts of oleic and palmitic acids), 1.5 ml of Ringer solution and 25 mg of bile salts are introduced into the intestinal segment in situ with intact nervous connections. For analysis, the mucosa of 2 rats are combined. Our first results, using α-monopalmitin (glycerol-C¹⁴, fatty acid-H³), demonstrated that in the mucosa the istotopic ratio H³/C¹⁴ increased from monoglycerides to diglycerides to triglycerides; there was, therefore, acylation of the monopalmitin by labeled palmitic acid. Since this palmitic acid could arise only by hydrolysis of part of the labeled monopalmitin our experiments furnished new proof of the existence of a monoglyceride lipase in the intestinal mucosa and demonstrated that this enzyme is active in vivo on α-monopalmitin.

The biosynthesis of triglycerides from α-monoglycerides could be explained satisfactorily by direct acylation according to Clark and Hubscher(2). However, since we had observed at the same time in this study that the phosphatidic acids were highly radioactive and had a H³/C¹⁴ ratio intermediate between those of the monoglycerides and diglycerides, the existence of a phosphorylated intermediate in the synthesis of triglycerides from monoglycerides seemed reasonable.

To clarify this point, we therefore repeated the experiment with α-monopalmitin (glycerol-1-C¹⁴, 1 μc, and palmitic acid 9,10-H³, 3.6 μc) but with separation of the phospholipids by methods particularly designed to obtain the phosphatidic acids and lysophosphatidic acids in as pure a state as possible.