| For medical practitioners and the general public - Cholesterol Journal Article Catalog. |
| First Page | Previous Page | Next Page | Last Page |
| A novel variant of lysosomal acid lipase (Leu336-->Pro) associated with acid lipase deficiency and cholesterol ester storage disease Seedorf, U., H. Wiebusch, et al. (1995), Arterioscler Thromb Vasc Biol 15(6): 773-8. Abstract: Cholesterol ester storage disease (CESD) is associated with premature atherosclerosis, hepatomegaly, elevated LDL cholesterol levels, and in most cases, low HDL cholesterol levels. Previous studies have shown a G-->A mutation at the 3' splice junction of exon 8 (E8SJM) of the gene encoding lysosomal acid lipase (LAL) in two kindreds with CESD. In a Canadian-Norwegian kindred with this disease, we show this mutation in conjunction with an as yet unknown T-->C transition in exon 10 predicting a Leu336-->Pro (L336P) replacement and an A-->C transversion in exon 2 predicting a T-6P replacement in the prepeptide. Identification of the L336P rather than the T-6P replacement as the second defect underlying CESD in our patient is deduced from three lines of evidence. First, the E8SJM allele is located in cis with the mutation predicting the T-6P-encoding allele but in trans with the L336P-encoding allele; second, the L336P but not the T-6P replacement cosegregates with low LAL activity in the family; third, the T-6P replacement was found in 6 of 28 alleles from subjects with normal lysosomal acid lipase activity, suggesting that this variant represents a frequent nonfunctional polymorphism. Since the residual LAL activity is higher and the clinical phenotype based on plasma lipid values and severity of hepatosplenomegaly is milder in this case than in a previously studied case who was homozygous for the E8SJM allele, we conclude that the L336P variant appears to be associated with a phenotypically mild form of CESD. |
| A novel vesicular assay to study factors affecting cholesterol crystallization in vitro Zijlstra, A. I., B. J. Van de Heijning, et al. (1996), J Lipid Res 37(4): 877-83. Abstract: We present an assay to study cholesterol crystallization that is fast, facile, and highly reproducible. Cholesterol crystallization from metastable vesicles was induced upon addition of bile salts and depended on the hydrophobicity of the bile salt used and the cholesterol-to-phospholipid ratio of the vesicles. Bile salt-induced crystallization was stimulated upon addition of Con A-binding glycoproteins (CABGs), comparable to the results of the same CABGs in a crystal growth assay. The onset time and total measuring time, however, were much shorter. This assay might, moreover, provide a tool to study the mechanism of cholesterol crystallization in more detail. |
| A one-year look at the impact of cholesterol screening, education, and counseling Love, J. E., Jr. and J. L. McBride (1993), Mil Med 158(7): 474-7. Abstract: During 1989 and 1990, we offered cholesterol screening in conjunction with National Medical Laboratory Week. The proportions of subjects categorized as hypercholesterolemic decreased in 1990 in each eligibility category except family members. Active duty subjects decreased from 39% to 30%, retired subjects from 60% to 55%, and civilian subjects from 51% to 43%. These changes were also reflected in significant decreases in the mean cholesterol levels for retired and civilian subjects. The data suggest a downward trend that we hope will continue with active screening, education, and counseling. |
| A paradoxical severe decrease in serum HDL-cholesterol after treatment with a fibrate Crook, M. A., J. Lynas, et al. (2000), J Clin Pathol 53(10): 796-7. Abstract: There have been a handful of reports in the literature of a paradoxical decrease in serum high density lipoprotein (HDL)-cholesterol in patients on fibrate drugs. The reason for this decline in cardioprotective HDL-cholesterol is not known and may have potential deleterious effects on the patient. This report describes a decrease in serum HDL-cholesterol in a patient on both simvastatin and bezafibrate. This patient also developed abnormal renal function, probably interstitial nephritis. In addition, the literature of fibrate induced serum HDL-cholesterol decline is reviewed and possible mechanisms for this phenomenon discussed. |
| A partial estrogen receptor agonist with strong antiatherogenic properties without noticeable effect on reproductive tissue in cholesterol-fed female and male rabbits Holm, P., M. Shalmi, et al. (1997), Arterioscler Thromb Vasc Biol 17(10): 2264-72. Abstract: Estrogen replacement therapy retards the development of cardiovascular disease and osteoporosis in postmenopausal women. However, long-term unopposed use increases the risk of cancer in endometrium and possibly in breast. The racemic compound ormeloxifene, widely used in India as an antifertility agent, is a partial estrogen receptor agonist with antiosteoporotic properties. The present study was undertaken to investigate the effect of the L-enantiomer (levormeloxifene) and the d-enantiomer (d-ormeloxifene) on the development of atherosclerosis. In a short-term experiment (6 weeks), 4 x 10 ovariectomized female rabbits were fed a 0.25% cholesterol-enriched diet and the effect on plasma cholesterol levels was studied. In a long-term experiment (13 weeks), 4 x 15 ovariectomized female and 4 x 15 shamoperated male rabbits were maintained at a similar plasma cholesterol level of 25 mmol/L and the effect on undamaged and balloon-injured arterial wall was studied. In both experiments, the rabbits were treated with levormeloxifene, d-ormeloxifene, 17 beta-estradiol, or placebo, respectively. In the short-term experiment, levormeloxifene, in contrast to d-ormeloxifene, significantly reduced plasma cholesterol by 30% compared with the placebo group. In the long-term experiment, levormeloxifene, in contrast to d-ormeloxifene, significantly reduced atherosclerosis by 50% in the undamaged arterial wall of both female and male rabbits. Because these rabbits were cholesterol-clamped, the antiatherogenic effect was not mediated via plasma cholesterol lowering. Like estrogen, levormeloxifene did not inhibit atherosclerosis in the endothelium-denuded site of aorta. The antiatherogenic effects of levormeloxifene were thus similar to those of estrogen, but produced in the absence of any noticeable estrogenic effect on uterine or testicular tissue. |
| A patient with an unexplained low level of high density lipoprotein cholesterol Castro Cabezas, M., D. van Loon, et al. (1997), Ned Tijdschr Geneeskd 141(4): 200-2. Abstract: A non smoking male patient 42 years old complained of pain in the calves after exercise and had a low 'high density'-lipoprotein (HDL) cholesterol serum concentration. Angiography of the leg vessels revealed no abnormalities. Treatment with simvastatin and gemfibrozil did not affect HDL cholesterol concentrations. Blood tests of relatives made familial hypo-alpha-lipoproteinaemia unlikely. It appeared that the patient had used anabolic steroids; these increase hepatic lipase activity leading to a higher metabolism of HDL and reduced HDL cholesterol levels. |
| A patient with an unexplained low level of high-density-lipoprotein cholesterol Gebhardt, D. O. (1997), Ned Tijdschr Geneeskd 141(13): 651-2. |
| A PEST deletion mutant of ABCA1 shows impaired internalization and defective cholesterol efflux from late endosomes Chen, W., N. Wang, et al. (2005), J Biol Chem 280(32): 29277-81. Abstract: ATP-binding cassette transporter A1 (ABCA1) promotes the efflux of cellular cholesterol and phospholipids to apoA-I. We described previously a cytoplasmic PEST sequence in ABCA1 and showed that deletion of the PEST sequence results in a prominent increase in the cell surface concentration of ABCA1. In the current study we evaluated the hypothesis that the PEST sequence-deleted ABCA1 might display defective internalization and trafficking to the late endosomes/lysosomes. As assessed by monensin treatment and cell surface biotinylation, the internalization rate of PEST sequence-deleted ABCA1 (ABCA1-dPEST) was markedly decreased compared with wild-type ABCA1 (ABCA1-wt). Immunofluorescence confocal microscopy of ABCA1-wt showed both plasma membrane localization and substantial co-localization with LAMP2 in late endosomes. In contrast, ABCA1-dPEST showed more prominent plasma membrane localization but little co-localization with LAMP2. To assess cholesterol efflux from late endosomes, HEK293 cells were transiently co-transfected with scavenger receptor A (SR-A) and incubated with 3Hcholesterol/acetyl low density lipoprotein (acLDL). Although ABCA1-dPEST showed higher cholesterol efflux than did ABCA1-wt following cell surface labeling (3Hcholesterol/acLDL in the absence of SR-A co-transfection), it showed impaired cholesterol efflux after late endosomal labeling (3Hcholesterol/acLDL in the presence of SR-A). Thus, deletion of the PEST sequence leads to a decrease in the internalization of ABCA1 and decreased cholesterol efflux from late endosomal cholesterol pools, providing evidence that the internalization and trafficking of ABCA1 is functionally important in mediating cholesterol efflux from intracellular cholesterol pools. |
| A pharmacoeconomic evaluation of the Myocardial Ischaemia Reduction with Aggressive Cholesterol Lowering (MIRACL) study in the United Kingdom Buller, N., D. Gillen, et al. (2003), Pharmacoeconomics 21 Suppl 1: 25-32. Abstract: OBJECTIVE: To determine the short-term healthcare costs associated with intensive lipid lowering with atorvastatin initiated within 24-96 hours of the occurrence of acute coronary syndrome (ACS) in patients in the UK. METHODS: Patient-level clinical outcome data from the Myocardial Ischaemia Reduction with Aggressive Cholesterol Lowering (MIRACL) trial and standard cost data were used to compare the total expected 16-week cost per patient on atorvastatin 80 mg/day versus placebo. Clinical outcomes assessed included the following: death; cardiac arrest with resuscitation; nonfatal myocardial infarction; worsening angina pectoris with objective evidence of myocardial ischaemia requiring rehospitalisation; surgical or percutaneous coronary revascularisation; nonfatal stroke; hospitalisation for angina without objective evidence of myocardial ischaemia; and new or worsening congestive heart failure requiring rehospitalisation. All relevant direct medical costs from the perspective of the NHS were considered. RESULTS: The total expected cost was pound 784.05 per patient in the placebo cohort and pound 851.59 per patient in the atorvastatin cohort, resulting in an incremental cost of pound 67.54 per patient in the atorvastatin group. The cost per event avoided was pound 1762.04. A third of the cost of atorvastatin treatment was offset within 16 weeks by the cost savings resulting from the reduction in the number of events in the atorvastatin cohort compared with the placebo cohort. CONCLUSION: The clinical benefits of short-term intensive atorvastatin treatment administered after ACS is attainable through a marginal increase in 'upfront' costs. |
| A pharmacy-based approach to cholesterol management Ditusa, L., A. B. Luzier, et al. (2001), Am J Manag Care 7(10): 973-9. Abstract: OBJECTIVE: To determine the clinical and economic impact of a pharmacy-based cholesterol management program in patients with cardiovascular disease. STUDY DESIGN: Demonstration project. Patients and METHODS: From January 1, 1999, through June 30, 1999, 300 patients with a documented history of cardiovascular disease were enrolled in a pharmacy-based cholesterol program. A similar group of 150 randomly selected patients receiving usual care during the same period served as the comparator group. The following were collected for both groups: patient demographics, comorbidities, fasting lipid profiles, cholesterol medication, cost of medication, and cardiovascular events. The McNemar symmetry chi2 test was used to compare appropriate laboratory monitoring, receipt of cholesterol medication, and achievement of target low-density lipoprotein cholesterol levels at baseline and 1 year for both groups. Kruskal-Wallis analysis of variance was used to compare the cost of therapy for both groups at baseline and follow-up. RESULTS: Mean +/- SD age of program and usual care patients was 67 +/- 10 and 69 +/- 11 years, respectively. At 1 year, >95% of program patients were receiving appropriate laboratory monitoring. In 1 year, the percentage of patients reaching target low-density lipoprotein cholesterol levels increased from 45% to 72% (P<.01) and from 33% to 43% (P =.26) in program and usual care patients, respectively. Despite increased medication use among program patients, their cost per patient per month was lower at 1-year follow-up vs baseline. CONCLUSION: Regular patient interaction and close patient monitoring allowed the pharmacy-based lipid management program to improve cholesterol management in patients with cardiovascular disease. |
| A physical and functional link between cholesterol and tetraspanins Charrin, S., S. Manie, et al. (2003), Eur J Immunol 33(9): 2479-89. Abstract: By interacting with each others, the tetraspanins are thought to assemble a network of molecular interactions, the tetraspanin web. These tetraspanin/tetraspanin interactions involve in part the palmitoylation of the proteins. We show that tetraspanins interact with cholesterol as indicated by the precipitation of tetraspanin/tetraspanin complexes by digitonin, a cholesterol-precipitating reagent, and the labeling of the tetraspanins CD9, CD81 and CD82 with a photoactivatable cholesterol in vivo. Cholesterol may participate to the interaction of tetraspanins with each other since digitonin-precipitation of tetraspanins was correlated with their mutual interaction, and because these interactions were disrupted following cholesterol depletion by methyl-beta-cyclodextrin (MbetaCD) treatment, or cholesterol sequestration by saponin. A mutant CD9 molecule lacking all palmitoylation sites was not precipitated by digitonin under conditions in which wild-type CD9 was precipitated, indicating a role of palmitoylation for the interaction with cholesterol. Finally, upon ligation of tetraspanins on the surface of a lymphoid B cell line, the tyrosine phosphorylation of several proteins, including the vav nucleotide exchange factor, was inhibited when cells were pretreated with MbetaCD, and increased when they were treated with MbetaCD/cholesterol complexes. Thus, there is a physical and functional link between tetraspanins and cholesterol. |
| A pilot scheme for improving the accuracy of serum cholesterol measurement in Scotland and Northern Ireland Packard, C. J., M. A. Bell, et al. (1993), Ann Clin Biochem 30 (Pt 4): 387-93. Abstract: The measurement of plasma cholesterol is subject to particularly stringent requirements of bias and imprecision because clinically important action limits lie within the bulk of the population distribution. A pilot scheme was conducted with the aim of improving the accuracy of the assay of cholesterol in 26 laboratories in Scotland and Northern Ireland. This involved the distribution of three sets of authentic human sera with varying cholesterol concentrations assigned by the reference Abell-Kendall method which was established and validated in a central laboratory. The precision (coefficient of variation) of cholesterol testing was 1.71% across all laboratories at the first distribution and this did not alter subsequently. Mean bias was initially 4.07% (median 4.14%) relative to the Abell-Kendall assigned values and this figure had fallen to 1.12% (median 0.92%) by the third distribution. We conclude that through a standardization scheme it is possible in a relatively short time to improve bias significantly and meet the criterion of < 2-3% bias required for optimum cholesterol measurement. |
| A pilot study of antioxidant intake in patients with cholesterol gallstones Worthington, H. V., L. P. Hunt, et al. (1997), Nutrition 13(2): 118-27. Abstract: Whereas macronutrient intake has been extensively investigated in an attempt to unravel the pathogenesis of human cholesterol gallstones, theoretical considerations and animal models suggest that deficits in micronutrient antioxidants may be more relevant. We report a pilot study of this aspect. The plan was to obtain 7-d weighed food inventories over a 6-mo period from equal numbers of patients who had not consciously changed their diets, patients who were on low-fat diets and age- and gender-matched controls. Food tables would be used to derive daily intakes of 16 known antioxidants, essential amino acids, and essential fatty acids. Under-reporting of food intake, a recognized drawback of this dietary method, would be sought retrospectively by reference to a key publication giving minimum cut-off limits for ratios of energy intakes to basal metabolic rates. There were 18 pairs for study. Analysis of data for the 9 pairs involving patients on their normal diets showed no differences in the intakes of energy macronutrients, and cholesterol, but the patients ingested lower amounts of 10 among 16 antioxidants (P < 0.05 for methionine, alpha-tocopherol, manganese, and vitamin D; 0.05 < P < 0.10 for cysteine, beta-carotene, vitamin C, selenium, zinc, and phosphorus). Both subsets of patients ingested lower amounts of linoleic acid (diet unchanged P = 0.009, changed P = 0.026) and several essential amino acids than did matched controls. Institution of a low-fat diet caused the expected fall in intakes of energy and saturated fatty acids such that the deficit in alpha-tocopherol was amplified, but substitution of fruit and vegetables by the patients resulted in a fortuitous increase in vitamin C, beta-carotene, and manganese intake. Retrospective analysis confirmed under-reporting of food intake by all four subsets of subjects but there was no significant difference in the mean ratio of energy intake to estimated basal metabolic rate in the subset of patients who had not consciously altered their diets and the subset of matched controls. Furthermore, the lower daily intake of alpha-tocopherol and linoleic acid by these patients persisted when results were expressed relative to total fat consumption. The results support the hypothesis that insufficiency of dietary antioxidants, particularly alpha-tocopherol, may be germane to human gallstone disease; they also suggest that low intakes of linoleic acid and essential amino acids may be relevant. Because of the small sample sizes, however, these deductions should be regarded as tentative, pending confirmation by biochemical analysis of blood and especially of hepatic bile. |
| A pilot study of the effect of cholesterol measurement on the eating patterns of adolescents Rohwer, J., B. Wandberg, et al. (1992), Am J Health Promot 7(2): 93-5. Abstract: Epidemiologic research demonstrates that a diet high in total fat, saturated fat, and cholesterol is linked to elevated cholesterol levels and increased risk for heart disease in adults. It is thought that this relationship also holds true for children and adolescents. The literature shows strong support for combining instruction strategies with cardiovascular risk factor screening within the educational setting. Reasons cited include: enhancing the effectiveness of classroom curriculum in achieving health behavior modification; presenting a motivational component; providing important physiological feedback; and offering norm-creating potential. Teaching the adolescent the importance of controlling blood cholesterol through a low-fat diet can be difficult, but producing a change in behavior can be even more challenging. The purpose of this project was to determine whether or not incorporating a cholesterol measurement into instructional strategies would enhance learning. Project objectives included: 1) increasing students' knowledge of the health effects of a high blood cholesterol; and 2) promoting a change in dietary behaviors related to low-fat dietary choices. |
| A pilot study with simvastatin and folic acid/vitamin B12 in preparation for the Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine (SEARCH) MacMahon, M., C. Kirkpatrick, et al. (2000), Nutr Metab Cardiovasc Dis 10(4): 195-203. Abstract: BACKGROUND AND AIM: This study was conducted in preparation for the Study Evaluating Additional Reduction in Cholesterol and Homocysteine (SEARCH). SEARCH is a 12,000 patient 2X2 factorial study in post-myocardial infarction patients that will compare simvastatin 20 mg with simvastatin 80 mg to evaluate whether greater LDL-C reductions with simvastatin provide greater coronary event reductions. SEARCH will also test the hypothesis that lowering plasma homocysteine with folic acid and vitamin B12 will reduce coronary events. This pilot study was performed to determine whether any clinically meaningful interaction between simvastatin and folic acid/vitamin B12 exists. METHODS AND RESULTS: Following a 2-week diet/placebo run-in period, 141 patients with primary hypercholesterolaemia were randomised to one of three treatments for 6 weeks: 80 mg/day simvastatin and 2 mg folic acid/0.8 mg vitamin B12 daily (combination group); or 80 mg/day simvastatin and placebo vitamins (simvastatin alone group); or 2 mg folic acid/0.8 mg vitamin B12 daily and placebo simvastatin (vitamins alone group). The combination group and simvastatin alone group experienced similar serum lipid changes with reductions in LDL-cholesterol of 55.2% and 51.5% respectively. The combination group and vitamins alone group experienced similar homocysteine lowering with reductions in homocysteine of 25.3% and 23.1% respectively. All therapies were well tolerated. CONCLUSIONS: There was no detectable antagonistic effect when simvastatin and folic acid/vitamin B12 were administered concomitantly. |
| A placebo-controlled parallel study of the effect of two types of coffee oil on serum lipids and transaminases: identification of chemical substances involved in the cholesterol-raising effect of coffee van Rooij, J., G. H. van der Stegen, et al. (1995), Am J Clin Nutr 61(6): 1277-83. Abstract: In a randomized double-blind parallel study in 36 subjects the effect on serum cholesterol of a daily dose of 2 g lipid extracted from green Arabica and Robusta coffee beans was studied. Arabica oil elevated serum total cholesterol by 1.1 mmol/L (95% CI for the difference from placebo: 0.41, 1.73 mmol/L); the effect of robusta oil (+0.5 mmol/L) was not statistically significant (95% CI: -0.01, 0.92 mmol/L). Arabica oil also raised plasma triglycerides by 0.8 mmol/L (95% CI: 0.26, 1.25 mmol/L). The effect of robusta oil on triglycerides was +0.14 mmol/L and not significant (95% CI: -0.26, 0.42 mmol/L). In the 12 subjects taking arabica oil an average serum alanine amino-transferase elevation of 18 U/L (95% CI: 9.4, 28.4 U/L) was observed. Because only arabica oil contains kahweol and arabica coffee contains more cafestol than does robusta oil, this is further evidence for the role of diterpenes in the rise of serum cholesterol and alanine aminotransferase after consumption of boiled coffee. |
| A plasma lipoprotein containing only apolipoprotein E and with gamma mobility on electrophoresis releases cholesterol from cells Huang, Y., A. von Eckardstein, et al. (1994), Proc Natl Acad Sci U S A 91(5): 1834-8. Abstract: Previous studies have identified lipid-poor high density lipoproteins with electrophoretic pre-beta mobility as the initial acceptors of cell-derived cholesterol in human plasma. These lipoproteins contain apolipoprotein A-I (apo A-I) as their sole apolipoprotein. In the present study, incubation of human plasma with 3Hcholesterol-laden skin fibroblasts has led to the identification of another lipoprotein that serves as a potent initial acceptor of cell-derived cholesterol. This lipoprotein, which we term gamma-LpE, exhibits gamma mobility on agarose gel electrophoresis. As determined by nondenaturing PAGE and by electron microscopy, the size of the spherical particle ranges between 12 and 16 nm. SDS/PAGE and subsequent immunoblotting identified apoE as its sole apolipoprotein. Plasma from normal and apoA-I-deficient mice, but not from apoE-deficient mice, released 3Hcholesterol from fibroblasts into a gamma-migrating lipoprotein. Cell culture media from hepatoma cells or mouse peritoneal macrophages, both of which contain apoE of cellular origin, also promoted efflux of 3Hcholesterol from fibroblasts into a gamma-migrating fraction. This was not observed with cell culture medium from fibroblasts alone. In conclusion, our results strongly indicate the presence in human plasma of a lipoprotein containing only apoE, gamma-LpE, which is secreted by peripheral cells and is a potent acceptor of cell-derived cholesterol. |
| A polymorphism in the glucocorticoid receptor gene, which decreases sensitivity to glucocorticoids in vivo, is associated with low insulin and cholesterol levels van Rossum, E. F., J. W. Koper, et al. (2002), Diabetes 51(10): 3128-34. Abstract: We investigated whether a polymorphism in codons 22 and 23 of the glucocorticoid (GC) receptor gene GAGAGG(GluArg) --> GAAAAG(GluLys) is associated with altered GC sensitivity, anthropometric parameters, cardiovascular risk factors, and sex steroid hormones. In a subgroup of 202 healthy elderly subjects of the Rotterdam Study, we identified 18 heterozygotes (8.9%) for the 22/23EK allele (ER22/23EK carriers). In the highest age group, the number of ER22/23EK carriers was higher (67-82 years, 12.9%) than in the youngest age group (53-67 years, 4.9%; P < 0.05). Two dexamethasone (DEX) suppression tests with 1 and 0.25 mg DEX were performed, and serum cortisol and insulin concentrations were compared between ER22/23EK carriers and noncarriers. After administration of 1 mg DEX, the ER22/23EK group had higher serum cortisol concentrations (54.8 +/- 18.3 vs. 26.4 +/- 1.4 nmol/l, P < 0.0001), as well as a smaller decrease in cortisol (467.0 +/- 31.7 vs. 484.5 +/- 10.3 nmol/l, P < 0.0001). ER22/23EK carriers had lower fasting insulin concentrations (P < 0.001), homeostasis model assessment- insulin resistance (IR) (index of IR, P < 0.05), and total (P < 0.02) and LDL cholesterol concentrations (P < 0.01). Our data suggest that carriers of the 22/23EK allele are relatively more resistant to the effects of GCs with respect to the sensitivity of the adrenal feedback mechanism than noncarriers, resulting in a better metabolic health profile. |
| A polymorphism of the gene coding for cholesterol ester transfer protein (CETP) that affects transfer of plasma cholesterol ester and its sensitivity to regulation Girard-Globa, A. (1997), Biomed Pharmacother 51(9): 404-5. |
| A pool of Y2 neuropeptide Y receptors activated by modifiers of membrane sulfhydryl or cholesterol balance Parker, S. L., M. S. Parker, et al. (2002), Eur J Biochem 269(9): 2315-22. Abstract: The cloned guinea-pig Y2 neuropeptide Y (NPY) receptors expressed in Chinese hamster ovary (CHO) cells, as well as the Y2 receptors natively expressed in rat forebrain, are distributed in two populations. A smaller population that is readily accessed by agonist peptides on the surface of intact cells constitutes less than 30% of Y2 receptors detected in particulates after cell homogenization. A much larger fraction of cell surface Y2 sites can be activated by sulfhydryl modifiers. A fast and large activation of these masked or cryptic sites could be obtained with membrane-permeating, vicinal cysteine-bridging arsenical phenylarsine oxide. A lower activation is effected by N-ethylmaleimide, an alkylator that slowly penetrates lipid bilayers. The restricted-access alkylator, 2-(trimethylammonium)ethylmethanethiosulfonate, was not effective in unmasking these sites. Some of the hidden cell surface Y2 sites could be activated by polyene filipin III through complexing of membrane cholesterol. The results are consistent with the presence of a large Y2 reserve in a compartment that can be accessed by alteration of sulfhydryl balance or fluidity of the cell membrane, and by treatments that affect the anchoring and aggregation of membrane proteins. |