Radioactive Cholesterol Research Articles

Metabolism of 1-(14)C linolenic acid in developing brain: II. Incorporation of radioactivity from 1-(14)C linolenate into brain lipids.

Metabolism of 1-(14)C linolenic acid was studied in growing animals by injecting the tracer intraperitoneally into 12-13 day old suckling rats and following up the results by sacrificing groups of animals at 8 hr, 48 hr, 15 day, and 45 day intervals. In the first 15 days, there was a greater decrease in radioactivity of brain total lipids compared to the later period, although the earlier age period is characterized by lipid deposition rather than breakdown. Since the 18∶3 ω3 family of fatty acids occurs largely in the brain total phosphatidyl ethanolamine fraction, we expected that, in the initial period, total phosphatidyl ethanolamine would be the most highly radioactive component. However, results showed that 8 hr after the tracer phosphatidyl choline had the highest specific radioactivity. When the total phosphatidyl ethanolamine fraction was resolved into diacyl and alk-1-enyl species, it was found that radioactivity was not distributed evenly between the two species. There was a progressive increase in radioactivity of the alkenyl and a decrease in the diacyl species. Forty-eight hr after the tracer, however, the radioactivity of phosphatidyl ethanolamine increased and at 45 days remained slightly higher than phosphatidyl choline. Radioactivity of cholesterol, a result of synthesis from acetate undoubtedly derived from the breakdown of tracer linolenate, was also high 48 hr after tracer and remained high until 45 days.

An evaluation of some of the potential immediate sources of cholesterol for bile acid synthesis in swine

An experiment with cholesterol-fed and mash-fed swine, untreated or treated with hypocholesterolemic drugs, was terminated with a single dietary pulse of radioactive cholesterol followed by a seven-day period of observation for a balance study.The appearance of radioactivity in fecal bile acids was consistent with a transit time of three days or less. Comparison of measured quantity and specific activity of fecal bile acids with quantity and specific activities calculated on the assumption that serum cholesterol is the more or less immediate precursor cast doubt on this very assumption. The inconsistency remains if calculations are based on free cholesterol only or ester cholesterol only.With the data available, the simplest assumption leading to reasonable agreement with observation was that the precursor is a mixture (unweighted average) of dietary and endogenously synthesized cholesterol.

Isotope kinetics of human skin cholesterol secretion.

Specific radioactivity (SA) time curves of plasma and skin surface cholesterol collected at several skin areas were recorded in 10 patients on formula diets after single intravenous injections of radioactive cholesterol. Earliest detectable radioactivity on skin surface was found in 4-6 days; depending on the skin site, SA's peaked in 13-75 days. SA's of free cholesterol were almost always higher than those of esterified cholesterol. The general forms of the SA time curves were in keeping with the idea that plasma cholesterol is carried to the skin surface in association with the epidermal and sebaceous cells, whereby (a) cholesterol synthesized de novo is mixed with derived from plasma and (b) appearances of plasma cholesterol on the skin surface is delayed by a time factor that corresponds to the movement of epidermal and sebaceous cells from the basal layer to the skin surface. The shorter mean transit times of plasma cholesterol on skin areas rich in sebaceous glands (22-24 days on the head) than on those poor in these glands (38 days on forearms and 72 days on feet) suggest that cholesterol passes faster through the sebaceous glands than through the epidermis, and faster through thin than thick epidermis. The fraction of skin surface cholesterol (f) that is derived from plasma cholesterol was estimated by three independent methods, and comparable results were obtained. Values of f were lower on skin areas rich in sebaceous glands (0.29-0.46 on forehead) than on areas poor in these glands (0.41-0.70 for forearms; 0.60 on feet) and lower for esterified (0.27-0.33) than for free (0.39-0.48) cholesterol. These data suggest that higher proportions of sebaceous gland and of esterified cholesterol, respectively, are synthesized de novo than epidermal and of free cholesterol. In two patients it was possible to calculate that f of total skin surface cholesterol was 0.49 and 0.37, respectively, and that the maximum amount of plasma cholesterol lost through the skin was 29 and 22 mg/day, respectively. Knowing the total daily excretion of total neutral and acidic steroids in feces in these patients, and assuming a total daily urinary steroid excretion 50 mg, we estimated that no more than 3.2% of total steroid excretion occurred via the skin.

Direct counting of tissues containing radioactive cholesterol by a two-phase liquid scintillation method

A rapid and simple method for counting radioactivity in tissue samples containing [ 3H]- or [ 14C]-cholesterol is described. Up to 500 μl of the specimen to be counted (plasma, tissue homogenate) is measured into a counting vial. The lipids of the tissue are extracted into 15 ml of a toluene-based scintillation mixture containing 37.5% ethylene glycol monomethyl ether that is added to the same vial. With the addition of 1 ml water, two phases form: the upper toluene phase containing all cholesterol together with the scintillating phosphor and the lower water phase containing most of the quenching material. Bleaching to reduce color quenching is not necessary. Chemiluminescence is negligible. The counting efficiencies are appreciably higher than those obtained in aqueous one-phase scintillation systems but lower than those obtained with pure standards in one-phase pure toluene scintillation systems.

Increased cholesterol-ester formation during forced cholesterol synthesis in rat hepatocytes.

By comparing the incorporation of 3H20 and [14C]mevalonate into cholesterol in suspensions of rat hepatocytes, it was calculated that the cholesterol biosynthesis could be stimulated 4--7-fold by addition of mevalonate. The addition of 3.3--6.7 mM mevalonate also caused a 5--6-fold increase in the proportion of newly synthesized cholesterol that was esterifield. The esterification of radioactive cholesterol, entering the cells by exchange with surrounding plasma lipoproteins was also increased, indicating that a true increase in the rate of cholesterol ester formation rather than a more selective utilization of newly synthesized cholesterol for esterification, occurred. The increase in cholesterol esterification was not abolished by cycloheximide, indicating that it did not require an increased synthesis of cholesterol esterifying enzyme. Instead the data suggest that the supply of cholesterol to the esterifiable pool may be an important factor, regulating the rate of cholesterol ester formation in rat liver. The addition of 0.5 mM oleic acid to the medium did not increase the degree of cholesterol esterification significantly, whereas 2 mM oleic acid bound to 1% albumin increased the proportion of newly synthesized cholesterol that was esterified, by about 70%. The cells secreted radioactive cholesterol esters into the medium. Cycloheximide inhibited this secretion to about 80% but did not affect the rate at which newly synthesized cholesterol was transferred to surrounding plasma lipoproteins.

Experimental atherosclerosis in rabbits fed cholesterol-free diets: 4. Investigation into the source of cholesteremia

The metabolism of exogenous and endogenous cholesterol was studied in rabbits fed a normal diet of rabbit chow or an atherogenic, semisynthetic diet ( SS) containing 40% carbohydrate, 25% casein, and 14% hydrogenated coconut oil. The SS diet was more cholesteremic, betalipoproteinemic and atherogenic. More cholesterol was absorbed and more synthesized by rabbits on the SS diet and more radioactive cholesterol was recovered from their lesions. The chow-fed rabbits excreted 4 times as much feces over a 3 day period. Biliary cholesterol and total bile acids were 2.2 and 2.6 times higher in the SS group. The biliary cholesterol-specific activity was significantly higher in the SS group but cholanoic acid-specific activities and primary/secondary bile acid ratios were significantly lower. The data suggest that one reason for the cholesteremic effect of the SS diet is a reduced level of degradation of endogenous cholesterol to bile acids.

Transport of cholesterol in the chick optic system.

Abstract—After injection of radioactive cholesterol into the vitreous humour of chicks, cholesterol is axonally transported along the optic nerve to the contralateral optic tectum. In the optic nerve of young chicks cholesterol is only transported at the slow rate whereas in the sciatic nerve of adult chicks it is transported at both the fast and slow rates. This difference is not due to age or a peculiarity of avian species as the same difference exists in mice. Estimates of the cholesterol content of tissues and the amount of cholesterol transported suggests the existence of small rapidly turning over pools of cholesterol in nerves. Due to its great metabolic stability, cholesterol is a useful and specific marker for slow transport in the optic nerve and thus may prove suitable for the mapping of neuronal projections.

Effect of cholesterol feeding on cholesterol biosynthesis in maternal and foetal rat liver.

Hepatic cholesterol biosynthesis has been studied in rat foetuses whose mothers had been fed on a cholesterol rich diet during the last week of gestation. Foetal liver was found to be capable of synthesizing cholesterol from acetate in vitro. The rate of incorporation of labelled acetate into digitonin precipitable sterols, fatty acids and CO(2) in foetal liver was much higher than that found in maternal liver. Cholesterol feeding reduced the rate of sterol synthesis in maternal liver but it did not have any appreciable effect on foetal liver. In order to investigate whether this lack of feed-back control in foetal liver could be attributable to an obstacle to the placental transfer of dietary cholesterol. 14-C-cholesterol was administered to the pregnant rats and its distribution in maternal and foetal liver and plasma was studied. Our results indicate that placental transfer of cholesterol from mother to foetus occurs very slowly so that only a small proportion of labelled cholesterol is found in foetal plasma over a 48 hour period following the administration of radioactive cholesterol. Cholesterol transferred from the mother into the foetal plasma is efficiently taken up by the foetal liver. These findings would suggest that the low amount of dietary cholesterol transferred from the mother into the foetal plasma is not sufficient to activate the control mechanism of the cholesterol biosynthetic pathway in the foetal liver.

The Effect of Insulin on the Incorporation of D-Glucose-U-14C into the Lipids of the Rat Aorta In Vivo

Recent studies have shown that an excessive insulin response to glucose is common in subjects with premature atherosclerosis. To test whether insulin may influence the metabolism of the arterial wall, the effect of insulin on the incorporation of radiolabeled glucose into arterial lipids in vivo was studied in rats. Intravenous injection of 10,000 µ units of insulin per 100 gram body weight resulted in increased incorporation of D-glucose-U-14C into triglyceride 60 and 90 minutes after injection, and into triglyceride and free fatty acids 180 and 360 minutes after injection. No effect on radioactive cholesterol was found. There was no correlation between serum and aortic radioactivities 90 and 360 minutes after injection, suggesting that the effect of insulin on arterial lipids did not originate from changes in serum lipids. Thus the hyperinsulinism found in human subjects with atheromatous vascular disease may have a role in the pathogenesis of atherosclerosis.

Cholesterol exchange between the red blood cells and plasma of young and old rats

The exchange of unesterified cholesterol between plasma and red blood cells of young (1.5 month) and old (1–2 years) rats was studied using [ 3H]-cholesterol as label. Using labeled plasma and unlabeled red blood cells, no equilibrium of specific activity of unesterified cholesterol with the specific activity of cholesterol of red blood cells occurred in either young or old rats. Using labeled red blood cells and unlabeled plasma, equilibration of radioactivity of cholesterol was observed in both young and old rats. However, a more rapid exchange of free cholesterol was observed when labeled old red blood cells were incubated with unlabeled plasma. This apparent paradox may be explained by the existence of a larger, more readily exchangeable pool of free cholesterol in red blood cells of old rats than of young rats. Red blood cells and plasma of old rats contained significantly more free cholesterol than those of young rats but phospholipid levels were comparable.

Changes in plasma lipoprotein lipids in hypercholesterolaemic patients treated with the bile acid-sequestering resin, colestipol.

1. Seven patients with type II hyperlipoproteinaemia were treated with the bile acid-sequestering resin, colestipol (5 g three times daily), after a prolonged period of taking placebo. 2. After 8–9 weeks of treatment, the plasma concentration of the non-esterified cholesterol of very-low-density lipoprotein (VLDL) had risen by a mean of 0.09 mmol/l (43% increase, P < 0.001), that of the esterified cholesterol of VLDL had risen by a mean of 0.11 mmol/l (38% increase, P < 0.01), and that of the triglyceride of VLDL had risen by a mean of 0.40 mmol/l (53% increase, P<0.001). During the same period, the plasma concentration of the non-esterified cholesterol of low-density lipoprotein (LDL) decreased by a mean of 0.44 mmol/l (26% decrease, P < 0.01), that of the esterified cholesterol of LDL decreased by a mean of 1.28 mmol/l (30% decrease, P< 0.001), and that of the triglyceride of LDL decreased by a mean of 0.04 mmol/l (8% decrease, P < 0.01). No significant changes occurred in the plasma concentration of either the cholesterol or triglyceride of high-density lipoprotein (HDL) during treatment. 3. During the early period of treatment with colestipol, changes took place in the specific radioactivity of plasma cholesterol (labelled by intravenous injection of [3H]cholesterol), which, together with the changes in the mass of cholesterol within the individual plasma lipoproteins, were consistent with an increased influx into plasma of non-esterified cholesterol within VLDL, and an increased efflux of cholesterol from plasma within LDL.

The turnover of cholesterol in human atherosclerotic arteries.

The equilibration of cholesterol between plasma and atherosclerotic arteries was studied in 13 patients with obstructive atherosclerosis 2-96 days after the intravenous and/or oral administration of isotopic cholesterol. Arterial specimens were obtained in 12 patients during surgery for arterial reconstruction and in a 13th patient at autopsy. Equilibration was calculated as the specific radioactivity of cholesterol in the arterial tissue relative to that in the plasma (percent).In specimens obtained 2-4 days after pulse labeling, the specific activity of cholesterol in atheroma ranged from 0.3 to 4.5% of that in the plasma. By 17-27 days, the relative specific activity ranged from 6 to 20% in different arteries. In contrast, cholesterol of skeletal muscle had a relative specific activity of 96% by 22 days. By 61-96 days, atheroma cholesterol in the abdominal aorta, common iliac, and femoral arteries had equilibrated to 55, 30, and 26%, respectively. In the patient who died at 96 days, the cholesterol in the coronary arteries had a mean equilibration of 66%, similar to the values for the abdominal (66%) and thoracic (57%) aortas. The route of administration of the isotope did not influence the equilibration. Within the atheromatous plaque, the superficial layers equilibrated better than the deeper layers (75% vs. 22%). The free cholesterol in the atheroma equilibrated to a significantly higher extent than did esterified cholesterol (59% vs. 38%). There was a fourfold higher specific activity of cholesterol in the media than in the corresponding intima (916 vs. 230 dpm/mg). The estimated minimal influx rates of plasma cholesterol into the atheromatous intima ranged from 0.065 to 0.274 mg of cholesterol/g dry tissue per day for different arteries. The approximated turnover times of atheroma cholesterol ranged from 442 days for the abdominal aorta and the coronary arteries to 580 days for the common iliac and 821 and 934 days, respectively, for the femoral and the carotid arteries. These data indicate a definite, though slow, exchange of cholesterol between the plasma and severely atherosclerotic human arteries. Within the atheroma, there are multiple pools of cholesterol, each turning over differently and more slowly than the cholesterol of most other tissues, such as the skeletal muscle. The estimates of influx rate and turnover time of atheroma cholesterol suggest the possibility that this cholesterol is mobilizable, an indication of potential regression of atheromatous lesions in man.

A physiological method for labelling plasma lipoproteins with radioactive cholesterol

To study the metabolism of plasma cholesterol, it is necessary to have the tracer in the same physical state as the plasma cholesterol itself. A number of in vitro methods have therefore been designed to incorporate radioactive cholesterol into plasma, but most of them have not proven satisfactory, except perhaps one which appears promising in this regard. The plasma in this method is labelled by transfer of radioactive cholesterol from red blood cells through a process of exchange, so that on theoretical grounds all the radioactive cholesterol should be present in plasma lipoproteins; and this was supported by the experimental data. On agarose electrophoresis of the labelled samples, the radioactive cholesterol added through exchange with red blood cells was present only in association with lipoproteins; whereas, significant fractions of the tracer incorporated by other methods in current use were not present with the lipoproteins. The rate of disappearance from circulation of radioactive cholesterol incorporated into plasma by the former method was similar to that of cholesterol native to lipoproteins, but the rates of disappearance of radioactive cholesterol incorporated into plasma by other methods were much faster than normal. Preliminary work has also suggested that the red blood cell exchange method can be successfully employed to study the metabolism of free cholesterol in isolated lipoproteins.

The Enzymology of Cholesterol Esterification

Formation of cholesteryl esters are catalyzed by several different enzymes, the most important are acyl-CoA: cholesterol acyltransferase, lecithin: cholesterol acyltransferase, and cholesterol ester hydrolase. Because the substrates of these enzymes are mostly water-insoluble and bound to biomembranes, and because the products are transferred to other compartments, the determinations of these enzymes are very difficult. The problems concerning lecithin: cholesterol acyltransferase are discussed in more detail. The formation of cholesteryl ester in plasma is dependent both on the enzyme and on the availability of substrates. It is almost impossible to separate these two factors in an enzyme assay. Using radioactive cholesterol in the assay presupposes a known degree of isotopic equilibrium. This is difficult, especially in abnormal sera. Assay for the activity of lecithin: cholesterol acyltransfer is not a routine method, and should be performed only in connection with other measurements of lipid metabolism.

In vivo incorporation of 14C into liver and kidney sterols from parenterally administered (2-14C)D,L-mevalonic acid.

SummaryTwenty-four hr after the iv administration of [2-14C]d,l-mevalonic acid to dogs and to sheep, the specific radioactivity of digitonin precipitatible sterol expressed as cholesterol in the kidney cortex was respectively 10.0- and 5.6-fold greater than the specific radioactivity of plasma cholesterol. Cholesterol of the kidney medulla, by contrast, attained a specific radioactivity which was 20% and 10% less than that of plasma cholesterol. Parenteral administration of [2-14C]d,l-mevalonic acid to gerbils and to hamsters resulted in kidney cortical sterol specific radioactivities, 24 hr after the mevalonic acid had been given, which were, respectively, 34- and 18.7-fold greater than the specific radioactivity of liver cholesterol. Hamster kidney medullary cholesterol, at the same time interval attained a specific radioactivity 30% less than that of liver cholesterol. The apparent half life of the 14C-labeled kidney cortical sterols was 4.3 days in gerbils and 5.5 days in hamsters while the apparent h...

The lipids and lipoproteins of human peripheral lymph, with observations on the transport of cholesterol from plasma and tissues into lymph.

1. The lipids and lipoproteins of lymph obtained from the dorsum of the foot were examined in seven human subjects. 2. The concentration of total cholesterol in lymph was about one-tenth that in plasma and was significantly correlated with the plasma total cholesterol concentration. The ratio of esterified to total cholesterol in lymph was similar to that in plasma. 3. Triglyceride was detectable in lymph, but the concentration was less than one-tenth that in plasma and was unrelated to the plasma triglyceride concentration. 4. No lipase activity was detectable in lymph, either before or after intravenous injection of heparin. 5. Cholesterol-esterifying activity was detected in four samples of lymph. 6. The major lipoprotein antigens of human plasma (apo-A, apo-B and apo-C) were present in whole lymph, but their distribution in fractions of different density was different from that in plasma. 7. [14C]Cholesterol, injected intravenously, appeared in lymph within 30 min of the injection, indicating that some of the cholesterol in lymph is derived directly from plasma. 8. At intervals greater than 29 days after a single intravenous injection of [14C]-cholesterol, the specific radioactivity of lymph cholesterol was greater than that of plasma cholesterol, indicating that some of the cholesterol in lymph is derived from tissue pools of cholesterol with slow turnover.

Time course of biosynthetic cholesterol in the adult rat brain

In a first experiment, the time course of biosynthetic cholesterol was studied in the whole brain of adult rats, 1 h to 15 months after an intraventricular injection of 5-[ 14C]mevalonic acid. Radioactivities of nonsaponifiable compounds, sterols and cholesterol purified by gas-liquid chromatography were measured in the brain and those of precursor sterols and squalene fraction were calculated. In the other experiments, only cholesterol radioactivities were measured from 3 days to 1–3 months after injection (1) in the whole brain and in a piece of skull, (2) in the cerebellum and cerebral hemispheres and (3) in a fraction of whole brain homogenate and in cerebral myelin. The most striking results are the following: 1. Brain cholesterol specific radio-activity reaches two maxima, one at 3 days, and the other, slightly lower, at 1 month after injection. 2. After these two peaks, brain cholesterol specific radioactivity decreases as an exponential function of time, with a slope of 0.004 day −1. 3. Cholesterol specific radioactivity of myelin increases up to 40 days after injection whereas that of the other subcellular fractions decreases up to 6 days and increases from 6 to 40 days after injection. Two complementary explanations of these results are discussed. The first involves the existence of two groups of subcellular structures, those of group 1 exchanging their cholesterol with that of plasma, those of group 2 exchanging their cholesterol between themselves and with cholesterol of the first group. The second interpretation involves two chains of biosynthesis for brain cholesterol, working at different rates.

Labeling plasma lipoproteins with radioactive cholesterol.

Abstract Cholesterol-4- 14 C suspended in alcoholic physiologic saline was injected intravenously into 13 subjects in order to compare this with other more "physiologic" methods for labeling plasma lipoproteins with radioactive cholesterol. Cholesterol in suspension was removed from the circulation within a few minutes. It was then reincorporated into plasma lipoproteins slowly so that peak incorporation was reached in about 12 hours. Esterification of free cholesterol incorporated into different lipoproteins was similar to those of endogenous cholesterol synthesized in the body. The slopes of the specific activity curves of plasma cholesterol obtained from 1 to 50 days after the injection of particulate cholesterol were identical to those obtained after other more physiologic methods of labeling plasma cholesterol; i.e., (1) injection of radioactive precursors for endogenous synthesis of cholesterol, (2) feeding of radioactive cholesterol, (3) injection of plasma labeled by a modification of Sodhi and Kalant's method, and (4) injection of plasma labeled with Porte and Havel's method. The ratios of specific activity obtained after injection of the plasma labeled with 3 H-cholesterol by the last two methods to the 14 C specific activity obtained after the injection of particulate 14 C-cholesterol were identical to the ratio of the doses of the isotopes initially administered, and so were different parameters of cholesterol kinetics analyzed from the specific activity curves. The values of cholesterol turnover determined from the specific activity curves after the intravenous injection of cholesterol-4- 14 C suspended in physiologic saline were also similar to the values obtained on the same subjects by the cholesterol balance techniques, thus providing additional support for the data obtained from the specific activity curves. Our studies suggest that after having once been incorporated into plasma lipoproteins, the cholesterol injected intravenously as an aqueous suspension is metabolized in the same manner as the cholesterol incorporated into plasma lipoproteins biosynthetically or by other physiologic means.

Exogenous cholesterol transport in rabbit plasma lipoproteins

1. The appearance of exogenous cholesterol in free cholesterol and ester cholesterol of plasma chylomicra, very-low-density (VLD), low-density (LD) and high-density (HD) lipoproteins was studied in unanaesthetized rabbits after ingestion of a meal containing 5% fat and 0.08% [(3)H]cholesterol. 2. The specific radioactivity of ester cholesterol of VLD lipoproteins reached the highest value of any lipoprotein fraction and for each lipoprotein it increased at a faster rate and reached a higher maximum than that of free cholesterol; the maximum in VLD lipoproteins occurred later than in chylomicra. 3. The pattern of appearance of exogenous cholesterol in chylomicra and VLD lipoproteins of plasma was similar to the pattern previously observed in lymph. The specific radioactivity of ester cholesterol in plasma VLD lipoproteins was higher than that in chylomicra in spite of a larger pool size and dilution of cholesteryl esters from VLD lipoproteins produced by the liver. These results support the concept that during absorption the major portion of exogenous cholesterol is transported in VLD lipoproteins as ester cholesterol. 4. The specific radioactivity of ester cholesterol of chylomicra and VLD lipoproteins increased at a faster rate than that of LD and HD lipoproteins. However, the rate of increase and the absolute values of the specific radioactivity in LD and HD lipoproteins were identical. Since cholesteryl esters are thought not to exchange between lipoproteins, this observation supports the hypothesis that a result of VLD lipoprotein and chylomicron metabolism is the formation of LD and HD lipoproteins. 5. Results in vivo showed that the free cholesterol of individual plasma lipoproteins does not equilibrate within a period of 24h.

Improved estimation of body masses and turnover of cholesterol by computerized input–output analysis

In 23 patients, the decay curves of serum cholesterol specific activity after a single intravenous dose of radioactive cholesterol were measured for 16-66 wk and were subjected to computerized input-output analysis. Of 17 patients with decay curves followed for longer than 50 wk, a three-exponential curve fit was better in 12, and a two-exponential curve fit in 5, according to computerized F tests. Of six patients with decay curves followed for less than 50 wk, a two-exponential curve fit was better in five and a three-exponential curve fit in one. In the 13 patients who exhibited three-exponential curve fits, the third exponential appeared after 13-43 wk of observation (average, 25 wk). In 12 patients of this group who were followed for 50 wk or more, turnover rates and exchangeable masses of cholesterol were measured at maximum lengths of the curves (50-66 wk), and these parameters were then compared with measurements made with curves successively shortened down to 10-12 wk. The average differences between analyses of the minimum vs. the maximum lengths of the curves were: It (input rate: absorbed dietary plus biosynthesized cholesterol), 14% larger (1.24 vs. 1.09 g/day); M(a) (rapidly exchangeable mass of cholesterol), no change (34 vs. 33 g); M (total exchangeable mass), 26% smaller (67 vs. 91 g); M - M(a) (remaining exchangeable mass), 39% smaller (40 vs. 65 g). Significant differences in It, M, and M - M(a) (minimum vs. maximum curve lengths) were found in both normolipidemic and hypercholesterolemic patients, and the differences were of similar magnitude in the two groups. Since only 12 of 17 patients followed for 50 wk or longer demonstrated three-exponential curve fits, various means were sought by which it might be predicted at the outset whether a given patient must be studied for so long a time; none was found. However, in the group with two exponentials the value of M(a) was significantly larger than those with three-exponential curve fits, and this difference was apparent at as early as 10-12 wk.