[The lipid metabolism of the small intestine and its correlation to the lipid and lipoprotein metabolism of the total organism].

It is attempted to discuss the intestinal lipid metabolism particularly as it relates to systemic lipid and lipoprotein metabolism. In an introductory review of recent concepts of intestinal fat absorption special reference is given to the significance of the mixed lipid micelle during the intraluminal phase of fat absorption, to special morphological and functional features of the lumen-absorptive cell interface and to the proposed role of a fatty acid binding protein in the intracellular transport of long chain fatty acids. Furthermore the metabolic fate of absorbed fatty acids within the absorptive epithelial cell is reviewed and pointed out, that certain proteins (f.i. apoprotein B) are of crucial significance for the export of intestinal lipids into lymph. Absorption of dietary fat, however, is not the only function of the intestine in lipid metabolism. Intestinal mucosa is also capable of de novo synthesis of lipids, a function which is elaborated upon in the discussion of intestinal biosynthesis of cholesterol. Since in addition biosynthesis of cholesterol in the liver has been shown to be under sensitive control of cholesterol in intestinal lymph lipoproteins and the intestine represents the only organ, in which serum cholesterol can be excreted (after conversion to bile salts in the liver), the intestine occupies the central role in cholesterol metabolism. The intensive interrelation of intestinal functions and systemic lipoprotein metabolism is underscored by the more recent finding, that the intestine also synthesizes endogenous lipoproteins, which are secreted constantly into intestinal lymph even in the fasting state, thus contributing to plasma very low density lipoproteins. The intestine is the only organ, in which extrahepatic production of endogenous lipoproteins has been demonstrated. In the special section of this paper own original work is presented, elaborating on intestinal metabolism of plasma free fatty acids (FFA) and dealing with the important question, whether plasma FFA are incorporated into intestinal lymph lipoproteins. Studies were conducted in rats initially, which were given a rapid i.v. injection of labelled fatty acids. Mucosal radioactivity of the small intestine was greatest 2 minutes after i.v. 14C-palmitate, and accounted for 1% of administered isotope. Of mucosal 14C, 42% were present in water soluble metabolites, including CO2 and ketoacids, 28% in phospholipids and only 16% in triglycerides. The specific activity of mucosal triglyceride fatty acids (TGFA) was 11 times that of serum TGFA, confirming triglyceride synthesis by intestinal mucosa. The unexpectedly low percent incorporation of 14C-palmitate into triglycerides and its preferential conversion to water soluble metabolites, reflecting oxidation, were confirmed in double label experiments (3H-palmitate intraluminally, 14C-palmitate i.v.), which showed marked differences in the metabolism of fatty acids entering mucosa simultaneously from the two sources. While i.v...