Leon Swell

Dynamic Aspects of Cholesterol Metabolism in Different Areas of the Aorta and Other Tissues in Man and Their Relationship to Atherosclerosis

Tracer doses of cholesterol-4-C14 were administered to each of 8 patients with limited life expectancies. Chemical determinations of free and esterified cholesterol and determinations of their C14-specific activities were made in serially collected sera, in tissues, and in the different layers of the abdominal aorta, the intimas of which were separated where possible, into normal-appearing intima, thickened intima, and free-lying lipid material. The turnover times of the free and esterified cholesterol fractions of the various tissues and the portions of those fractions derived from the plasma and by synthesis were calculated from these data. The plasma and liver free cholesterol fractions were in equilibrium at the earliest time, 2.5 days. The free cholesterol fractions of other tissues were in equilibrium with the plasma free cholesterol at later dates. The esterified cholesterol fractions of the various tissues, including liver, were in equilibrium with that of the plasma at later dates than were their free fractions. The findings indicate that a large portion of the cholesterol ester of the plasma is not derived from the liver. The greater part of both the free and ester cholesterols of other tissues was derived from the plasma, although a significant portion was derived by synthesis, except for both fractions of skeletal muscle. The adrenal glands were the most active in the synthesis of free cholesterol. The data indicate that a major portion of adrenal cortical hormones is synthesized directly from acetate, without cholesterol as an intermediary product. A major portion of the cholesterol of the intima was derived from plasma through interchange but a considerable amount of cholesterol was synthesized in the intima. The free-lying lipids in the intima appeared to be inert. Data concerning the changes of the lipid contents of the intima with the development of atherosclerosis have been presented. One patient, who died of a second myocardial infarction, differed from the others in having a longer turnover time of the cholesterol of his intima.

Correlation of arachidonic acid of serum cholesterol esters in different species with susceptibility to atherosclerosis.

Summary Serum CEFA composition of 8 different species has been determined by gas-liquid chromatography. Different CEFA spectrums were found for each species. The outstanding variation was the proportions of arachidonic acid in CEFA of the serums. That of the rat had 50% of arachidonic acid and that of the dog had 17%. All species also had a large proportion of linoleic acid in the CEFA of their serums. The rat and the dog, with high arachidonic acid levels in CEFA of their serums, are known to be highly resistant to development of atherosclerosis. Those species having small proportions of arachidonic acid in the CEFA of their serums are susceptible to atherosclerosis and develop the disease spontaneously. This study provides evidence, by association, that level of arachidonic acid in the CEFA may be related to susceptibility to atherosclerosis. It remains open to investigation whether the proportion of arachidonic acid in the CEFA is a characteristic of the species or whether it, and the susceptibility to atherosclerosis, can be altered by dietary measures.

Sterol specificity of pancreatic cholesterol esterase.

Summary Pancreatic cholesterol esterase catalyzes the esterincation and hydrolysis of a number of different sterols and sterol esters. The order of activity observed in the esterifying system with oleic acid was as follows: dihydrocholesterol † cholesterol † β-sitosterol † sitosterol † stigmasterol † ergosterol. Of these sterols, only dihydrocholesterol and cholesterol were esterified with butyric acid. In the hydrolytic system all of the sterol butyrates tested were split very rapidly, with the exception of ergosterol butyrate. the order of splitting of the oleates in the first 4 hours was: cholesterol oleate † sitosterol oleate † stigmasterol oleate † ergosterol oleate. The relation of enzyme activity to sterol structure is discussed.

The mechanism of cholesterol absorption.

In reviewing the literature directly related to sterol absorption one is impressed by the very limited area of agreement. Most investigators agree that absorbed cholesterol is transported via the lymph, that bile is obligatory for absorption, and that a major portion of the cholesterol in lymph is in the esterified form.’-b There is also evidence both for and against the participation of pancreatic juice and esterification in the absorption process.6-12 There are three puzzling aspects of cholesterol absorption that cannot be explained on the basis of a direct transfer of cholesterol from the lumen of the intestine to lymph. These are: first, the appearance of fed cholesterol-4-CI4 in lymph for periods up to several days; second, the endogenous dilution of fed cholesterol-PCI4 in its transfer from the lumen to the lymph and, third, the poor absorption of cholesterol-4-C14 when large or small doses are fed. Recently Glover and his co-worker~~~~ l4 have postulated that cholesterol absorption takes place at a molecular level by way of a rapid exchange and transfer process between the lipoproteins of the cell membrane, organelles, and ground plasm. Endogenous dilution occurs due to interchange of the labeled cholesterol with inactive cholesterol on the lipoproteins. According to these workers, esterification acts in absorption only as an accelerating factor. Also, one explanation offered for the absorption of plant st9rols is based on the fact that these have a certain degree of a fh i ty for the acceptor lipoproteins that allow these sterols to participate in exchange reactions during passage across the mucosal cells. The mechanism proposed by Glover and his colleagues accounts neither for the obligatory requirement of bile nor for the appearance of labeled cholesterol in the lymph for periods up to several days after its feeding. in which cholesterol-PC14 and the lymph fistula animal have been used to study cholesterol absorption, it has been common practice to administer 1 to 3 mg. cholesterol-4-C14 dissolved in corn or cottonseed oil. In a fasted rat the amount of cholesterol appearing in the thoracic duct lymph during a 2Phour period is 8 to 10 mg.16* Is When fat alone is administered there is an increase of approximately 2 mg. in the lymph cholesterol level.16 Thus, the administration of small amounts of cholesterol-4-Cl4 (1 to 3 mg.) dissolved in fat does not produce a chemical increase in the lymph cholesterol over that normally expected from the feeding of fat alone. However, as demonstrated by several workers, the feeding of these amounts of cholesterol-PC14 is followed ,by the appearance of labeled cholesterol in lymph.4* 6 , 7-9 While this certainly demonstrates absorption of the labeled * The work reported in this paper was supported in part by research grants from the American Heart Association, Inc., New York, N. Y., and Grants H-1897 and H-2746 from the National Heart Institute, Public Health Service, Bethesda, Md.

Lipid Fatty Acid Composition of Several Areas of the Aorta in Subjects with Atherosclerosis.

Summary The cholesterol ester, triglyceride and phospholipid fatty acid composition (gas-liquid chromatography) of aortic media, thickened intima and plaque material were determined in 6 human subjects. For comparison, the fatty acid composition of those fractions in serum was determined in 6 subjects (aged 55-70 years) with occlusive atherosclerosis in a good nutritional status. The triglyceride fraction of those tissues and serum were similar in their fatty acid composition. Some slight differences were noted in the fatty acid composition of the phospholipid fractions, most notably in the arachidonic and long chain fatty acids. The cholesterol ester fraction of the tissues studied showed the greatest differences in fatty acid composition. Both early and advanced plaques had significantly less linoleic and more oleic acid in that fraction than serum or media. Media and serum CEFA were similar in their fatty acid composition. The significance of these findings in relation to atherogenesis is discussed.

Adrenal Cholesterol Ester Fatty Acid Composition of Different Species.

Summary The fatty acid composition of adrenal cholesterol ester fraction has been determined by gas-liquid chromatography in 7 species. Characteristic CEFA patterns for each type of animal were apparent. Rat and dog had the highest content of polyunsaturated fatty acids in the adrenal cholesterol ester fraction. The remaining species had considerably less of those acids and the major CEFA of dog, man, goose, chicken and rabbit was oleic acid. In the dog, rat and human, an acid tentatively identified as a C22 tetraenoic acid was found in the adrenal CEFA. The significance of these results in relation to hormone synthesis and polyunsaturated fatty acid metabolism is discussed.

Specific function of bile salts in cholesterol absorption.

Summary Entrance of cholesterol-4-C14 into the free cholesterol pool of mucosa and its subsequent transfer to the lymph requires the presence of a specific type of bile salt. This process does not require the presence of dietary fatty acid. It is suggested that free cholesterol in the lumen of the intestine complexes with bile salt which then enters the free cholesterol pool of mucosa.

Influence of Dietary Bile Salts on Blood Cholesterol Levels

Summary Different bile salts were fed to rats on high cholesterol-fat diets. The blood cholesterol increased very markedly with the feeding of sodium cholate and taurocholate, somewhat less with the feeding of desoxycholate and no increase was observed in those fed dehydrocholate. When sodium dehydrocholate, taurocholate, and dehydrocholate were fed successively to the same animal, taurocholate increased the blood cholesterol very markedly while dehydrocholate first had no effect and later lowered it. The results of this study agree in general with the findings of the preceding study on the in vitro effect of bile salts on cholesterol esterase activity, in that a comparable pattern of activity was observed in relation to the effect of bile salts on the blood cholesterol. It is suggested that the bile salts are necessary for cholesterol absorption because they are a necessary co-factor for cholesterol esterase activity.

Fatty Acid Composition of Tissue Cholesterol Esters in Elderly Humans with Atherosclerosis.

Summary The fatty acids esterified with cholesterol have been determined by gasliquid chromatography in the media, free-lying lipid containing material of far-advanced plaques, and livers obtained at autopsy from 6 atherosclerotic subjects. For comparison, serum CEFA in 6 well-nourished subjects with occlusive atherosclerosis was also determined. Of tissues studied, composition of CEFA of the aortic media was very similar to that of serum. Plaque material contained a greater proportion of saturated and monoenoic fatty acids than either media or serum. Liver CEFA contained considerably less polyunsaturated fatty acids and more saturated and monoenoic acids than did serum or media. The major CEFA of media and serum was linoleic acid and major fatty acid of plaque material and liver was oleic acid. The significance of these findings in relation to the role of unsaturated fatty acids in atherogenesis is discussed.

Electrolyte Changes in Ileal Contents and in Feces during Restriction of Dietary Sodium with and without the Administration of Cation-Exchange Resin

Sodium and potassium levels of the contents of the terminal ileum and feces were determined for four dogs at different dietary sodium intakes, both before and during administration of a carboxylic cation-exchange resin. With restriction of dietary sodium, there was more sodium bound by the resin in the terminal ileum than was provided by the diet. There was considerable conservation of sodium in the upper gastrointestinal tract, but the colon always absorbed a major part of the sodium presented to it. During resin administration with very low sodium diets there was more sodium in the feces than in the diets.