The biosynthesis, absorption, and origin of cholesterol and plant sterols in the Florida land crab

In order to study the biosynthesis, composition, and origin of sterols in the Florida land crabs, Cardisoma guanhumi (Latreille), we fed 17 male crabs either a cholesterol-free or a high cholesterol diet for 2 to 7 weeks. The origin of sterols in these crabs, whether from biosynthesis or from the diet, was determined by tahree procedures: the incorporation of isotopic mevalonate into the cholesterol when the diet was cholesterol-free; the absorption of isotopic cholesterol and sitosterol from the diet; the cholesterol and plant sterol concentrations of hepatopancreas, plasma, and muscle under conditions of cholesterol-free and high cholesterol diets. In addition, the interconversion of cholesterol and sitosterol was investigated. Dietary sterols of plant and animal sources were readily absorbed and provided the major source of sterols for this species of crab. The biosynthesis of cholesterol from mevalonate in this crab was minimal. However, cholesterol was synthesized from dietary sitosterol by dealkylation. Cholesterol and the three plant sterols (24 epsilon-methyl cholesterol, stigmasterol, and sitosterol) were found in the hepatopancreas, plasma, and muscle of the crab. Plant sterols contributed from 9 to 37% of the total sterols in the hepatopancreas, plasma, and muscle of the crabs fed a cholesterol-free diet.

Absorption and transport of shellfish sterols in human subjects

While individually cholesterol is the most abundant sterol in clams, oysters, and scallops, five different major noncholesterol sterols (a C-26 sterol, 22-dehydrocholesterol, brassicasterol, 24-methylene cholesterol, and a C-29 sterol) comprise some 60% of the total sterols, a situation unique among foods of animal origin that humans consume. In order to measure the intestinal absorption and transport of these noncholesterol shellfish sterols by humans, clams, oysters, and scallops were fed daily for 3-4 wk to 9 subjects and in a single meal to 8 subjects. The major shellfish sterols in the diet and in the plasma, individual plasma lipoproteins, red blood cells, and feces were analyzed by digitonin precipitation, thin-layer and gas-liquid chromatography-mass spectrometry. The absorption of these shellfish sterols was ascertained both by their appearance in the blood of the subjects fed shellfish and by their intake and output in the feces. After daily feeding for 3--4 wk, shellfish sterols were identified for the first time in the plasma lipids, in lipoproteins (chiefly in low-density lipoprotein), and in red blood cells of all subjects. The absorption of the dietary C-26 sterol, 22-dehydrocholesterol, brassicasterol, 24-methylene cholesterol was 54%, 35%, 4% and 11%, respectively. Our data indicated that the intestinal mucosa of humans absorbs these unusual sterols, and then they enter into the tissues and overall sterol metabolism of the body.

Lymphatic absorption of shellfish sterols and their effects on cholesterol absorption

Studies have been conducted on the absorbability of individual sterols from a mixture of oyster sterols when administered intragastrically to rats with indwelling catheters in the left thoracic duct. In addition, the effect of oyster sterols on cholesterol absorption has been assessed using [4-14C] cholesterol in the mixture, and comparison against absorption of cholesterol alone. The order of absorbability (percentage absorption) of individual sterols from the mixture of oyster sterols was: cholesterol greater than or equal to 26-carbon sterols greater than or equal to dehydrocholesterol greater than 24-methylene cholesterol greater than brassicasterol greater than plant sterols. The absorption of noncholesterol sterols was 8.2 +/- 0.8% of the fed dose, or less than half of that for an equivalent level of cholesterol alone. The presence of these sterols in mixtures containing cholesterol reduced lymphatic absorption of cholesterol by 25 to 40% compared to absorption of the same amount of cholesterol administered alone, or to an amount of cholesterol equal to the total oyster sterols, respectively. These studies suggest that shellfish sterols are poorly absorbed, and, like plant sterols, effectively reduce dietary and/or endogenous cholesterol absorption from the intestine.

Cholesterol and bile acid balance in Macaca fascicularis. Effects of alfalfa saponins

We determine the effects of alfalfa top saponins on cholesterol and bile acid balance in eight cynomolgus macaques (Macaca fascicularis). The monkeys ate semipurified food containing cholesterol with or without added saponins. The saponins decreased cholesterolemia without changing the levels of high density lipoprotein-cholesterol; hence, they reduced the total cholesterol/high density lipoprotein-cholesterol ratio. Furthermore, they decreased intestinal absorption of cholesterol, increased fecal excretion of endogenous and exogenous neutral steroids and bile acids, and decreased the percent distribution of fecal deoxycholic and lithocholic acids. The fecal excretion of fat was also slightly increased, but steatorrhea did not occur. We saw no signs of toxicity in the monkeys after 6 or 8 wk of saponin ingestion. The data suggest that alfalfa top saponins may be of use in the treatment of patients with hypercholesterolemia, but long-term studies on possible toxicity are needed before this therapy can be recommended for humans.

The long term effects of dietary cholesterol upon the plasma lipids, lipoproteins, cholesterol absorption, and the sterol balance in man: the demonstration of feedback inhibition of cholesterol biosynthesis and increased bile acid excretion

In order to study the metabolic responses of humans consuming a diet moderately high in cholesterol content, we carried out a long-term sterol balance study, up to 25 weeks in duration. Two subjects, one normocholesterolemic and one hypercholesterolemic, were given, in sequence, a very low cholesterol diet and then a diet containing 1000 mg cholesterol per day. The plasma lipids, lipoproteins, cholesterol absorption and synthesis, and fecal steroid excretion were then measured during the different dietary periods (10-14 weeks of a very low cholesterol diet and 11 weeks of a moderately high cholesterol diet). During the high cholestrol dietary period, the plasma cholesterol level increased from 280 to 427 mg/dl for Subject 1 and from 123 to 166 mg/dl for Subject 2. The low density lipoprotein (LDL) cholesterol increased from 215 to 318 mg/dl and from 76 to 112 mg/dl. The high density lipoprotein (HDL) cholesterol also increased. Of the possible compensatory mechanisms against cholesterol overloading from the diet, two mechanisms were partially effective: cholesterol biosynthesis decreased (feedback inhibition) and bile acid excretion increased. Cholesterol absorption remained unchanged after the high cholesterol diet and was not a compensatory mechanism despite earlier assumptions that it might be. In spite of these compensatory mechanisms, the cholesterol feeding led to a 44% increase in the plasma cholesterol levels of these subjects. The predominant component of the plasma cholesterol increase was in the cholesterol transported by LDL and with presumably greater atherogenicity as a result. In the hypercholesterolemic subject, the LDL/HDL ratio increased and there was a net storage of cholesterol in the body. Storage of cholesterol did not occur in the normal subject.

Sterols of human gallstones: the recent identification of eight different digitonin precipitable sterols

In this study, we report for the first time the presence of many noncholesterol sterols in human gallstones. Gallstones were obtained from 17 patients at cholesystectomy. They were rich in sterol composition, some 52.8% by weight. A number of different sterols were found: 22-dehydrocholesterol 0.29% of total sterols; cholestanol, 1.54%; lathosterol, 1.92%; 24-methylene cholesterol and campesterol, 0.64%; stigmasterol, 0.02%; beta-sitosterol, 0.21%; and cholesterol, 95.33%. These individual sterols were identified by a variety of methods which included final verification by combined gas chromatography-mass spectroscopy. These unusual sterols were derived either from the diet or from the metabolism of cholesterol in the body. Their presence might well affect the lithogenicity of the bile and raises questions about their metabolism and function generally in the body.

A comparison of the turnover and metabolism cholesterol in normal and atherosclerotic monkey aortas

Rhesus monkeys were fed high cholesterol and cholesterol-free diets for 21-24 months. The animals were then given isotopic cholesterol intravenously and autopsied from 1 to 51 weeks later. The plasma and aortic cholesterol contents were 633 +/- 130 mg/dl and 35.6 +/- 11.4 mg/g dried tissue (45.5% in ester form) for atherosclerotic monkeys and 135 +/- 25 mg/dl and 9.9 +/- 3.6 mg/g (15.0% in ester form) for control monkeys, respectively. The minimal influx rate of cholesterol from plasma into the aorta was much greater for atherosclerotic animals, 0.470 +/- 0.20 mg/g per day versus 0.088 +/- 0.031 for control monkeys. There was a rapid turnover of both free and esterified cholesterol in the atherosclerotic aortas, greater than for normal aortas. These studies of cholesteryl ester metabolism indicated a likely origin of aortic cholesteryl ester from the plasma cholesteryl esters. Our data indicated a dynamic cholesterol metabolism and turnover in the aorta during atherogenesis.

Lipid metabolism in abetalipoproteinemia: a study of cholesterol absorption and sterol balance in two patients

The concept that an absence of apoprotein B in plasma may result in increased cholesterol biosynthesis was investigated by sterol balance techniques in 2 male patients with abetalipoproteinemia, one an adult, the other a child. Total body synthesis of cholesterol in both the adult patient (19.3 +/- 3.8 mg/kg/day vs. 10.8 +/- 0.9 mg/kg/day in controls) and the child with abetalipoproteinemia (34.9 mg/kg/day vs. 14.5 +/- 3.8 mg/kg/day in control children) was significantly higher than in controls whereas bile acid synthesis was similar in both groups. Absorption of orally administered [1,2-3H]cholesterol was lower in the abetalipoproteinemic subjects than the controls and subsequent labeling of plasma cholesterol in the former patients was minimal (less than 3% of controls). The mechanisms for the increased sterol synthesis in abetalipoproteinemia may relate to the absence of chylomicrons and low density lipoproteins in plasma, but the magnitude of the increase can be largely explained on the basis of enhanced sterol losses that occur secondary to malabsorption of biliary cholesterol.

Sterol balance in abetalipoproteinemia: studies in a patient with homozygous familial hypobetalipoproteinemia

A new case of homozygous familial hypobetalipoproteinemia is reported in a 16-yr-old girl. Apoprotein B was absent from plasma and the patient had acanthocytes and steatorrhea, but minimal neurologic dysfunction. Total body cholesterol synthesis was assessed intermittently over a 30-mo period by sterol balance techniques. The rate of synthesis of cholesterol was higher (15.0 +/- 2.9 mg/kg/day) in the patient (8.3 +/- 0.4 mg/kg/day than in 3 control children, p less than 0.005). Bile acid synthesis was similar (4.6 +/- 1.8 versus 4.0 +/- 1.7 mg/kg/day) in the patient and controls, but total body sterol synthesis was significantly higher (19.6 +/- 3.0 versus 12.2 +/- 2.0, p less than 0.005). The absorption of orally administered [1,2,(3)H] cholesterol in the patient was low and less than 0.5% of the label appeared in the total plasma volume at all times up to 48 hr. Estimates of the extent that malabsorption of biliary cholesterol contributes to the enhanced excretion of neutral sterols in this case indicate that all of the increase can be explained on this basis. Thus, although the mechanisms for the increased sterol synthesis in this case may relate to the absence of chylomicrons and low density lipoproteins in plasma, the magnitude of the increase can be fully explained on the basis of a compensatory mechanism to maintain cholesterol homeostasis in the face of enhanced fecal losses.

The cholesterol turnover, synthesis, and absorption in two sisters with familial hypercholesterolemia (type IIa)

To explore the mechanisms of the profound plasma cholesterol elevations in familial homozygous hypercholesterolemia (type IIa), cholesterol turnover, sterol balance, cholesterol absorption, and low density lipoprotein studies were carried out under controlled dietary conditions in two sisters (aged 13 and 16). Cholesterol turnover was prolonged. The half-life of the first exponential of the plasma cholesterol specific activity decay curve was double that of normal adults. The rate constants for the removal of cholesterol from pool A (KAA = 0.0652) and for the excretion of cholesterol from the system (Kaa = 0.0197) were less than half of normal. The production rates of cholesterol were low, only 6.30 and 6.86 mg/kg per day as measured by cholesterol turnover and sterol balance techniques, respectively. Fecal neutral steroid and bile acid excretion were 5.22 and 1.64 mg/kg per day, which is remarkably low in comparison to those of normal and heterozygous children. Cholesterol absorption was within the upper limit of the values reported for normal adults. THE HDL cholesterol values were extremely low (27 mg/dl) in contrast to profoundly elevated LDL levels. The fractional catabolic rate of LDL (0.127 per day) and the rate of synthesis and catabolism of apo-LDL (15 mg/kg per day) were low in comparison to previously reported values in homozygotes. These composite data indicated that the definable metabolic defects of these two sisters with homozygous familial hypercholesterolemia were the sluggish clearance of cholesterol from the body coupled with low total body synthesis of cholesterol.

The steroids of 2000-year-old human coprolites

Six samples of human coprolites, some more than 2,000 years old, were analyzed for fecal steroid composition. Despite this very lengthy period of storage, the fecal steroids of coprolites were remarkably similar to those of stool samples collected today. The sterol nucleus was clearly rather stable under the dry environmental conditions of the Nevada Caves. The steroid content (microgram/g dried weight) of coprolite was low in comparison to that of modern man. The bile acid/cholesterol and plant sterol/cholesterol ratios of the coprolite, however, were similar to these ratios of the stools of modern man. In the six coprolites, an average 73% of the neutral steroids was digitonin-precipitable. This precipitate was composed of cholesterol and three plant sterols (campesterol, stigmasterol, and beta-sitosterol) and their bacteria-modified products. A portion of the neutral steroids had been converted to products tentatively identified as epimers of these steroids. Individual bile acids were identified in the coprolite. The bile acid composition of the coprolite was similar to that of the stool of modern man.

The interaction of dietary fibers and cholesterol upon the plasma lipids and lipoproteins, sterol balance, and bowel function in human subjects

To identify any metabolic effects of dietary fiber upon cholesterol metabolism in man, six adult volunteer subjects were fed eucaloric cholesterol-free formula diets, with and without added dietary fiber for two 4-wk periods. A large quantity of dietary fiber was fed, some 60 g of plant cell wall material (or 16 g of crude fiber) derived from corn, beans, bran, pectin, and purified cellulose. This provided about five times the fiber intake of the typical American diet. The addition of fiber to the cholesterol-free diet did not change either the plasma cholesterol level (171+/-21 mg/dl, SEM, to 167+/-18) or the triglyceride (103+/-39 to 93+/-27 mg/dl). The excretion of both endogenous neutral steroids and bile acids were unchanged with fiber (505+/-41 to 636+/-75 mg/day and 194+/-23 to 266+/-47 mg/day, respectively.) However, total fecal steroid excretion was increased 699+/-29 to 902+/-64 mg/day, P < 0.025). With fiber, intestinal transit time was decreased (59+/-9 to 35+/-8 h, P < 0.005), and both the wet and dry stool weights were greatly increased.A second group of six subjects was fed similar diets containing 1,000 mg cholesterol derived from egg yolk. The addition of fiber to the 1,000-mg cholesterol diet did not alter either plasma cholesterol level (233+/-26 to 223+/-36 mg/dl) or triglyceride (102+/-19 to 83+/-11 mg/dl). The excretion of endogenous neutral steroids (618+/-84 to 571+/-59 mg/day), of bile acids (423+/-122 to 401+/-89 mg/day), and of total fecal steroids (1,041+/-175 to 972+/-111 mg/day) were unchanged by fiber. The absorption of dietary cholesterol was not altered when fiber was added to the 1,000-mg cholesterol diet (44.0+/-3.3 to 42.9+/-2.5%). A two-way analysis of variance utilizing both groups of subjects indicated a significant (P < 0.001) effect of dietary cholesterol upon the plasma cholesterol concentration. We concluded that a large quantity of dietary fiber from diverse sources had little or no effect upon the plasma lipids and sterol balance in man in spite of the fact that intestinal transit time and stool bulk changed greatly.

The intestinal absorption of dietary cholesterol by hypercholesterolemic (type II) and normocholesterolemic humans

The incomplete absorption of dietary cholesterol may represent an adaptive intestinal barrier that prevents hypercholesterolemia. To explore this mechanism, we compared cholesterol absorption in 15 normocholesterolemic and 6 hypercholesterolemic (type II) subjects fed background cholesterol-free formula diets with 40% of calories as fat. Each test meal consisted of a breakfast into which was incorporated scrambled egg yolk containing 300-500 mg of cholesterol and [4-(14)C]cholesterol (3-22 muCi), either naturally incorporated into the yolk cholesterol by previous isotope injection into the laying hen or added in peanut oil to the yolk of the test breakfast. In some instances [1alpha-(3)H]cholesterol was the radioactive marker. The radioactivity of the fecal neutral sterol fraction was determined in daily stool samples for the next 7 days to provide an estimate of unabsorbed dietary cholesterol. The amount of absorbed and reexcreted labeled cholesterol proved negligible. Most unabsorbed dietary cholesterol appeared in the stool on the second or third day after the meal, and 95% or more was recovered in the stool by 6 days. Plasma specific activity curves were usually maximal at 48 h. Normal subjects absorbed 44.5+/-9.3 (SD) of the administered cholesterol (range 25.9-60.3). Hypercholesterolemics absorbed the same percentage of cholesterol as normals: 47.6+/-12.6% (range 29.3-67.3). Absorption was similar whether the radiolabeled cholesterol was added to egg yolk or naturally incorporated in it (42.1+/-9.3 vs. 48.9+/-9.8%). Six normal subjects were fed a cholesterol-free formula for 4 wk, and then different amounts of cholesterol (110-610 mg/day) were added for another 4 wk. At the end of each period, single test meals containing either 110, 310, or 610 mg of cholesterol and [1alpha-(3)H]cholesterol were administered. Cholesterol absorption was 42.3+/-6.0% and 45.4+/-8.3% for the two dietary periods, respectively. The absolute cholesterol absorption was linearly related to the amount of cholesterol in the test meal, and absorption was not affected by background diets high or low in cholesterol content.

Cholesterol metabolism and placental transfer in the pregnant Rhesus monkey

The placental transfer of cholesterol (5-cholesten-3beta-ol) was investigated by giving pregnant rhesus monkeys cholesterol-1alpha-(3)H or cholesterol-4-(14)C and then determining the cholesterol specific activity (SA) in maternal serum and in fetal serum and tissues. An isotopic steady state was established in five pregnant animals by the daily feeding of a tracer dose of cholesterol-4-(14)C. Comparison of maternal and fetal serum cholesterol SA revealed that an average of 42.6% of the serum cholesterol in the term fetus originated by transfer from the maternal blood. The remainder presumably arose by fetal synthesis de novo. Fetal tissues had cholesterol SA equal to or slightly less than that of fetal serum, except for brain which had a SA only 5% that of fetal serum. In other studies a single intravenous dose of radioactive cholesterol was given to either mother or fetus in late pregnancy. The time for detectable passage across the placenta in either direction was between 4 and 24 hr. With maternal administration of the isotope, there was equilibration of maternal and fetal serum cholesterol SA after 10-12 days. With fetal injection of isotopic cholesterol, however, the maternal cholesterol SA never attained a level more than 5% of fetal SA. This indicated that the net cholesterol flux was strongly in the direction of mother to fetus. Serum cholesterol levels were significantly greater in maternal than in fetal serum (80.3+/-18.5 vs. 59.6+/-15.6 mg/100 ml). Maternal serum cholesterol concentration in the monkey was significantly lower in late pregnancy than during the puerperium. Studies of breast milk indicated that approximately two-thirds of milk cholesterol was transferred from the maternal blood.

Metabolism of cholesterol in the tissues and blood of the chick embryo

Three artificially inseminated laying White Leghorn hens were given 35-50 micro c of cholesterol-4-(14)C intravenously. Their subsequently produced eggs contained cholesterol-(14)C-labeled yolks. Some of the fertilized eggs were analyzed for cholesterol content and radioactivity. Other eggs were incubated until hatching. The specific activity of the cholesterol contained in the serum and tissues of newly hatched chicks was determined and compared with that of yolk sac, which was taken as representative of egg yolk cholesterol before its metabolic transfer into the chick embryo. The specific activities of cholesterol in intestine, liver, serum, heart, and skeletal muscle and the whole chick were 95-98% of that in yolk sac, but that of brain cholesterol was only 11% of this value. These results indicate that whereas most of the cholesterol in the chick originated from the egg yolk, cholesterol biosynthesis was active in the brain and provided about 90% of its cholestero content. Newly hatched chicks were found to be hyperlipemic compared with older chicks and had fatty livers with a high cholesterol content. Desmosterol was found in 9- and 15-day old chick embryos but not in the newly hatched chicks, in which the only sterol was cholesterol.

Placental transfer of cholesterol-4-14C into rabbit and guinea pig fetus

A tracer dose of cholesterol-4-(14)C was given daily in the diet of six pregnant guinea pigs to establish an isotopic steady state. At the time of parturition, maternal and fetal blood and fetal tissues were collected and analyzed for cholesterol content and cholesterol specific activity. A comparison of these specific activities in neonatal and maternal serum indicated that about 22% of the fetal serum cholesterol was transferred from maternal blood. In the newborn, tissues generally had the same cholesterol specific activity as serum. Brain tissue was an exception in having a specific activity only 8.4% of that of serum. Dietary cholesterol did not increase serum cholesterol levels in the newborn but did increase the percentage of fetal cholesterol derived from the maternal circulation. The rapid transfer of cholesterol-4-(14)C across the placenta was indicated by the appearance of this isotope in the newborn 2 days after its administration to pregnant rabbits. A considerable amount of the cholesterol content of newborn guinea pigs and rabbits originated from the maternal blood.