Phosphatidic acids as intermediates in fatty acid absorption

The nature of the stimulatory role of the supernatant fraction on triglyceride synthesis by the alpha-Glycerophosphate pathway

Evidence is presented as to the nature and mechanism of the stimulatory effect of the supernatant fraction on the biosynthesis of triglycerides via the alpha-glycerophosphate pathway in the intestinal mucosa. When microsomes are employed as the enzyme source, the major lipid formed from either labeled palmitic acid orL: -alpha-glycerophosphate is phosphatidic acid and only a limited amount of triglyceride is synthesized. The addition of the supernatant fraction to microsomes results in a stimulation of triglyceride biosynthesis at the expense of phosphatidic acid. Employing the same microsomal fraction, the reaction sequence was followed step by step and the intermediates were isolated. The results suggest that the stimulatory role of the supernatant fraction can be attributed to the presence ofL: -alpha-phosphatidate phosphohydrolase (EC 3.1.3.4). The hydrolysis of the biosynthesized microsomal phosphatidic acid by the supernatant enzyme occurs at a faster rate than the hydrolysis of added phosphatidic acid prepared from egg lecithin. The initial acylation steps in the biosynthesis of triglycerides or phosphatidic acid via the glycerophosphate pathway occur only in the presence of fatty acid and the cofactors necessary for its activation. Under these conditions, fatty acyl-CoA will not substitute for the fatty acid activation system.

Fine structural localization of acyltransferases. The monoglyceride and -glycerophosphate pathways in intestinal absorptive cells

A study of the fine structural localization of the acyltransferases of the monoglyceride and alpha-glycerophosphate pathways for triglyceride synthesis in the intestinal absorptive cell is reported. Glutaraldehyde-fixed tissue was found to synthesize diglyceride and triglyceride from monopalmitin and palmityl CoA, and parallel morphological studies showed the appearance of lipid droplets in the smooth endoplasmic reticulum of the absorptive cell. Glutaraldehyde-fixed tissue also synthesized triglyceride from alpha-glycerophosphate, although this enzyme system was more susceptible to fixation than the monoglyceride pathway acyltransferases. Cytochemical methods for the localization of free CoA were based (a) on the formation of the insoluble lanthanium mercaptide of CoA and (b) on the reduction of ferricyanide by CoA to yield ferrocyanide which forms an insoluble precipitate with manganous ions. By these methods the monoglyceride pathway acyltransferases were found to be located mainly on the inner surface of the smooth endoplasmic reticulum. The alpha-glycerophosphate pathway acyltransferases were localized mainly on the rough endoplasmic reticulum. Activity limited to the outer cisternae of the Golgi membranes occurred with both pathways. The possible organization of triglyceride absorption and chylomicron synthesis is discussed in view of these results.

A radioautographic study of glyceride synthesis in vivo during intestinal absorption of fats and labeled glucose

Radioautography was used to detect the synthesis of labeled glycerides in intestinal absorptive cells following injections of fatty chyme and glucose-6-H(3) into ligated segments of upper jejunum of fasting rats. Absorption intervals ranged from 2 to 20 min. Labeling is evident throughout the cells in as short a time as 2 min. Most grains are present over droplets of absorbed fat beginning with those in the endoplasmic reticulum immediately subjacent to the terminal web. With longer absorption periods, frequent grains are present over accumulations of fat droplets in the Golgi cisternae and intercellular spaces. A similar pattern of grains is seen following absorption of either linoleic acid or safflower oil. By comparison, considerably less label is present in the cells when the fat is extracted with alcohol prior to radioautographic procedures, or when labeled glucose alone is absorbed. A significant incorporation of glucose label into newly synthesized glycerides is indicated and confirmed by scintillation counts on saponified lipid extracts. The grain distribution implies an involvement of the extreme apical endoplasmic reticulum in this synthesis.

Intestinal metabolism of fatty acids

1. The effect of concentration on the oxidation and incorporation into lipids of lauric acid and linoleic acid by rings of rat small intestine has been studied in vitro. 2. In the absence of glucose, the oxidation of lauric acid in the range 0.01-5.0mm showed a maximum at 0.1mm. In the presence of glucose the maximum was at 0.5mm. The oxidation of linoleic acid in the presence of glucose increased throughout the concentration range 0.01-5.0mm. 3. The incorporation of lauric acid into lipids was maximal at 0.5-0.6mm in the presence of glucose, but at 10mm in the absence of glucose. At 0.8mm-lauric acid, in the presence of glucose, over 75% of the incorporated lauric acid was in triglycerides, but at 10mm they only contained 30%. The incorporation of glucose carbon into glycerides paralleled the incorporation of lauric acid. 4. In the range 0.01-2.5mm-linoleic acid the quantity incorporated into lipids increased. In the range 0.01-0.4mm linoleic acid was incorporated predominantly into triglycerides, but between 0.4 and 1.0mm most was in diglycerides, and between 2.5 and 5.0mm most was in monoglycerides. 5. The relationship of fatty acid concentration to the mechanism of absorption is discussed, together with the correlation between the distribution of the absorbed fatty acids within the tissue lipids and the lipase activity of intestinal mucosa.