Cholesterol 7 alpha-hydroxylase in isolated rat liver cells

Sulfhydryl groups responsible for extreme lability of human cholesterol 7alpha-hydroxylase identified by site-directed mutagenesis

Cholesterol 7alpha-hydroxylase (cholesterol-NADPH oxidoreductase, EC 1.14.13.17, 7alpha-hydroxylating) is known to have extremely sensitive sulfhydryl group(s). It is believed that a cysteine residue that has a sulfhydryl group plays an important role in the decrease of this enzyme activity. The amino acid sequences of cholesterol 7alpha-hydroxylase of five different mammalian species, human, rat, rabbit, hamster and mouse, revealed that these mammalian species contain eight cysteine residues that are well conserved. To identify which cysteine residues are responsible for the extremely high lability, we used the technique of the site-directed mutagenesis. Eight mutated genes of human cholesterol 7alpha-hydroxylase in which one codon for a cysteine residue was changed to that for alanine were prepared and expressed in COS-1 cells. The protein mass and enzyme activity of cholesterol 7alpha-hydroxylse obtained from these eight mutated genes were determined. While all mutated genes expressed the enzyme mass, two mutated genes did not express protein capable of catalyzing 7alpha-hydroxylation of cholesterol: in one mutant a codon for the 7th cysteine residue (Cys 444) was substituted to that for alanine and in the other mutant a codon for the 8th cysteine residue (Cys 476) was changed similarly. These results suggest that the 7th and 8th cysteine residues are important for expression of the enzyme activity. Based on the fact that Cys 444 exists in the heme binding region, Cys 476 was suggested to be responsible for enzyme lability.

The fatty acid composition of chylomicron remnants influences their binding and internalization by isolated hepatocytes

The binding and internalization of (125)I-labelled chylomicron remnants derived from palm, olive, corn, or fish oil (rich in saturated, monounsaturated, n-6, or n-3 polyunsaturated fatty acids, respectively) by hepatocytes from rats fed a low-fat diet or a diet supplemented with the corresponding fat for 21 days was investigated. In hepatocytes from rats fed the low-fat diet, the association of radioactivity with the cells at 4 degrees C (a measure of initial binding only) was similar with all types of remnants tested, but was more rapid at 37 degrees C (a measure of binding plus internalization) when fish oil, as compared to olive, corn or palm oil remnants, was used, and similar differences in the internalization of the particles were observed. In contrast, when hepatocytes from rats fed the fat-supplemented diets were used, the rate of association at 37 degrees C of remnants with cells from rats fed palm, corn or fish oil was similar, and higher than that found with cells from animals fed olive oil, and in this case these differences were mainly due to changes in the binding of the particles to the cells at 4 degrees C. Both excess low-density lipoprotein (LDL), which inhibits remnant uptake by the LDL receptor, and lactoferrin, which blocks the LDL receptor-related protein (LRP), were found to decrease the association of the remnants with cells from rats fed the low-fat and high-fat diets. However, in hepatocytes from animals given the low-fat diet, most of the differences between the various types of particle were retained in the presence of lactoferrin, but abolished in the presence of LDL. In contrast, in cells from rats fed the high-fat diets, the differences were reduced by both lactoferrin and LDL. These findings demonstrate that the hepatic uptake of chylomicron remnants is influenced both by the fatty acid composition of the particles, and by longer-term adaptive changes in liver tissue, and suggest that the former effects are mediated mainly by the LDL receptor, while the latter may involve both the LDL receptor and the LRP.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Dexamethasone regulates bile acid synthesis in monolayer cultures of rat hepatocytes by induction of cholesterol 7 alpha-hydroxylase

To study the effect of steroid hormones on bile acid synthesis by cultured rat hepatocytes, cells were incubated with various amounts of these compounds during 72 h and conversion of [4-14C]cholesterol into bile acids was measured. Bile acid synthesis was stimulated in a dose-dependent way by glucocorticoids, but not by sex steroid hormones, pregnenolone or the mineralocorticoid aldosterone in concentrations up to 10 microM. Dexamethasone proved to be the most efficacious inducer, giving 3-fold and 7-fold increases in bile acid synthesis during the second and third 24 h incubation periods respectively, at a concentration of 50 nM. Mass production of bile acids as measured by g.l.c. during the second day of culture (28-52 h) was 2.2-fold enhanced by 1 microM-dexamethasone. No change in the ratio of bile acids produced was observed during this period in the presence of dexamethasone. Conversion of [4-14C]7 alpha-hydroxycholesterol, an intermediate of the bile acid pathway, to bile acids was not affected by dexamethasone. Measurement of cholesterol 7 alpha-hydroxylase activity in homogenates of hepatocytes, incubated with 1 microM-dexamethasone, showed 10-fold and 90-fold increases after 48 and 72 h respectively, as compared with control cells. As with bile acid synthesis from [14C]cholesterol, no change in enzyme activity was found in hepatocytes cultured in the presence of 10 microM steroid hormones other than glucocorticoids. Addition of inhibitors of protein and mRNA synthesis lowered bile acid production and cholesterol 7 alpha-hydroxylase activity and prevented the rise of both parameters with dexamethasone, suggesting regulation at the mRNA level. We conclude that glucocorticoids regulate bile acid synthesis in rat hepatocytes by induction of enzyme activity of cholesterol 7 alpha-hydroxylase.

Bile acid synthesis and intracellular and extracellular cholesterol concentrations in isolated rat hepatocytes: the effect of dietary cholesterol

Bile acid synthesis in isolated hepatocytes prepared from rats given 1% cholesterol in the diet and incubated for 1 h in suspension was not increased compared to that in cells from control rats. When the hepatocytes were maintained in monolayer culture for 24 h, however, increased production of bile acid (X2.5) was observed in the cholesterol-fed group. The amount of bile acid synthesised during incubation in suspension was significantly correlated with intracellular unesterified cholesterol levels, but showed no correlation with intracellular esterified or medium cholesterol concentrations after 1 h. Bile acid production in hepatocytes maintained in monolayer culture was also significantly correlated with the intracellular unesterified, but not esterified, cholesterol content. In addition, in this case, there was a significant correlation with the levels of both unesterified and esterified cholesterol found in the medium after 24 h. These results suggest that the amount of cholesterol available to liver cells from extracellular sources has a role in the regulation of bile acid synthesis in cholesterol-fed rats, while the concentrations of esterified cholesterol stored within the cells are not important in this process.

Assay of cholesterol 7 alpha-hydroxylase activity in rat hepatocytes in primary monolayer culture

A sensitive and precise method is described to assay cholesterol 7 alpha-hydroxylase activity in homogenates of rat hepatocytes cultured in monolayers for up to 76 h. The assay is based on measurement of the amount of radioactive cholesterol converted into 7 alpha-[14C]-hydroxycholesterol. Since no subcellular fractionation was applied to measure enzyme activity, this method is rapid and can be performed with cell protein, corresponding to as little as 1 to 2 million hepatocytes. Optimal assay conditions were determined and the reproducibility of this cholesterol 7 alpha-hydroxylase determination was established. Exogenous cholesterol (105 microM), solubilized in Tween 80, was added to saturate the enzyme, giving an apparent Km of 56 microM. Under these conditions, 70% of the cholesterol present in the homogenates is directly accessible to the cholesterol 7 alpha-hydroxylase. The detection limit of the assay was found to be about 10 pmol per incubation. A time course of the cholesterol 7 alpha-hydroxylase activity in cultured hepatocytes revealed that after an initial loss of approximately 60% of the activity as compared with 287 pmol/h/mg for freshly isolated cells, the enzyme activity was increased to the initial level in hepatocytes cultured for 52 h. This result and the finding that the cholesterol 7 alpha-hydroxylase activity was diminished by 94% after a 24-h incubation with 5 microM cycloheximide suggest that the enzyme activity is associated with de novo protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of ionophore A23187, verapamil, and dibutyryl cyclic AMP on bile acid synthesis in isolated rat hepatocytes

The effect of the divalent cation ionophore A23187 and the calcium channel-blocker verapamil on bile acid synthesis in isolated hepatocytes in the presence and absence of dibutyryl cyclic AMP was studied. Both A23187 (1 microM) and verapamil (0.04 mM) caused a small (approximately 15-20%) but consistent decrease in total bile acid synthesis in the cells. When hepatocytes were incubated with dibutyryl cyclic AMP (1 mM) production of total bile acid was increased by about 25%, and this effect was unchanged by A23187 but abolished by verapamil. The relative proportions of the individual bile acids produced were not affected by either A23187 or verapamil. Dibutyryl cyclic AMP (1 mM) lowered the ratio of the amount of conjugated cholic acid to conjugated chenodeoxycholic + beta-muricholic acid formed in the cells by about 50%. Neither A23187 nor verapamil was able to prevent this change. These results suggest that the stimulatory effect of dibutyryl cyclic AMP on total bile acid synthesis is dependent on mobilisation of calcium from intracellular stores, but its effect on the relative proportions of bile acid formed via the cholic acid versus the chenodeoxycholic acid pathway is independent of calcium movement.

Characterisation of rat hepatocyte monolayers for investigation of the metabolism of bile salts

Rat hepatocyte monolayers were maintained for periods up to 24 h during which time their viability was greater than 85%. Using specific radioimmunoassays, the hepatocyte monolayers were shown to synthesise conjugated cholic, chenodeoxycholic and beta-muricholic acids. Feeding the bile salt sequestrant, cholestyramine, to donor animals increased synthesis of the major bile salt conjugates by the cells. Incubation of hepatocyte monolayers with bovine serum albumin decreased total synthesis of the three bile acids measured, but increased the amount of conjugated chenodeoxycholic acid detected. In order to test whether the effect of bovine serum albumin on bile salt synthesis was due to binding of bile salts, hepatocyte monolayers were incubated with antiserum to conjugated chenodeoxycholic acid. This treatment increased conjugated chenodeoxycholic acid production but had no effect on the other bile salt conjugates. It is concluded that the increase in conjugated chenodeoxycholic acid synthesis seen with bovine serum albumin and antiserum to conjugated chenodeoxycholic acid is caused by binding of the bile salt in the medium.

The effect of a rat plasma high-density lipoprotein subfraction on the synthesis of bile salts by rat hepatocyte monolayers

The effect of a rat high-density lipoprotein subfraction (HDL2) on the synthesis of bile salts by rat hepatocyte monolayers prepared from rats fed a diet containing cholestyramine, was investigated. The synthesis of bile salts as measured by radioimmunoassay of conjugated cholic, chenodeoxycholic and beta-muricholic acids was significantly increased when hepatocytes were incubated with a physiological concentration (500 micrograms HDL2 protein X ml-1) of HDL2.

Cholesterol ester turnover in isolated liver cells. Effects of cholesterol feeding

Isolated hepatocytes from rats that had been kept in a steady state of [3H]cholesterol were incubated in a salt medium with or without serum. The cells released esterified cholesterol into the incubation medium as lipoproteins. This secretion, 18.1 +/- 0.5 nmol/h per g of cells, was increased when the cells were incubated in a medium containing serum (46.3 +/- 4.9 nmol/h per g of cells). This secretion was strikingly enhanced by cholesterol feeding (1% in the diet, 30 days) to 323-620 nmol/h per g of cells, and inhibited by cycloheximide, colchicine or EDTA. After removal of EDTA and addition of calcium, the cholesterol ester secretion was restored. Free cholesterol of previously labelled high-density lipoproteins (HDL) was exchanged (t1/2 = 30 min) with that of liver cells and esterified. The esterification rate (25.8 +/- 2.5 nmol/h per g of cells) was increased by cholesterol feeding (1% in the diet, 8 days) to 63.2 +/- 2.8 nmol/h per g of cells. No cholesteryl ester hydrolysis was detected with the isolated liver cells. Consequently, it is suggested that the turnover of hepatic cholesteryl ester was caused mainly by secretion in lipoproteins.

The effect of dibutyryladenosine 3',5'-monophosphate on the synthesis of bile salts in isolated hepatocytes from rat

The effect of dibutyryladenosine 3',5'-monophosphate (Bt2cAMP) on the synthesis of conjugated cholic, chenodeoxycholic and beta-muricholic acids has been investigated. Hepatocytes were incubated with 1 mM Bt2cAMP for 3 h at 37 degrees C. In cells from rats with a basal rate of bile salt synthesis (soft-diet-fed rats) production of conjugated cholic acid was increased about two fold, synthesis of conjugated chenodeoxycholic acid was increased 10-20-fold but formation of its metabolite, conjugated beta-muricholic acid, was decreased by 30-50% in the presence of the cyclic nucleotide. The sum of the amounts of the three bile salts produced (total bile salt synthesis) was increased 30-50% by Bt2cAMP. When hepatocytes were prepared from rats in which bile salt synthesis had been stimulated by feeding the bile salt sequestrant, cholestyramine, Bt2cAMP had no effect on conjugated cholic acid synthesis, increased conjugated chenodeoxycholic acid production 3-5-fold and decreased conjugated beta-muricholic acid synthesis by about 50%. Total bile salt synthesis was unchanged. The ratio of the amount of conjugated cholic acid to conjugated chenodeoxycholic acid + conjugated beta-muricholic acid produced, an indication of the activity of 7 alpha-hydroxycholest-4-en-3-one 12 alpha-hydroxylase, was raised by Bt2cAMP in hepatocytes from soft-diet-fed but not in those from cholestyramine-fed rats. The effects of the cyclic nucleotide on the synthesis of the three bile salts in hepatocytes from soft-diet-fed rats were found to be saturable at a concentration of about 2 mM. Responses were half-maximal at concentrations of Bt2cAMP varying between 0.5 and 1.5 mM. These results suggest that in hepatocytes from rats with a basal rate of bile salt synthesis Bt2cAMP has effects at three different stages in the pathway, at the level of cholesterol 7 alpha-hydroxylase, 7 alpha-hydroxycholest-4-en-3-one 12 alpha-hydroxylase and chenodeoxycholine acid 6 beta-hydroxylase. In cells from rats in which bile salt synthesis has been stimulated only the effect at the chenodeoxycholic acid 6 beta-hydroxylase level is apparent. Bt2cGMP and Bt2cCMP had no effect on the synthesis of any of the bile salts measure, showing that the effects are specific for Bt2cAMP. The ratio of the amounts of the three bile salts found inside the cells to those found in the medium was decreased by about 90% when Bt2cAMP was present in the hepatocyte incubations. This effect was mimicked by Bt2cGMP and to a lesser extent by Bt2cCMP.(ABSTRACT TRUNCATED AT 400 WORDS)

The metabolism of chenodeoxycholic acid to beta-muricholic acid in rat liver

1. The synthesis of conjugated chenodeoxycholic acid and tauro-beta-muricholic acid in isolated rat hepatocytes was measured by radioimmunoassay. Production of tauro-beta-muricholic acid was linear over 4 h of incubation at 37 degrees C. The net synthesis of conjugated chenodeoxycholic acid was very much lower than that of tauro-beta-muricholic acid. 2. When hepatocytes were prepared from rats in which the enterohepatic circulation had been broken, either by feeding the bile salt sequestrant, cholestyramine or by total biliary drainage for 48 h, synthesis of tauro-beta-muricholic acid was increased compared to that in cells from control rats. Conjugated chenodeoxycholic acid accumulation during incubation of the hepatocytes was increased by cholestyramine feeding but not by total biliary drainage. These results suggest that there is a metabolic difference between the two methods of breaking the enterohepatic circulation with regard to chenodeoxycholic acid synthesis. 3. Hepatocytes prepared from rats given 1% cholesterol in the diet for at least 2 weeks synthesised significantly more tauro-beta-muricholic acid than those from control rats. The total amount of conjugated cholic, chenodeoxycholic and tauro-beta-muricholic acids synthesised by cells from cholesterol fed animals, however was not significantly different from that produced by hepatocytes from normal rats. 4. Exogenous taurochenodeoxycholic acid was metabolised to tauromuricholic acid by isolated hepatocytes. Production of tauro-beta-muricholic acid reached a maximum at a concentration of 20 microM taurochenodeoxycholic acid. The total metabolism of taurochenodeoxycholic acid, however, increased linearly up to the highest concentration measured, 50 microM. 5. The biliary content of tauro-beta-muricholic acid during total biliary drainage fell rapidly in the first 10 h and thereafter continued to decline, reaching a minimum after about 24 h. No significant rise was observed during the remainder of the experimental period. 6. It is concluded that a large proportion of the conjugated chenodeoxycholic acid synthesised by isolated hepatocytes in vitro is metabolised to tauro-beta-muricholic acid, and therefore it is necessary to take this into account when using this system to study bile salt synthesis.

The effect of dietary fat on bile salt synthesis in rat liver

The effect of dietary fat on conjugated cholic, chenodeoxycholic and tauro-beta-muricholic acid synthesis was studied using hepatocytes isolated from rats given a low-fat diet, or a low-fat diet mixed with 10% olive oil or 10% corn oil. The rats were totally biliary drained for 48 h prior to preparation of the cells in order to raise bile salt synthesis to a level which was measurable by radioimmunoassay. Synthesis of both conjugated cholic and chenodeoxycholic acid was raised in hepatocytes from rats given a fat supplement (either corn oil or olive oil) in the diet as compared to that in cells from low-fat-fed animals. Tauro-beta-muricholic acid synthesis, however, was unaffected by corn oil feeding. Production of conjugated cholic acid was increased to a greater extent when rats were given olive oil as opposed to corn oil, but these differences were not statistically significant. The conjugated cholic, chenodeoxycholic, and tauro-beta-muricholic acid and cholesterol content of bile collected at 2-h intervals during the biliary drainage of the same groups of rats was also determined. The pool size of both conjugated cholic and chenodeoxycholic acid in the enterohepatic circulation was found to be significantly decreased in rats given olive oil as compared to those given corn oil or the low-fat diet only. The pool size of tauro-beta-muricholic acid was also decreased in the olive oil-fed rats compared to the other two groups, but this difference was not statistically significant. After the pool had been drained out, animals which had received fat in the diet secreted more conjugated cholic and chenodeoxycholic acid into the bile than rats which had received the low-fat diet only. This effect was more marked when the fat given was olive oil rather than corn oil. Secretion of tauro-beta-muricholic acid into bile at this stage of biliary drainage was not changed by dietary fat supplements. Biliary cholesterol excretion was also increased in rats on diets containing 10% fat, with olive oil again having a greater effect than corn oil. The results show that supplementing the diet with fat leads to increased synthesis of conjugated cholic and chenodeoxycholic acids and biliary cholesterol secretion in the rat. The relatively more saturated fat, olive oil (85% oleate), gave a consistently larger increase than the more unsaturated, corn oil (50% linoleate), but the type of fat appeared less important than the presence of fat in the diet.

A radioimmunoassay for tauro-beta-muricholic acid suitable for use with isolated rat liver cells

A new radioimmunoassay which can be used to measure the amounts of tauro-beta-muricholic acid produced by isolated rat hepatocytes in vitro is described. Cross reactivities of other bile acids known to be present in rat liver with the antiserum used in the assay were not sufficient to interfere with the measurement of tauro-beta-muricholic acid. Exogenous taurochenodeoxycholic acid was metabolised by isolated rat hepatocytes concurrently with the appearance of tauro-beta-muricholic acid in the cell.

Persistent enhancement of bile acid synthesis in guinea pigs following stimulation of cholesterol catabolism in neonatal life

Cholesterol catabolism to bile acids was stimulated in neonatal guinea pigs by feeding 1.11% cholestyramine (CT)-containing diet for 8 weeks. The animals were then switched to standard laboratory diet for an additional 4 weeks. At the end of the laboratory diet period: a) CT-pre-treated guinea pigs continued to excrete significantly higher (p less than 0.05) amounts of bile acids, b) the activity of hepatic 7 alpha-hydroxylase was significantly elevated (p less than 0.01) in CT-pre-treated animals, and c) isolated hepatocytes from CT-pre-treated guinea pigs secreted significantly higher (p less than 0.05) amounts of bile acid when compared to controls during a 4-hour incubation. These data provide biochemical support for our contention that stimulation of cholesterol catabolism during neonatal life can have effects that persist into adult life.

The effect of the hypocholesteremic drug, AY 9944 on the synthesis of bile salts in rat liver

1. The compound trans-1,4 bis-(2-dichlorobenzylaminomethyl)cyclohexane dihydrochloride (AY9944) blocks cholesterol synthesis at a late stage. This leads to a decrease in cholesterol and accumulation of cholesta-5,7-diene-3-beta-ol (7-dehydrocholesterol) in tissues and plasma. 2. The effect of AY9944 on bile salt synthesis in rat liver was studied. The synthesis of conjugated cholic and chenodeoxycholic acids was measured in hepatocytes isolated from rats 2 h, 24 h and 48 h after administration of a single oral dose of AY9944. Production of the two bile salts was inhibited by 70-80% in hepatocytes from AY9944-treated as compared to untreated animals. 3. When AY9944 was added to the incubation medium in vitro of hepatocytes prepared from untreated rats the synthesis of conjugated cholic and chenodeoxycholic acids was not inhibited during the first hour of incubation, probably because of the presence of endogenous cholesterol. However when hepatocytes from untreated rats were incubated with AY9944 for periods of 2 h or longer, bile salt production was decreased markedly. 4. Bile salt synthesis is stimulated when rats are subjected to total biliary drainage for 24 h. The effect of AY9944 on this stimulation was studied. The content of conjugated cholic and chenodeoxycholic acid in the bile was measured as an indicator of bile salt synthesis. 5. In control animals the rate of secretion of biliary bile salts began to increase after about 24 h of total biliary drainage and reached a maximum after approximately 36 h. A single oral dose of AY9944 given 2 h after the start of total biliary drainage delayed and reduced this response. 6. The results show that inhibition of cholesterol synthesis by AY9944 resulting in the replacement of cholesterol by 7-dehydrocholesterol decreases but does not completely prevent bile salt synthesis.

Portal blood concentrations of conjugated cholic and chenodeoxycholic acids. Relationships to bile salt synthesis in liver cells

1. Rats were maintained in a strictly controlled environment of 12 h illumination and 12 h darkness. At regular intervals during the light/dark cycle the portal blood conjugated cholic acid and conjugated chenodeoxycholic acid concentrations were measured. The bile salt concentrations exhibited similar diurnal rhythms, the highest concentrations occurring in the middle of the dark phase. 2. The concentrations of conjugated cholic and chenodeoxycholic acids in the portal blood of rats fed a diet containing the bile salt sequestrant, cholestyramine, were significantly lower than those found in rats given a control diet. 3. During total biliary drainage the portal blood concentrations of conjugated cholic and chenodeoxycholic acids fell to a minimum 6--8 h after the start of the experiment, whereas bile salt synthesis in hepatocytes isolated from the rats was not increased until the least 13 h after the commencement of total biliary drainage. 4. These results suggest that the concentrations of bile salts in the portal blood do not affect directly the diurnal fluctuation in rates of bile salt synthesis, as the response of synthesis to a change in portal blood bile salt concentrations is too slow. 5. When the rats were given a small supplement of cholesterol in the diet to suppress hepatic cholesterologenesis prior to being subjected to total biliary drainage, the changes in the portal blood concentrations of conjugated cholic and chenodeoxycholic acids and the synthesis of the two bile salts by isolated hepatocytes were similar to those found in rats given the control diet. 6. The rise in bile salt production seen during biliary drainage may not be dependent exclusively on a preceding increase in cholesterol synthesis.

Effects of cholesterol on the properties of the membranes of isolated sheep liver nuclei and nuclear envelopes

The exchangeability of cholesterol between sheep liver nuclear membranes and liposomes, and the effect of cholesterol on the fluidity of the membrane lipid were studied. In intact nuclei, the cholesterol/phospholipid ratio increased from 0.102 to 0.145 mol/mol on incubation with cholesterol-rich liposomes, with a time for half-maximal uptake of 4.2 h. In isolated envelopes under the same conditions, the ratio increased from 0.110 to 0.266 mol/mol with a time for half-maximal uptake of about 1.9 h. Moreover, the approximate order parameter of the spin label 5-(N-oxyl-4',4'-dimethyloxazolidino)-stearic acid was 0.677 in intact nuclei and 0.723 in isolated envelopes prior to exchange; after exchange, these values increased to 0.717 and 0.756, respectively. These differences between the preparations could not be attributed to differences in the capacity for cholesterol uptake between the two nuclear membranes, or to a slow rate of exchange between them; the presence of an intact nuclear matrix appeared both to disorder the lipid partially and to inhibit cholesterol uptake. The differences indicate that conclusions based on physical studies of the membrane lipid in isolated envelopes are not necessarily applicable to the intact nucleus.