The lymphatic absorption of dihydrolanosterol and cholesterol in the rat.

Dihydrolanosterol (DHL) and its analogs have been studied extensively as cholesterol biosynthesis inhibitors. They inhibit specific steps in cholesterol synthesis by inhibiting lanosterol demethylase and by suppressing HMG-CoA reductase. The present study was designed to estimate the lymphatic absorption of DHL. For comparison, a cholesterol group was included. The left thoracic duct of male Wistar rats weighing between 210 and 230 g was cannulated. A lipid emulsion containing 0.75microCi of either [3H]-DHL or [3H]-cholesterol was given intragastrically. After the lipid meal, lymph was collected at 3 h intervals up to 9 h and then at 24 h. Radioactivity of DHL and cholesterol in the lymph was estimated. Lipid extracts of lymph specimens were also subjected to thin layer chromatography and fractions of DHL, cholesterol and their esters were isolated and the masses were estimated. There were no differences in lymph volumes between the two groups. However, absorption and esterification of DHL in lymph were significantly reduced compared with the cholesterol group. The marked decrease in the esterification of DHL is likely due to its poor absorption into the mucosal cell and subsequently into the lymphatic system. The amount of DHL available in the mucosal cell for esterification may be a limiting factor.

Alterations in rat intestinal mucin patterns following luminal infusion of acetylsalicylic acid and prostaglandin derivatives.

The secretion of gastrointestinal mucin and/or the formation of mucoid caps have been implicated in cytoprotective or repair mechanisms related to mucosal injury models. In this study, rats were treated with acetylsalicylic acid (ASA) or prostaglandins (PG), and their effects on the synthesis and secretion of small intestinal mucin were examined. A newly developed polyclonal antibody to rat intestinal mucin was used for immunoassay of rat intestinal luminal and tissue mucin. The mucin antigen source was obtained by vacuum aspiration of luminal mucus. A high-molecular-weight glycoprotein (2 x 10(6) Da) fraction injected into rabbits produced a primary mucin antibody. A sensitive and quantitative enzyme-linked immunosorbent assay (ELISA) was developed that yielded a highly reproducible and linear response with mucin aliquots containing 0-20 ng of protein/ml. Incorporation of the plasma tracers ([3H]glucose and [35S]sodium sulfate) into mucin derived from hexadecyltrimethylammonium bromide precipitation after treatment with ASA (100 mg/kg body wt) decreased, although administration of dimethylprostaglandin E2 (100 micrograms/kg body wt) significantly increased the specific tracer incorporation values for the sialomucin and sulfomucin indices in luminal mucin fractions. The immunoassay data pattern for the ELISA technique was virtually identical to the results of the radiolabeled tracer method obtained for the same pharmacologic treatments. These experiments demonstrate that the estimation of synthesized mucin (tissue source) or secreted mucin (luminal source) as determined by the ELISA technique is similar to that obtained by the time-consuming and labor-intensive tracer incorporation methodology.

A comparison of the digestion and absorption of cocoa butter and palm kernel oil and their effects on cholesterol absorption in rats.

Digestion and absorption of cocoa butter and palm kernel oil and their effect on cholesterol absorption were studied in adult male rats. Duodenal and thoracic duct catheters were inserted surgically into the anesthetized rats. After an overnight fast, animals were given a single duodenal dose of an aqueous emulsion containing [1,2-3H]cholesterol and one of the following: corn oil, cocoa butter or palm kernel oil. Digestion and absorption were estimated by recovering the total fatty acids in the thoracic duct lymph over a 24-h collection period (after subtraction of the baseline "endogenous fatty acids" in the lymph). Intestinal absorption of cholesterol into the thoracic duct lymph was reduced significantly (P less than 0.05) in the presence of cocoa butter, compared to absorption when palm kernel oil or corn oil was administered. Compared to the absorption of corn oil (arbitrarily defined as 100%), the absorption of palm kernel oil and cocoa butter was 82 and 63%, respectively. The present study suggests that palm kernel oil absorption was not significantly different from that of corn oil. The lower absorbability of cocoa butter and its inhibitory effect on cholesterol absorption may explain in part why cocoa butter is less hypercholesterolemic and atherogenic than other equally saturated fats.

Discrimination between cholesterol and sitosterol for absorption in rats.

The intestinal absorption of cholesterol and sitosterol was compared in rats. The intragastric administration of a single emulsified lipid meal containing either 50 mg of [4-14C]cholesterol or [4-14C]sitosterol resulted in the lymphatic absorption of 18.2% and 0.42% of each sterol, respectively, in 6 hr. This difference was unaltered when the mucosal sterol load was equalized by reducing the cholesterol to 1 mg in the emulsified lipid meal while maintaining the same sitosterol load or when the physical state in the lumen was equalized by infusion of a micellar solution containing both sterols into bile-diverted intestine. Lymphatic cholesterol was 90% esterified compared to 12% for sitosterol. Both sterols were associated predominantly (greater than 70%) with the chylomicron fraction. Eighty percent of the chylomicron cholesterol was recovered as ester with the core lipids, while 77% of the sitosterol was recovered as free sterol with the chylomicron coat. In mucosal homogenates at 6 hr, sitosterol recovery was one-eleventh that of cholesterol. When [3H]cholesterol (10 mg) and [14C]sitosterol (10 mg) were co-administered in an emulsified intragastric lipid meal, sitosterol associated with the brush border isolated 2 hr later was one-fifth that of cholesterol. Similar differences were seen when brush border membranes were incubated in vitro with micellar solutions containing either 50 microM [3H]cholesterol or [14C]sitosterol and the relative uptake of each sterol was unaffected by micellar phospholipid type (egg yolk phospholipids, phosphatidylcholine, or phosphatidylethanolamine).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of cholesterol absorption in rats by plant sterols.

The extent and site(s) of inhibition of cholesterol absorption by plant sterols, sitosterol and fucosterol, were studied in rats. The intragastric administration of a single emulsified lipid meal containing 25 mg [3H]cholesterol and 25 mg of either sitosterol or fucosterol inhibited the lymphatic absorption of cholesterol by 57% and 41%, respectively, in 24 hr. Less than 2% of each plant sterol was absorbed in the 24-hr period. In contrast, neither plant sterol (50 microM) inhibited cholesterol absorption when co-administered with equimolar amounts of cholesterol in phospholipid-bile salt micelles nor was either absorbed from the micellar solution. A series of in vitro studies was conducted to identify the site(s) of plant sterol inhibition of cholesterol absorption and to account for the difference in inhibitory effectiveness of sitosterol and fucosterol. A comparison of the micellar solubility of each sterol alone and in equimolar binary mixtures (to 2.0 mM) revealed that the solubility of individual sterols decreased in the following order: cholesterol, fucosterol, sitosterol, and that in binary mixtures cholesterol solubility was decreased by sitosterol and, to a lesser extent, by fucosterol relative to its solubility alone. A comparison between micellar-solubilized cholesterol and either sitosterol or fucosterol for binding to isolated brush border membranes, intestinal mucin, or for esterification by either cholesterol esterase or acyl coenzyme A:cholesterol acyltransferase revealed moderate to no competition. The data suggest that plant sterols displace cholesterol from bile salt (taurocholate) micelles and that sitosterol is more effective than fucosterol in this capacity.

Dietary fiber supplements: effects on serum and liver lipids and on liver phospholipid composition in rats.

Rats (6 per group) were fed semipurified diets containing either particulate fibers (alfalfa, 10%; cellulose, 10%; bran, 10%), a soluble ionic fiber (pectin 5%), soluble, nonionic fibers (guar gum, 5%; Metamucil, 10%), a mixed fiber preparation (Fibyrax, 10%, or an insoluble, ionic bile acid-binding resin (cholestyramine, 2%). The control group was fed the unsupplemented diet. The feeding period, during which diet and water were provided ad libitum, was 28 days. Compared with the control group, serum total cholesterol levels were increased by more than 10% in rats fed alfalfa and decreased by more than 10% in rats fed cellulose, guar gum, Fibyrax and cholestyramine. There were no significant differences in percentage of plasma HDL cholesterol. Serum triglycerides were elevated in the groups fed alfalfa, pectin, guar gum or Fibyrax and reduced in the group fed Metamucil. Plasma phospholipids were elevated in rats fed alfalfa or bran, unaffected in rats fed pectin or Metamucil and reduced in the other groups. Liver total cholesterol was elevated in all groups but those fed wheat bran and cholestyramine. The percentage of liver cholesterol present as ester was elevated in every group except that fed cholestyramine. Liver triglycerides were reduced in rats fed guar gum or Metamucil and elevated in those fed alfalfa. Liver phospholipids were lowered in the group fed cellulose. Liver phospholipids were fractionated by thin layer chromatography to give phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyelin (Sph), lysophosphatidylcholine (LPC) and phosphatidylinositol plus phosphatidylserine (PI + PS).(ABSTRACT TRUNCATED AT 250 WORDS)

Sterol carrier protein2-like activity in rat intestine.

A sterol carrier protein2 (SCP2)-like activity has been demonstrated in rat intestinal mucosal homogenates and in isolated intestinal cells from both crypt and villus zones. The results indicate the presence of a protein with similar molecular weight and antigenicity to that of authentic SCP2 purified from rat liver cytosol. Like liver SCP2, mucosal cytosol stimulates pregnenolone production in rat adrenal mitochondria and acyl coenzyme A:cholesterol acyltransferase activity of liver and mucosal microsomes. The distribution of SCP2-like activity as determined by radioimmunoassay indicates high levels in mitochondria and cytosol and relatively lower levels in microsomes and in brush-border membranes. The widespread distribution of SCP2-like protein in the intestine is consistent with potential transfer functions in all phases of cholesterol processing.

Dietary fiber and intestinal adaptation: effects on lipid absorption and lymphatic transport in the rat.

Adult male rats fed defined diets containing various fiber supplements or cholestyramine for 4 wk were surgically provided with lymphatic drainage catheters and starved overnight. After duodenal administration of a standard lipid test emulsion, absorption rates and lipoprotein distributions of cholesterol and oleic acid were determined. Prefeeding diets containing cellulose or alfalfa had no significant effect on oleic acid absorption. Diets containing pectin, guar gum, metamucil, mixed fibers (Fibyrax), or cholestyramine caused decreased lymphatic recovery in the initial period; except for the metamucil diet, no decrease was caused in the 24-h recovery, suggesting delayed but not impaired absorption. Fatty acid distribution among lipoproteins and chylomicron size were not altered by diet. All supplements caused a significant reduction in cholesterol absorption during the initial period, and cholesterol absorption remained depressed in animals prefed pectin, guar gum, mixed fibers, metamucil, and cholestyramine.

Dietary fiber supplementation and fecal bile acids, neutral steroids and divalent cations in rats.

Young adult rats were fed ad libitum for 4 wk on defined diets containing either no fiber, 10% levels of insoluble fiber sources [cellulose, wheat bran, alfalfa, mixed fibers (Fibyrax)], 5% levels of soluble fiber sources [pectin, guar gum, psyllium (Metamucil)] or 2% cholestyramine (Questran), a bile acid sequestrant. Fecal samples were obtained from paired rats over a 3-d period, were analyzed for neutral and acidic steroid levels and compositions and, together with the food, were assayed for divalent cations (Ca2+, Mg2+, Fe2+ and Zn2+). Animals in all groups were in balance for divalent cations, and there appeared to be no relationship between the extent of cation balance and the type of fiber fed. All insoluble fiber diets and the guar gum and psyllium diets resulted in significantly higher daily fecal mass and, in general, resulted in significant dilution of total fecal steroids. With the insoluble fibers, there was a general dilution of fecal neutral steroids, which was not apparent with the soluble fibers or cholestyramine. In addition, except for the pectin- and mixed fiber-containing diets, there was reduced bacterial conversion of the primary bile acids to secondary bile acids or metabolites.

Digestion, absorption and effects on cholesterol absorption of menhaden oil, fish oil concentrate and corn oil by rats.

Adult male rats were surgically provided with a drainage catheter in the left thoracic lymphatic channel and an indwelling duodenal catheter for constant infusion of physiological saline-5% glucose. After an overnight fast, animals were given a single duodenal dose of an aqueous emulsion containing one of the following: oleic acid, corn oil, menhaden oil or a fish oil concentrate (FOC) and [1,2-3H]cholesterol. Digestion and absorption were estimated by recovering the total fatty acids in the thoracic duct lymph over a 24-h collection period (after subtraction of the "baseline" endogenous fatty acids in the lymph). Cholesterol absorption in the thoracic duct lymph was significantly reduced (P less than 0.05) in the presence of menhaden oil or FOC compared to that in the presence of corn oil. With various fat feedings, the major increases in lymph fatty acids were directly related to the dietary fatty acid content. The relative amounts of eicosapentaenoic acid (EPA) and arachidonic acid (AA) in the thoracic lymph were influenced by the lipid content of the emulsion. The EPA/AA ratio in control, oleic acid and corn oil feedings ranged from 0.12 to 0.25. When marine oil was administered, the EPA/AA ratio was 0.78-0.98. The total amount of fatty acids found in the lymph after marine oil feeding was significantly less (P less than 0.01) than that found after corn oil feeding. The results suggested that the digestion and absorption of menhaden oil and FOC were decreased as compared with corn oil. The EPA/AA ratio was increased in the thoracic lymph after dietary fish oil feeding.

Reduction of colonic carcinogenesis by wheat bran independent of fecal bile acid concentration.

It has been reported previously that populations with a decreased concentration of fecal bile acids have a lower incidence of colon cancer. We examined the importance of fecal bile acid dilution by wheat bran (WB) in inhibiting colonic tumorigenesis in an experimental animal model. Male F344 rats received oral doses of the colon carcinogen 1,2-dimethylhydrazine [CAS: 540-73-8] and were assigned randomly to groups fed one of four semipurified diets for 26 weeks. The diets were fiber-free (FF), 10% WB, FF + bile salts, or WB + bile salts. The amount of bile salts added was adjusted to produce a fecal bile acid concentration in the group fed WB + bile salts equal to that found in the FF groups. Fecal bile acid concentrations at 12 and 24 weeks in the WB + bile salts group were similar to those in the FF group. Gross and microscopic findings at necropsy revealed a reduced total number and multiplicity of colon tumors in both bran-fed groups. Although the fecal bile acid concentrations of the FF and WB + bile salts groups were equal, the latter showed a significant reduction in tumor yield.

In vitro clearance of chylomicron triglycerides containing (omega-3) eicosapentaenoate.

Rat mesenteric lymph chylomicrons, containing triglycerides enriched with either [14C]oleic acid or [14C]eicosapentaenoic acid, were prepared by ultracentrifugation of lymph samples collected for 6 h after a single duodenal infusion of an emulsion containing 0.3 mmol of either fatty acid. After determination of protein and of total fatty acid content and composition, enriched chylomicrons were suspended in Krebs-bicarbonate buffer. Non-working hearts were perfused in a recirculating system for 45 min using the enriched chylomicron preparations. At 15 min intervals during perfusion, the media were assayed for total radioactivity, 14CO2 and 14C-labeled fatty acids associated with triglycerides, unesterified fatty acids, phospholipids, mono- and diglycerides. After perfusion, the hearts were extracted and assayed for total lipid radioactivity and isotope distribution among heart lipid fractions. With this membrane-supported lipoprotein lipase system, clearances of chylomicron triglycerides containing either fatty acid were identical, as were the myocardial uptakes of the fatty acids and oxidations to 14CO2. Furthermore, except for a significantly greater incorporation of eicosapentaenoate into myocardial phospholipids, tissue isotope distributions of the two labeled fatty acids were also the same. These studies suggest that at least the initial phases of peripheral clearance of chylomicrons enriched in omega-3 fatty acids is as efficient as with those containing oleate.

Comparison of the clearances of serum chylomicron triglycerides enriched with eicosapentaenoic acid or oleic acid.

Rat mesenteric lymph chylomicrons containing triglycerides enriched with either [14C]oleic acid (OA) or [14C]-eicosapentaenoic acid (EPA) were prepared by ultracentrifugation of lymph samples collected for 6 hr after a single duodenal infusion of an emulsion containing either fatty acid. These chylomicrons were injected into the jugular vein of recipient rats and, at various time intervals, blood was drawn and serum was assayed for radioactivity. In separate animals, serum lipoprotein fractions were separated by ultracentrifugation, and the redistribution of labeled fatty acid among circulating lipoproteins was determined by liquid scintillation spectrometry. When the early disappearance rates (10 min) of either total serum radioactivity or specifically the chylomicron fraction were compared, there were no differences between the groups receiving OA- or EPA-enriched chylomicrons. However, disappearance rates of EPA-enriched chylomicrons were slower than those of OA-enriched chylomicrons from 25 to 90 min. The small but significant differences in the disappearance rates for the longer time periods cannot be ascertained without further studies. At 5 min after injection of either type of chylomicron, the d less than 1.006 g/ml lipoprotein fraction of serum chylomicrons and very low density lipoproteins contained almost 90% of the original radioactivity. By 240 min, when less than 2% of the radioactivity remained, this radioactivity in the d less than 1.006 g/ml fraction was 43-46%, with concomitant increases in the low and high density lipoprotein fractions and in the lipoprotein-free serum.

Normal cholesterol absorption in rats deficient in intestinal acyl coenzyme A:cholesterol acyltransferase activity.

Acyl coenzyme A:cholesterol acyl transferase and/or cholesterol esterase may regulate the esterification and absorption of exogenous cholesterol. To assess this, mucosal acyl coenzyme A:cholesterol acyl transferase activity was inhibited selectively with three different drugs [Sandoz #58-035, inhibitor 1; Lederle inhibitor 2 and inhibitor 3] and the effect upon the absorption of a [4-14C]cholesterol meal was studied in the lymph fistula rat. Compared to control rats, ACAT activity measured in mucosal homogenates from the drug-treated rats was reduced 80-90%, 40%, and 30%, respectively, during the predicted time-frame for maximum mucosal esterification of cholesterol (i.e., after cholesterol is fed and before it appears in lymph). In contrast, [14C]cholesterol absorption in the drug-treated animals was unchanged from controls [5.7 +/- 1.2 (inhibitor 1) vs. 5.4 +/- 1.6 mumol/6 hr (control); 6.1 +/- 2.1 (inhibitor 2) and 5.2 +/- 1.5 (inhibitor 3) vs. 4.1 +/- 1.3 mumol/6 hr (control)]. Of the absorbed [14C]cholesterol, approximately 75% was esterified in all groups. Cholesterol esterase activity measured in the drug-treated rats was unchanged compared to controls nor did the drugs inhibit this enzyme in vitro. Under the conditions of this study, drugs causing substantial inhibition of acyl coenzyme A:cholesterol acyl transferase activity had no effect on the absorption of exogenous cholesterol.

Studies on the synthesis of sterol carrier protein-2 in rat adrenocortical cells in monolayer culture. Regulation by ACTH and dibutyryl cyclic 3',5'-AMP.

The effects of ACTH or dibutyryl cyclic AMP (Bt2cAMP) on the synthesis of sterol carrier protein-2 (SCP2) have been studied in rat adrenocortical cells in monolayer culture. Radiolabeling of total cellular proteins with [35S]methionine and immunoprecipitation with antibodies directed against rat liver SCP2, followed by polyacrylamide gel electrophoresis and fluorography, showed a 3-4-fold increase in the rate of synthesis of SCP2 in cells treated for 48 h with ACTH (1 microM) or Bt2cAMP (0.1 mM). The induction of SCP2 synthesis depended upon the concentrations of ACTH or Bt2cAMP with an ED50 of 8 and 100 nM, respectively, and increased linearly with time between 12 and 48 h of treatment. Immunoprecipitation of SCP2 synthesized in a rabbit reticulocyte in vitro translation system programmed with RNA isolated from cells treated with ACTH or Bt2cAMP revealed increased synthesis of SCP2 compared to RNA from control cells. The immunoprecipitable rat adrenal SCP2, synthesized in a cell-free translation system, showed mobility corresponding to Mr of 14,400 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was clearly larger than immunodetectable SCP2 synthesized in cultured adrenal cells (Mr = 11,300). The electrophoretic mobilities of rat liver SCP2 synthesized in cultured cells and in a cell-free translation system were the same as the respective forms from rat adrenal. It is concluded that the synthesis of SCP2 in rat adrenocortical cells is induced by ACTH and that the induction is mediated by cAMP and may involve increased levels of translatable mRNA encoding a higher molecular weight precursor form of SCP2, which presumably undergoes post-translational processing yielding the mature form.

Sterol carrier protein2: further evidence for its role in adrenal steroidogenesis.

Homogeneous rat liver sterol carrier protein (SCP2) has been implicated in adrenal steroidogenesis by studies utilizing as a model system various sub-cellular fractions of rat adrenals. Levels of SCP2 were measured in rat adrenal subcellular fractions and various rat tissues using a highly sensitive radioimmunoassay. The levels of SCP2 in various tissues correlate well with the capacity of each tissue to either synthesize or metabolize cholesterol. The high level of SCP2 in adrenal mitochondria (46% of total tissue SCP2) is consistent with its proposed role of enhancing transfer of cholesterol from the outer to the inner mitochondrial membrane. Neither ACTH nor cycloheximide treatment of rats had a significant effect on SCP2 levels or distribution in the adrenal subcellular fractions. Western blot analysis of adrenal subcellular fractions indicates the presence of a protein of identical molecular weight and at least similar antigenicity as homogeneous rat liver SCP2. In the present studies, intact dispersed rat adrenal fasciculata cells fused with liposomal encapsulated anti-SCP2 IgG showed a 40-65% reduction in their ability to produce corticosterone when stimulated with ACTH. The steroidogenic competence of these anti-SCP2 IgG treated cells can be restored by treatment of the cells with liposomal encapsulated SCP2 prior to ACTH stimulation. These findings provide direct evidence for the involvement of SCP2 in ACTH stimulated steroidogenesis in rat adrenocortical cells, and suggests that SCP2 may not be the putative high turnover "labile protein" involved in acute steroidogenesis.

Adrenal cholesterol esters as substrate source for steroidogenesis.

Adrenocortical cells obtained from tissues of unstimulated rats and which contained a high concentration of endogenous esterified cholesterol, were labeled in vitro with unesterified [4-14C]cholesterol, or incubated in the presence of [2-14C]acetate or lipoprotein [4-14C]cholesterol oleate (LP-CE). Incubations were conducted in the absence and presence of ACTH, and changes in the specific radioactivity (SA) of the secreted corticosterone were used to assess the primary sources of cholesterol substrate used for steroidogenesis. Incubations of cells containing [4-14C]cholesterol with ACTH resulted in a marked increase in the output of corticosterone mass, but not of labeled corticosterone. Thus, the SA of corticosterone when cells were incubated with ACTH was only 6.5% of that obtained from cells incubated in the absence of ACTH. During incubations with [2-14C]acetate, the ACTH-induced increase in the output of corticosterone mass was not associated with increased isotope output, and the SA of corticosterone was only 15% of that in control incubations. This dilution was not altered in cells isolated from adrenals of rats treated with 4-aminopyrazalopyrimidine (4-APP), in which increased cholesterogenesis was demonstrable. The uptake, and hydrolysis of LP-CE, and formation of labeled corticosterone was lipoprotein concentration dependent, and was not influenced by ACTH. However, in the presence of ACTH, the SA of the secreted corticosterone was only 4-8% of that in unstimulated cells. The consistent dilution of the SA of corticosterone in ACTH-treated cells in all studies suggest that the large stores of cytoplasmic cholesterol esters in these cells may normally serve as a primary source of the immediate precursor sterol used for steroidogenesis.

Sterol carrier and lipid transfer proteins.

The discovery of the sterol carrier and lipid transfer proteins was largely a result of the findings that cells contained cytosolic factors which were required either for the microsomal synthesis of cholesterol or which could accelerate the transfer or exchange of phospholipids between membrane preparations. There are two sterol carrier proteins present in rat liver cytosol. Sterol carrier protein 1 (SCP1) (Mr 47 000) participates in the microsomal conversion of squalene to lanosterol, and sterol carrier protein 2 (SCP2) (Mr 13 500) participates in the microsomal conversion of lanosterol to cholesterol. In addition SCP2 also markedly stimulates the esterification of cholesterol by rat liver microsomes, as well as the conversion of cholesterol to 7 alpha-hydroxycholesterol - the major regulatory step in bile acid formation. Also, SCP2 is required for the intracellular transfer of cholesterol from adrenal cytoplasmic lipid inclusion droplets to mitochondria for steroid hormone production, as well as cholesterol transfer from the outer to the inner mitochondrial membrane. SCP2 is identical to the non-specific phospholipid exchange protein. While SCP2 is capable of phospholipid exchange between artificial donors/acceptors, e.g. liposomes and microsomes, it does not enhance the release of lipids other than unesterified cholesterol from natural donors/acceptors, e.g. adrenal lipid inclusion droplets, and will not enhance exchange of labeled phosphatidylcholine between lipid droplets and mitochondria. Careful comparison of SCP2 and fatty acid binding protein (FABP) using six different assay procedures demonstrates separate and distinct physiological functions for each protein, with SCP2 participating in reactions involving sterols and FABP participating in reactions involving fatty acid binding and/or transport. Furthermore, there is no overlap in substrate specificities, i.e. FABP does not possess sterol carrier protein activity and SCP2 does not specifically bind or transport fatty acid. The results described in the present review support the concept that intracellular lipid transfer is a highly specific process, far more substrate-specific than suggested by the earlier studies conducted using liposomal techniques.