H.S. Sodhi

Absorption of dietary cholesterol in man

Abstract Cholesterol absorption was studied by balance methods in 10 hyperlipemic subjects. The cholesterol (401–1214 mg/day) and β-sitosterol (171–454 mg/day) were integral parts of the diets. The absorption was 37% ± 5% and the amounts of cholesterol absorbed were 149–508 mg/day. The amounts absorbed had an excellent correlation with the dietary intake of cholesterol (r = 0.98), and there was no leveling off of the amounts absorbed even at an intake of 1214 mg/day. Assuming that the percentage absorption of dietary and endogenous cholesterol in the gastrointestinal tract is identical, the amounts of endogenous cholesterol secreted into the gastrointestinal tract were calculated from the amounts of fecal neutral steroids of endogenous origin. The relationship of total (dietary plus endogenous) cholesterol in the gastrointestinal tract and its absorption was also excellent (r = 0.97) and also without any evidence of leveling off of the slope up to 2300 mg. These data indicate that within the normal range for North America, the amounts of dietary cholesterol absorbed are proportional to its intake.

Effects of plant sterols oncholesterol metabolism in man

Abstract Plant sterols (Cytellin, 3 g t.i.d.) were administered to 3 normal and 3 hyperlipemic subjects for periods of 12 or 15 days. There was a moderate decrease (9 ± 4%) in plasma cholesterol; and in 5 of 6 subjects there was a modest increase (6 ± 1%) in plasma triglycerides, and in the sixth, the levels were decreased by 20%. On cessation of treatment, plasma lipids returned gradually to pretreatment levels. Treatment with plant sterols increased the fecal excretion of metabolites of endogenous cholesterol by 35–73% over the control values. The increment was generally limited to the neutral metabolites except in one normal subject who showed significant increases both in neutral sterols and bile acids. The increment in fecal excretion was considerably in excess of the decrement in plasma cholesterol pool, suggesting one or more of the following: (1) a reduction in absorption of cholesterol from the intestinal tract, (2) an increase in cholesterol synthesis, and (3) mobilization of cholesterol from tissues. In all subjects the decay in plasma cholesterol specific activity showed an increase after commencement of treatment. Since treatment with plant sterols decreases the absorption of dietary (unlabelled) as well as endogenous (labelled) cholesterol in the intestinal lumen, the increased slope of the plasma cholesterol specific activity during this period may be a reflection of the increase in synthesis to compensate for the decrease in absorption. Similarly, the decreased slope on stopping the treatment in all probability reflects decreased synthesis of cholesterol.

Turnover of plasma cholesteryl esters and its relationship to other parameters of lipid metabolism in man.

Studies on plasma cholesteryl esters were made in 32 normal and hyperlipidaemic subjects in an attempt to elucidate further the metabolic relationships between cholesterol and other plasma lipids and lipoproteins. Fractional and net turnover rates of plasma cholesteryl esters were measured using two independent (in vivo and in vitro) methods, both of which determined the prevailing activity of plasma lecithin: cholesterol acyltransferase in the ambiance of the patients own plasma. The results obtained by the two methods were in excellent agreement.

Synthesis of cholesterol in hypercholesterolemia and its relationship to plasma triglycerides

Abstract Cholesterol synthesis was estimated by three independent methods in a number of subjects with hypercholesterolemia with or without hypertriglyceridemia. Cholesterol balance studies indicated that the synthesis of cholesterol in those who had hypertriglyceridemia was almost three times greater than in those who did not. Similar differences in synthetic rates were also suggested by data on kinetics of plasma cholesterol and by relative rates of incorporation of radioactive acetate and mevalonate into plasma cholesterol. This difference in endogenous synthesis of cholesterol between the two groups appeared mainly to be related to the hepatic cholesterogenesis. Although the subjects with hypertriglycerdemia had greater body weights than those who did not, the relative body weights did not show significant correlation with cholesterol synthesis. Neither did the latter correlate well with plasma cholesterol levels. The correlation of cholesterol synthesis was best with concentrations of plasma triglycerides. It is therefore suggested that increased synthesis of cholesterol is primarily related to the increased concentrations of plasma triglycerides, which in turn may be dependent on increased hepatic synthesis of very low density lipoproteins in those subjects.

Effects of chlorophenoxyisobutyrate on the synthesis and metabolism of cholesterol in man

Abstract The effects of an oral administration of clofibrate 500 mg. four times a day on plasma cholesterol were studied in 16 subjects on constant dietary intake. The lipoprotein patterns were normal in four, Type II in five and Type IV in seven subjects. There was a rapid decline in the levels of plasma cholesterol and in 15 days these had stabilized at 78 ± 5 per cent of the pretreatment values. No differences between the Type II, Type IV or normals were seen in these short-term studies. Eight subjects (three normal, two Type II and three Type IV) were given a mixture containing sodium acetate-1-14C and DL mevalonate-2-3H lactone intravenously, once before and once after 10 days of treatment with clofibrate. The peak specific activity of 14C (from acetate-1-14C) in plasma cholesterol was decreased (28.5 ± 14.6%) but that of 3H (from mevalonate-2-3H) was not affected by the treatment. The 14C specific activity of biliary cholesterol was decreased even more than that of plasma cholesterol supporting the hypothesis that the site of action of clofibrate in the hepatic synthesis of cholesterol in man is between acetate and mevalonate. The drug did not have any effect on the fractional turnover rates of plasma cholesterol esters; but, their net turnover rates were reduced on account of the decrease in their pool size. The ratios of 3 H 14 C in cholesterol during the first few hours after the injection of the radioactive precursors were not constant from hour to hour. There was also a steady difference in the 3 H 14 C ratios between plasma and bile cholesterol at all times. This suggested to us that the relative incorporation of the two precursors in different cholesterol tissue pools in rapid equilibrium with plasma cholesterol was different.

Thin-layer chromatographic separation of chenodeoxycholic and deoxycholic acids

Abstract An efficient thin-layer Chromatographie system for separation of chenodeoxycholic and deoxycholic acids is proposed.

Catabolism of cholesterol in hypercholesterolemia and its relationship to plasma triglycerides

Various parameters of catabolism of cholesterol were measured in hypercholesterolemic subjects with and without hypertriglyceridemia. Amounts of endogenous cholesterol secreted into intestinal tract in patients who had hypertriglyceridemia (1140 ± 585 mg/day) were much greater (P < 0.025) than in those with only hypercholesterolemia (460 ± 142 mg/day). Most of this difference was in all probability caused by the biliary cholesterol. Total fecal (neutral plus acidic) steroids derived from endogenous cholesterol in the former group (1140 ± 452 mg/day) were also greater (P < 0.05) than in the latter group (445 ± 124 mg/day). The ratios of neutral to acidic steroids in the feces were similar m the two groups of patients. Although the total fecal bile acids were more in the subjects who had hypertriglyceridemia than in those who did not, the predominant bile acid in the former was deoxycholic acid and in the latter lithocholic acid. These data suggest that the amounts of cholesterol entering the catabolic pools in the liver were greater in hypercholesterolemic subjects who had hypertriglyceridemia than in those who did not. These data also suggest that the synthesis of chenodeoxycholic acid does not increase to the same degree as that of cholic acid when confronted with increased amounts of cholesterol for catabolism.

Hypocholesterolemic agents and mobilization of tissue cholesterol in man

Abstract Plasma cholesterol was pulse-labelled with [4- 14 C]cholesterol in 11 hyperlipemic subjects on cholesterol balance studies. When the decline in the specific activity of plasma cholesterol became exponential, various parameters of cholesterol metabolism were measured during steady state conditions of the control period. The subjects were then given clofibrate, nicotinic acid, or plant sterols and the changes in these parameters were noted. There was a prompt decline in the plasma cholesterol concentration and a marked upswing in the specific activity slope; the latter could only occur by the entry of cholesterol from the tissues. The amounts of tissue cholesterol entering plasma were estimated (a) from the increases in the fecal excretion of endogenous cholesterol and its metabolites, and (b) from the increases in the secretion of endogenous cholesterol into the lumen of the gastrointestinal tract. Both estimates were in good agreement. The increases in plasma cholesterol specific activity had excellent correlations with the amounts of cholesterol mobilized from the tissues. These parameters in turn correlated well with fall in plasma cholesterol concentration suggesting that the mobilization was secondary to the acute reduction in the plasma cholesterol cencentrations and was not caused directly by the drugs.

Mechanisms of Action of Cholestyramine in the Treatment of Hypercholesterolemia

Cholestyramine (12 g/day) was administered to four subjects with familial hyperchloesterolemia (type II) for 12 to 15 days. Plasma cholesterol values fell by 24 to 28% in all subjects. Total endogenous fecal steroids increased from 2.0 to 2.5 times over the control values. This increment was mainly in the acidic fraction which increased from 1.4 to 6.5 times during treatment. The neutral steroid fraction showed a slight increase (nonsignificant) in two subjects and a significant increase (P < 0.05) in one subject. The total fecal steroid increment was considerably in excess of the decrement in plasma cholesterol, thus indicating either (a) a substantial increase in cholesterol synthesis, (b) a transfer of cholesterol from depots, or (c) both. Plasma cholesterol specific activity time curves showed a sharp increase in the slope immediately after the commencement of treatment, reflecting an increase in the rate of entry of unlabeled cholesterol into the readily miscible pool. Since cholestyramine did not change the absorption of the dietary cholesterol, the increase in the contribution of unlabeled cholesterol could only be from an increase in endogenous synthesis. In accordance with previous findings from this laboratory, no evidence of degradation of the steroid nucleus was detected during its passage through the gastrointestinal tract.

Current concepts of cholesterol metabolism and their relationship to lecithin:cholesterol acyltransferase.

Fig. 1 represents the general consensus on some of the most important aspects of cholesterol metabolism in man. According to the published literature, there is a single pool of free cholesterol in the liver; most of the dietary, biliary, and intestinal cholesterol absorbed from intestines and removed by the liver enters this pool. Cholesterol synthesized by the liver also enters the same pool. Biliary cholesterol and recently synthesized bile acids come from this pool, and the free cholesterol for synthesis of new lipoproteins is also derived from the same pool. In addition. this pool of free cholesterol undergoes isotopic exchange with plasma free cholesterol. The published literaturc does not indicate the presence of any mechanism for transport of net amounts of plasma cholesterol into the liver. This concept (of a single hepatic pool of free cholesterol) suggests that in conditions associated with increased turnover of plasma lipoproteins increased requirements of free cholesterol for synthesis of new lipoproteins can, after a short lag, be met by increased amounts of cholesterol returning to this pool from catabolism of lipoproteins. Over the last few years we investigated the absorption (l), synthesis (2), catabolism (3), and mobilization (4) of tissue cholesterol in hyperlipemic subjects with and without hypertriglyceridemia. There were no differences in the absorption of dietary cholesterol between them, but studies on cholesterol synthesis and turnover indicated that cholesterol synthesis was significantly greater in hypertriglyceridemic subjects than in those who had normal plasma triglycerides. Similarly, fecal metabolites of endogenous cholesterol were also significantly greater in hypertriglyceridemic than in normotriglyceridemic subjects. Since elevation of plasma triglycerides above normal is primarily due to an increase in plasma very low density lipoproteins (VLDL), and since modest to moderate elevations of VLDL are associated with significant increase in their turnover rates (5), it was suggested by us that cholesterol synthesis as well as its catabolism are related to the turnover of plasma lipoproteins (6). The fact that the continued increases in synthesis and catabolism were observed in conditions associated with hypertriglyceridemia suggested to us that at least some of the cholesterol catabolized in the liver was not available for synthesis of new lipoproteins. On the basis of such reasoning, we proposed that the cholesterol synthesized in the liver and the cholesterol in chylomicra and other intestinal lipoproteins removed by the liver enter into a single pool which supplies the free cholesterol for synthesis of new plasma lipoproteins (Fig. 2). On the other hand, cholesterol derived from catabolism of plasma lipoproteins is transported back into a second pool from which biliary cholesterol and bile acids are derived. Although there may be some transfer of cholesterol from one pool to another, functionally the two pools are viewed as