Diurnal variation in the fraction of 3-hydroxy-3-methylglutaryl-coenzyme A reductase in the active form in the mammary gland of the lactating rat.

'Expressed' and 'total' activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) were measured in freeze-clamped samples of mammary glands from lactating rats at intervals throughout the 24 h light/dark cycle. 'Expressed' activities were measured in microsomal fractions isolated and assayed in the presence of 100 mM-KF. 'Total' activities were determined in microsomal preparations from the same homogenates but washed free of KF and incubated with exogenously added sheep liver phosphoprotein phosphatase before assay. Both 'expressed' and 'total' activities of HMG-CoA reductase underwent a diurnal cycle, which had a major peak 6 h into the light phase and a nadir 15 h later, i.e. 9 h into the dark period. Both activities showed a secondary peak of activity (around 68% of the maximum activity) at the time of changeover from dark to light, with a trough in the value of the 'expressed' activity that was close to the nadir value. 'Expressed' activity was lower than 'total' at all time points, indicating the presence of enzyme molecules inactivated by covalent phosphorylation. Nevertheless the 'expressed'/'total' activity ratio was comparatively constant and varied only between 43% and 75%. Immunotitration of enzyme activity, with antiserum raised in sheep against purified rat liver HMG-CoA reductase, confirmed the presence of both active and inactive forms of the enzyme and indicated that at the peak and nadir the variation in 'expressed' HMG-CoA reductase activity resulted from changes in the total number of enzyme molecules rather than from covalent modification. The sample obtained after 3 h of the light phase exhibited an anomalously low 'total' HMG-CoA reductase activity, which could be increased when Cl- replaced F- in the homogenization medium. The result suggests that at that time the activity of the enzyme could be regulated by mechanisms other than covalent phosphorylation or degradation.     

菊花等十五种中药对大鼠胆固醇代谢的影响

 大鼠口服菊花、郁金及刺五加水煎剂(剂量按药典成人用量折算)三周后,抑制其肝微粒体羟甲基戊二酰辅酶A还原酶的活力,并激活肝微粒体胆固醇7α-羟化酶。在相同状况下,首乌及川芎可抑制羟甲基戊二酰辅酶A还原酶,虽然对胆固醇7α-羟化酶亦有激活作用,但统计学上无意义。泽泻、蒲黄、丹参、黄精、虎杖、延胡索及菌陈等则只抑制肝微粒体羟甲基戊二酰辅酶A还原酶,而对胆固醇7α-羟化酶无作用。黄芪及枸杞对肝微粒体羟甲基戊二酰辅酶A还原酶的活力虽然稍有激活作用,但统计学上无意义。我们实验状况下,上述十五种中药只有菌陈能显著地提高大鼠血清高密度酯蛋白胆固醇的含量。刺五加水煎液对肝微粒体羟甲基戊二酰辅酶A还原酶和胆固醇7α-羟化酶活力调节作用是通过可逆的磷酸化及脱磷酸化作用而实现的。 山楂及刺五加水煎剂,在体外对大鼠肝微粒体羟甲基戊二酰辅酶A还原酶具有强烈的抑制作用。其作用机理亦是通过可逆的磷酸化及脱磷酸化作用进行的。     

3-Hydroxy-3-methylglutaryl coenzyme A reductase activity in beef adrenal cortex.

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) activity (mevalonate:NADP+ oxidoreductase )CoA-acylating) EC 1.1.1.34) was demonstrated in beef adrenal cortex. Most of the HMG-CoA reductase activity is in the microsomal fraction while a small percentage of the activity is associated with the mitochondria, Mitochondria purified on a linear sucrose gradient are enriched in HMG-CoA reductase and cytochrome c oxidase activities. The reductase present in microsomal preparations from the whole adrenal cortex has an apparent Km of 5.6 X 10(-5) M for (R,S)-HMG-CoA. Reductase activities found in the microsomal fractions from the zona glomerulosa, the zona fasciculata, and the zona reticularis were 1.32, 7.37, and 9.74 nmol mevalonate formed per milligram protein in 30 min respectively.     

Postnatal developmental profile of 3-hydroxy-3-methylglutaryl-CoA reductase, squalene synthetase and cholesterol 7 alpha-hydroxylase activities in the liver of domestic swine

Activities of 3-hydroxy-3-methylglutaryl-CoA reductase, squalene synthetase and cholesterol 7 alpha-hydroxylase, measured in liver microsomal preparations from domestic swine between birth and adolescence, correlated strongly in individual animals. A synchronous increase was observed between 4 and 6 weeks after birth, i.e., immediately after weaning. Rise in activity was highest for HMG-CoA reductase (30-fold), and smallest for squalene synthetase (5-fold). In pubertal pigs (16 to 30 weeks old), activities of these enzymes had the same low values as in suckling piglets. The increase of both HMG-CoA reductase and squalene synthetase activities may be caused by the shift from high-cholesterol milk intake to a chow diet with low-cholesterol content. The rise in cholesterol 7 alpha-hydroxylase activity might be due to other dietary or hormonal factors.     

Diurnal changes in the fraction of 3-hydroxy-3-methylglutaryl-CoA reductase in the active form in rat liver microsomal fractions.

'Initial' and 'total' activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) were measured in cold-clamped samples of liver from rats at 2h intervals throughout the 24h light/dark cycle. Initial activities were obtained in microsomes (microsomal fractions) isolated and assayed in the presence of 100mM-KF, whereas 'total' activities were measured in microsomes prepared from the same homogenates but washed free of KF and incubated with exogenous partially purified rat liver protein phosphatase. The initial/total-activity ratio for HMG-CoA reductase underwent a diurnal cycle, which had a nadir 4h into the light phase (when initial activity was 28% of total activity) and a peak 12h later, i.e. 4h into the dark phase (when initial activity was 80% of total activity). These low and high points of the cycle were separated by gradual steady changes in the ratio. The characteristics of this diurnal cycle were different from those of the cycle observed for total activity, which had a plateau of high activity between 2 and 10h into the dark cycle preceded and succeeded by a very rapid increase and decrease, respectively, in the total activity of HMG-CoA reductase. The combination of the two cycles resulted in the dampening of the resultant cycle for the initial or effective activity of HMG-CoA reductase, such that the changes in initial activity around the beginning and and end of the dark phase were more gradual than would otherwise have been the case if the initial/total-activity ratio for HMG-CoA reductase were constant throughout the diurnal cycle. The physiological implications of the observed diurnal variation in the fraction of hepatic HMG-CoA reductase in the active form are discussed.     

Isolation and immunological characterization of 3-hydroxy-3-methylglutaryl coenzyme A reductase from human liver

3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) has been isolated from human liver utilizing HMG-CoA affinity chromatography. The apparent monomer molecular weight of purified human HMG-CoA reductase by SDS-gel electrophoresis was 53,000, and the oligomeric molecular weight determined by sucrose density centrifugation was 104,000. A monospecific antibody prepared against rat liver HMG-CoA reductase inhibited the enzymic activity of microsomal and purified human liver enzyme and formed a single immunoprecipitin line by radial immunodiffusion. These results represent the initial isolation and characterization of human liver HMG-CoA reductase.     

The activity of beta-hydroxy-beta-methylglutaryl-CoA reductase in the soluble fraction of rat liver]

Assay conditions are worked out for determination of activity of beta-hydroxy-beta-methylglutaryl-CoA reductase (HMG-CoA reductase) in 140.000 g supernatant fraction of the rat liver. Some kinetic properties of the enzyme are studied: the activity dependency on the incubation time, protein concentration, pH, glutathione, dithiothreitol and HMG-CoA contents in the incubation medium. The effect of Triton WR 1339 on the activity of HMG-CoA reductase in the liver 140.000 g supernatant and microsomal fractions is comparatively studied. Diurnal activity variations of soluble and microsomal enzymes are also investigated. It is suggested that the rat liver HMG-CoA reductase in the 140.000 g supernatant fraction is not identical to the enzyme located in the microsomal fraction.     

Hydroxymethylglutaryl CoA reductase and the modulation of microsomal cholesterol content by the nonspecific lipid transfer protein

The influence of membrane cholesterol content on 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase, EC 1.1.1.34) in rat liver microsomes was investigated. Microsomes were enriched in cholesterol by incubation with egg phosphatidylcholine-cholesterol vesicles and the nonspecific lipid transfer protein from rat liver. By this method, the microsomal cholesterol content was 2.5-fold enhanced up to final concentrations of 140 nmol cholesterol per mg microsomal protein. In another experiment, microsomes isolated from rats fed a cholesterol-rich diet were depleted of cholesterol by incubation with egg phosphatidylcholine vesicles and the transfer protein. Both cholesterol enrichment and depletion had virtually no effect on the microsomal HMG-CoA reductase activity. In another set of experiments, normal rat liver microsomes were incubated with human serum, resulting in a rise of microsomal cholesterol content. This was reflected in an increase of acyl-CoA:cholesterol acyltransferase activity but failed to have an effect on HMG-CoA reductase.     

Inhibitors of sterol synthesis. Effects of dietary 5 alpha-cholest-8(14)-en-3 beta-ol-15-one on early enzymes in hepatic cholesterol biosynthesis

The effects of dietary administration (0.1% in diet for 8 days) of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one on the levels of activity of cytosolic acetoacetyl coenzyme A thiolase, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, and microsomal HMG-CoA reductase in liver have been studied in male Sprague-Dawley rats. Significant increases in the levels of activity of acetoacetyl-CoA thiolase and of HMG-CoA synthase were observed. The levels of microsomal HMG-CoA reductase activity were increased, relative to pair-fed control animals, in three experiments and increased, relative to ad libitum control animals, in one of three experiments. When compared with other agents for which the primary mode of action is an inhibition of the intestinal absorption of cholesterol, the magnitude of the increases in the levels of hepatic microsomal HMG-CoA reductase activity in the 15-ketosterol-fed rats was considerably smaller. In view of the previously described marked activity of the 15-ketosterol in the inhibition of the intestinal absorption of cholesterol, as well as its known effects in lowering HMG-CoA reductase activity in mammalian cells in culture, it is proposed that the 15-ketosterol may suppress the elevated levels of hepatic microsomal HMG-CoA reductase activity induced by the reduced delivery of cholesterol to liver as a consequence of the inhibition of the intestinal absorption of cholesterol.     

Cholesterol-mediated regulation of HMG-CoA reductase in microsomes from human skin fibroblasts and rat liver

3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase activity was determined in microsomes from human skin fibroblasts and rat liver that had been variously manipulated in vivo or in tissue culture to up- and down-regulate the enzyme. The cholesterol content of these microsomal preparations was then altered by depletion to or enrichment from either cholesterol-free or cholesterol-rich lipid vesicles. Microsomes from human skin fibroblasts responded to cholesterol depletion by increasing HMG-CoA reductase activity and by decreasing it in response to cholesterol enrichment. This was independent of the initial enzyme activity or the tissue culture conditions. Alterations in cholesterol content of rat liver microsomes in vitro failed to demonstrate any significant changes in HMG-CoA reductase activity whether the microsomes started with low enzyme activity (cholesterol-fed rats) or with high enzyme activity (cholestyramine-treated rats). The results are discussed in relation to previously published data and in respect to differences in the control of the human skin fibroblast and rat liver enzymes.     

3-Hydroxy-3-methylglutaryl coenzyme A reductase localization in rat liver peroxisomes and microsomes of control and cholestyramine-treated animals: quantitative biochemical and immunoelectron microscopical analyses

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a key regulatory enzyme involved in cholesterol biosynthesis, has recently been reported to be present in rat liver peroxisomes (Keller, G.A., M.C. Barton, D.J. Shapiro, and S.J. Singer, 1985, Proc. Natl. Acad. Sci. USA, 82:770-774). Immunoelectron labeling of ultrathin frozen sections of normal liver, using two monoclonal antibodies to purified rat liver microsomal HMG-CoA reductase, indicated that the enzyme is present in the matrix of peroxisomes. This study is a quantitative biochemical and immunoelectron microscopical analysis of HMG-CoA reductase in rat liver peroxisomes and microsomes of normal and cholestyramine-treated animals. Cholestyramine treatment produced a six- to sevenfold increase in the specific activity of peroxisomal HMG-CoA reductase, whereas the microsomal HMG-CoA reductase specific activity increased by about twofold. Using a computer program that calculates optimal linear combinations of marker enzymes, it was determined that between 20 and 30% of the total reductase activity was located in the peroxisomes of cholestyramine-treated animals. Less than 5% of the reductase activity was present in peroxisomes under control conditions. Quantitation of the immunoelectron microscopical data was in excellent agreement with the biochemical results. After cholestyramine treatment there was an eightfold increase in the density of gold particles per peroxisome, and we estimate about a threefold increase in the labeling of the ER.     

Characterization of active and inactive forms of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase

Rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was purified to homogeneity using agarose-HMG-CoA affinity chromatography. Additional protein was isolated from the affinity column with 0.5 M KCl that demonstrated no HMG-CoA reductase activity, yet comigrated with purified HMG-CoA reductase on sodium dodecyl sulfate-polyacrylamide gels. This protein was determined to be an inactive form of HMG-CoA reductase by tryptic peptide mapping, reaction with anti-HMG-CoA reductase antibody, and coelution with purified HMG-CoA reductase from a molecular-sieving high-performance liquid chromatography column. This inactive protein was present in at least fourfold greater concentration than active HMG-CoA reductase, and could not be activated by rat liver cytosolic phosphoprotein phosphatases. Immunotitration studies with microsomal and solubilized HMG-CoA reductase isolated in the presence and absence of proteinase inhibitors suggested that the inactive protein was not generated from active enzyme during isolation of microsomes or freeze-thaw solubilization of HMG CoA reductase.     

Activities of 3-hydroxyl-3-methylglutaryl-CoA reductase and acetyl-CoA carboxylase and the rate of mevalonic acid, squalene, sterol and fatty acid biosynthesis from [1-14C]acetyl-CoA and [2-14C]malonyl-CoA in rat liver: effects of Triton WR 1339, starvation and cholesterol diet

The effects of Triton WR 1339, starvation and cholesterol diet on the activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acetyl-CoA carboxylase and on the rates of mevalonic acid (MVA) biosynthesis from acetyl-CoA and malonyl-CoA in the soluble (140 000 g) and microsomal fractions of rat liver, on the rate of incorporation of these substrates into squalene, cholesterol and lanosterol in the rat liver postmitochondrial fraction and on the rate of fatty acid biosynthesis was studied. The administration of Triton WR 1339 (200 mg per 100 g of body weight twice) stimulated the activity of HMG-CoA reductase and MVA biosynthesis from acetyl-CoA and malonyl-CoA in the intact and solubilized microsomal fractions and had no effect on these parameters in the soluble fraction. Starvation for 36 hrs did not cause inhibition of the reductase activity or MVA biosynthesis from both substrates in the soluble fraction. Alimentary cholesterol significantly increased the activity of HMG-CoA reductase, had no effect on the rate of MVA biosynthesis from acetyl-CoA and stimulated the malonyl-CoA incorporation in to MVA in the soluble fraction. Starvation an alimentary cholesterol inhibited the HMG-CoA reductase activity and MVA biosynthesis from both substrates in the solubilized microsomal fraction. Triton WR 1339 stimulated 4--19-fold the lipid formation in the total unsaponified fraction and its components i.e. squalene, lanosterol, cholesterol, from acetyl-CoA and only insignificantly (1,2--1,7-fold) increased malonyl-CoA incorporation into these compounds. Starvation and alimentary cholesterol repressed lanosterol and cholesterol biosynthesis from acetyl-CoA, decreased malonyl-CoA incorporation into these sterols and had no influence on squalene biosynthesis from the two substrates. Triton WR 1339 and starvation inhibited the acetyl-CoA carboxylase activity, unaffected by alimentary cholesterol. No significant changes in the rate of fatty acid biosynthesis from the substrates were observed. The data obtained provide evidence for the existence of autonomic pathways of MVA biosynthesis localized in the soluble and microsomal fractions of rat liver. The pathway of MVA biosynthesis in the soluble fraction is less sensitive to regulatory factors. Sterol biosynthesis from malonyl-CoA is also more resistant to regulatory effects than sterol biosynthesis from acetyl-CoA. This suggests that HMG-CoA reductase localized in the soluble fraction takes part in MVA and sterol biosynthesis from malonyl-CoA.     

Diurnal variation of HMG-CoA reductase activity in rat liver peroxisomes

The specific activity of hepatic microsomal and peroxisomal 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) was determined at different times during a 24 hour cycle from cholestyramine treated rats. The microsomal HMG-CoA reductase activity displayed a peak at D-6 (6th hour of the dark cycle) as previously reported, whereas, the peroxisomal HMG-CoA reductase activity was the highest at L-2 (2nd hour of the light cycle). Immunoblots of the peroxisomal HMG-CoA reductase suggest that the increase in enzyme activity at L-2 is due to changes in enzyme mass. The different cyclic variations observed in microsomal and peroxisomal HMG-CoA reductase activity may suggest different mechanisms of regulation.     

Role of reversible phosphorylation in mediating changes in the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase during pregnancy and lactation in the rat.

1. The expressed and total (completely dephosphorylated) activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase were measured in microsomal fractions isolated from cold-clamped liver samples from female rats in various stages of the reproductive cycle. 2. There was little change in total HMG-CoA reductase activity during pregnancy and early lactation, but after 2 days post partum there was a marked increase in total activity. 3. The expressed/total activity ratio of HMG-CoA reductase showed a profound decrease during the last 2 days of pregnancy. The fraction of the enzyme in the active form increased progressively during the first 2 days of lactation. 4. The combined effect of these changes was that the expressed activity of HMG-CoA reductase changed in parallel with the known changes in the hepatic rate of cholesterogenesis during pregnancy and lactation in vivo.     

Inactive 3-hydroxy-3-methylglutaryl-coenzyme A reductase in broken cell preparations of various mammalian tissues and cell cultures.

Preincubation of broken cell preparations from a variety of tissues and cell cultures resulted in an apparent increase in the level of 3-hydroxy-3-methylglutaryl-CoA reductase activity. However, apparent activation of the reductase in mouse liver, hepatomas and primary liver cell cultures was attributed largely to the loss, during the preincubation period, of an interfering enzyme, 3-hydroxy-3-methylglutaryl-CoA lyase. Among non hepatic cells and tissues (which did not contain appreciable lyase activity) the proportion of latent reductase was high in sonicates of fetal brain and in L cells and was independent of the level of total enzyme activity present. Activation of the reductase was blocked by hydroxymethylglutaryl-CoA and NADPH as well as by KF so that activation did not occur under the conditions of the enzyme assay. The enzyme was activated slowly at 4 degrees C, so that partial activation of the latent form occurred during isolation of the microsomal fraction by differential centrifugation. The reductase present in sonicates of cells with either a high or low proportion of the latent enzyme was inactivated by incubation with ATP and Mg2+. Suppression of reductase activity in L cell cultures by treatment with 25-hydroxycholesterol and an age-related decline in brain enzyme activity did not involve reversible conversion of the reductase to an inactive form.     

3-hydroxy-3-methylglutaryl coenzyme A reductase from rat intestine: subcellular localization and in vitro regulation

The subcellular localization of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in rat intestine was reinvestigated. Highly enriched fractions of endoplasmic reticulum and mitochondria were prepared from mucosal cells. The highest specific activity of HMG-CoA reductase was located in the endoplasmic reticulum fraction with recovery of 25% of the total activity. The mitochondria had low specific activity and low recovery of reductase activity relative to whole homogenate (2-5%). Despite attempts to maximize cell lysis, much of the activity (about 60%) was recovered in a low speed pellet which consisted of whole cells, nuclei, and cell debris as determined by light microscopy. Taken together, the evidence strongly suggests that much of the cellular HMG-CoA reductase activity is present in the endoplasmic reticulum fraction and that mitochondria have little or no intrinsic HMG-CoA reductase. The in vitro regulation of intestinal microsomal HMG-CoA reductase was studied. The intestine possesses a cytosolic HMG-CoA reductase kinase-phosphatase system which appears to be closely related to that present in the liver. Intestinal reductase activity in microsomes prepared from whole mucosal scrapings was inhibited 40-50% by the presence of 50 mM NaF in the homogenizing buffer. It was less susceptible to the action of the kinase than liver reductase. The effects of NaF were reversed by incubation with partially purified intestinal or liver phosphatases. These results suggest that the kinase-phosphatase system could play a role in the regulation of intestinal sterol and isoprene synthesis in vivo.     

Allosteric activation of rat liver cytosolic 3-hydroxy-3-methylglutaryl coenzyme A reductase kinase by nucleoside diphosphates

Extensively purified rat liver cytosolic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase kinase was used to examine the role of ADP in inactivation of HMG-CoA reductase (EC 1.1.1.34). Solubilized HMG-CoA reductase was a suitable substrate for HMG-CoA reductase kinase. At sufficiently high concentrations of solubilized HMG-CoA reductase, reductase kinase activity approached that measured using microsomal HMG-CoA reductase as substrate. Inactivation of solubilized HMG-CoA reductase by HMG-CoA reductase kinase required both MgATP and ADP. Other nucleoside diphosphates, including alpha, beta-methylene-ADP, could replace ADP. HMG-CoA reductase kinase catalyzed phosphorylation of bovine serum albumin fraction V by [gamma-32P]ATP. This process also required a nucleoside diphosphate (e.g. alpha, beta-methylene-ADP). Nucleoside diphosphates thus act on HMG-CoA reductase kinase, not on HMG-CoA reductase. For inactivation of HMG-CoA reductase, the ability of nucleoside triphosphates to replace ATP decreased in the order ATP greater than dATP greater than GTP greater than ITP, UTP. TTP and CTP did not replace ATP. Both for inactivation of HMG-CoA reductase and for phosphorylation of bovine serum albumin protein, the ability of nucleoside diphosphates to replace ADP decreased in the order ADP greater than CDP, dADP greater than UDP. GDP did not replace ADP. Nucleoside di- and triphosphates thus appear to bind to different sites on HMG-CoA reductase kinase. Nucleoside diphosphates act as allosteric activators of HMG-CoA reductase kinase. For inactivation of HMG-CoA reductase by HMG-CoA reductase kinase, Km for ATP was 140 microM and the activation constant, Ka, for ADP was 1.4 mM. The concentration of ADP required to modulate reductase kinase activity in vitro falls within the physiological range. Modulation of HMG-CoA reductase kinase activity, and hence of HMG-CoA reductase activity, by changes in intracellular ADP concentrations thus may represent a control mechanism of potential physiological significance.     

Instructions for authors

The aim of the present study was to examine hypothesis that the enhanced cholesterologenesis, found in rats with experimental chronic renal failure (CRF) resulted from the increased gene expression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase – the rate limiting enzyme in the cholesterologenesis pathway, responsible for mevalonate synthesis. Wistar rats were used and experimental CRF was achieved by 5/6 nephrectomy model. We examined: (a) the changes in the rat liver microsomal HMG-CoA reductase activity, (b) the rat liver HMG-CoA reductase mRNA abundance in various times of day. Obtained data indicates that the increased activity of HMG-CoA reductase in the liver of rats with experimental CRF parallel enhanced mRNA level and suggests that enhanced cholesterol biosynthesis, observed in experimental CRF is at least in part due to the increased HMG-CoA reductase gene expression. The results also indicate that the physiological diurnal rhythm of HMG-CoA reductase activity is preserved in the course of experimental CRF.     

Homeostatic restoration of microsomal lipids and enzyme changes in HMG-CoA reductase and Acyl-CoA : cholesterol acyltransferase in chick liver

We have studied the correlation between changes in the lipid composition in chick liver microsomes and the activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acyl-CoA : cholesterol acyltransferase (ACAT) by in vivo and in vitro experiments with 21-day-old chicks. A 5% cholesterol diet for 3 hr produced an increase in the microsomal and plasmatic cholesterol content, a decrease in HMG-CoA reductase activity and a concomitant increase in ACAT activity. The effect produced by the short-term treatment virtually disappeared 27 hr after ending the cholesterol diet. In vitro experiments were carried out by using vesicles constituted by phosphatidycholine/cholesterol and phosphatidylcholine.