Education & Certifications
MBA, Santa Clara University, Finance
BA, Duke University, Zoology
The binding of chylomicron remnants to rat liver membranes was investigated using radioiodinated lipoproteins. The specific activity of binding increased in parallel with increased enrichment in plasma membrane markers. The yield of receptor activity, however, decreased with enrichment. Accordingly, a partially purified plasma membrane preparation was used for routine studies. Binding was saturable, with half maximal binding achieved at 4.6 micro g tetramethylurea-precipitable protein per ml. The rate of binding was time- and temperature-dependent. It could be inhibited only moderately by 10 mM EDTA. Chylomicron remnants appeared to bind to the membrane as a unit. The bound particle was richer in apoproteins of 20,000-50,000 molecular weight relative to low molecular weight apoproteins than the particles that were not bound. Lipoprotein particles containing only human apoB did not bind to liver membranes nor did they compete for the remnant binding site. Rat lipoproteins of d 1.019-1.063 g/ml did compete for remnant binding. When they were separated into apoB-rich (LDL) or apoE-rich (HDL(c)) fractions by block electrophoresis, the apoE-rich fraction was a more potent competitor. ApoE purified and reconstituted into dimyristoyl phosphatidylcholine vesicles was a potent competitor for the remnant binding site. Vesicles containing (125)I-labeled apoE bound to the membranes, and they could be displaced by unlabeled remnants. Dimyristoyl phosphatidylcholine vesicles themselves did not compete with either remnants or apoE-phospholipid vesicles. These results offer strong support for the hypothesis that the liver membrane chylomicron remnant receptor recognizes apoE with a high affinity, and this initiates the rapid removal of lipoproteins that contain this apoprotein.-Cooper, A. D., S. K. Erickson, R. Nutik, and M. A. Shrewsbury. Characterization of chylomicron remnant binding to rat liver membranes.
View details for Web of Science ID A1982NB03800005
View details for PubMedID 6799601
Chylomicrons were isolated from intestinal lymph and very low-density lipoproteins (VLDL) from the perfusate of isolated perfused livers. In vivo the initial phase of clearance of these particles was very rapid. Chylomicrons appeared to be cleared more quickly than VLDL (t1/2 = 3.7 +/- 1.4 vs. 10.6 +/- 4.0 min). Remnants were prepared from these particles in eviscerated rats and isolated using conditions under which contamination of particles from one organ by particles from the other organ was minimal. The removal of these remnant particles by isolated perfused livers was studied. VLDL remnants were removed more rapidly than the nascent VLDL. The removal of 125I-labeled VLDL remnants was inhibited by the presence of unlabeled VLDL remnants or chylomicron remnants in the perfusate. A 15- to 20-fold excess of either particle inhibited about 50% of the uptake of the labeled lipoprotein. The two types of remnants had comparable potency as competitors of uptake. Similarly, the two types of remnants inhibited uptake of a trace of labeled chylomicron remnants. The binding of these particles to rat liver plasma remnants. The binding of these particles to rat liver plasma membranes was also investigated. Both labeled chylomicron remnants and VLDL remnants bound specifically to the membranes, and either type of remnant displaced the binding of the other with equal potency. Taken together, these results indicate that chylomicron and VLDL remnants share the same hepatic removal mechanism and suggest that the rate of removal of a remnant is not a function of the organ of origin of the precursor lipoprotein.
View details for Web of Science ID A1982PQ53100037
View details for PubMedID 7137355
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.
View details for Web of Science ID A1982NB03800012
View details for PubMedID 6276481
To gain insight into the role of the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT) in cellular cholesterol homeostasis, its regulation in rat liver was investigated both in vivo and in vitro. In vitro assay conditions were optimized and some properties of the microsomal enzyme in vitro was also studied. Arrhenius plots of microsomal ACAT activity showed discontinuities at about 28-29 degrees C and 16-17 degrees C. Detergents above their critical micelle concentrations and organic solvents both inhibited the enzyme. Addition of progesterone or SC 31769, a 7-keto-cholesterol analogue, to microsomes inhibited activity while addition of 25-hydroxycholesterol increased the rate of cholesterol esterification, suggesting that the enzyme is susceptible to both negative and positive modulation by steroids, steroid analogues, or their metabolic products. Increasing the rate of cholesterol biosynthesis had a variable effect on ACAT activity. It was higher at the circadian peak of sterol biosynthesis than at the nadir. Increasing sterol biosynthesis by intragastric administration of mevalonolactone resulted in increased activity. In contrast, increasing the rate of sterol biosynthesis by feeding cholestyramine or administration of Triton WR 1339 had little effect on ACAT. Increasing hepatic cholesterol content by feeding cholesterol, cholate, or an atherogenic diet, fasting or intragastric administration of mevalonolactone all resulted in increased ACAT activity. ACAT activity showed a positive correlation with changes in microsomal free and esterified cholesterol contents. The response of ACAT to changes in hepatic cholesterol concentration in vivo and its response to changes in the rate of cholesterol synthesis support the hypothesis that this enzyme plays an important role in maintenance of hepatic cholesterol homeostasis.
View details for Web of Science ID A1980KJ27400012
View details for PubMedID 7441061
A rapid, biphasic inhibition of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 18.104.22.168) was induced by intragastric administration of R,S-mevalonolactone. The initial phase had a t1/2 of 5.3 min. 30 min after drug administration the inhibition could be reversed in vitro by cytosol or a partially purified cytosolic activator. The reactivation was prevented by 50mM NaF. Thus the initial inhibition appeared to be the result of reversible inactivation possibly by phosphorylation of the enzyme. Consistent with this was the finding that the net reductase activator (phosphatase) activity present in cytosol was decreased 64% in these animals. The rapid reversible inhibition could not be reproduced in vitro by incubating microsomes or postmitochondrial supernatants with mevalonate suggesting the intact cell was necessary for expression of the effect. The second phase of inhibition due to mevalonate administration had a t1/2 of 1.3 h and was not reversible. It was attributed to inhibition of synthesis of reductase probably as the result of sterol accumulation in the cell. Perfusion of 25-hydroxycholesterol through livers isolated from animals at the circadian peak of cholesterol biosynthesis resulted in a rapid, 75-80% inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase. This inhibition was not reversed by incubation with cytosol or partially purified activator. Further, there was no apparent change in net activator levels in cytosol from the livers perfused with 25-hydroxycholesterol. This suggests the effect of this sterol on reductase does not involve reversible phosphorylation-dephosphorylation. On the basis of this study it is postulated that there are at least two mechanisms by which 3-hydroxy-3-methylglutaryl coenzyme A reductase activity can be rapidly suppressed in the intact liver. One is reversible and appears to be the result of alteration in the reductase kinase-phosphatase system. The second is irreversible and may be due to acceleration of the normal degradation system.
View details for Web of Science ID A1980KM41800009
View details for PubMedID 7417482
3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase, EC 22.214.171.124) was purified from rat liver microsomes after solubilization by a slow freezing and thawing method. The purification was accomplished by a five-step procedure involving incubation at 37 degrees, ammonium sulfate fractionation, ultrafiltration, and column chromatography on Bio-Gel A-0.5m and Sephadex G-200. The specific activities of the purified enzyme preparations were up to 480 nmol of mevalonate formed/min/mg of protein, which represented an increase of 350-fold above that of the microsomes. The purified enzyme was found to be essentially homogeneous as evidences by the usual criteria. A subunit molecular weight of 120,000 was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On gel filtration a number of different molecular weight forms were observed which seem to be influenced by temperature, method of purification, and possibly an enzyme-Bio-Gel A-0.5m interaction. The solubilized enzyme consisted predominantly of a species with a molecular weight slightly greater than 200,000 by gel filtration and may be a dimer of two 120,000 subunits. The purified preparation contained lipids; the total cholesterol content was 18 mug/mg of enzyme protein and corresponds to a ratio of 5 mol of cholesterol/enzyme subunit of 120,000 daltons. The specificity of rabbit antiserum prepared against the purified enzyme was demonstrated by double diffusion analysis and quantitative precipitin reactions with solubilized enzyme. The antiserum, in addition to inhibiting the activity of solubilized enzyme also blocked the activity of intact microsomes. The microsomal HMG-CoA reductase is accessible to the antibody, indicating a localization of the enzyme on the outer cytoplasmic surface of the membranes. Intestinal microsomal HMG-CoA reductase was shown to cross-react with antibody to the liver enzyme.
View details for Web of Science ID A1976BV47800043
View details for PubMedID 819431