Acyl-CoA synthetase and the peroxisomal enzymes of beta-oxidation in human liver. Quantitative analysis of their subcellular localization.

The presence of acyl-CoA synthetase (EC 6.2.1.3) in peroxisomes and the subcellular distribution of beta-oxidation enzymes in human liver were investigated by using a single-step fractionation method of whole liver homogenates in metrizamide continuous density gradients and a novel procedure of computer analysis of results. Peroxisomes were found to contain 16% of the liver palmitoyl-CoA synthetase activity, and 21% and 60% of the enzyme activity was localized in mitochondria and microsomal fractions respectively. Fatty acyl-CoA oxidase was localized exclusively in peroxisomes, confirming previous results. Human liver peroxisomes were found to contribute 13%, 17% and 11% of the liver activities of crotonase, beta-hydroxyacyl-CoA dehydrogenase and thiolase respectively. The absolute activities found in peroxisomes for the enzymes investigated suggest that in human liver fatty acyl-CoA oxidase is the rate-limiting enzyme of the peroxisomal beta-oxidation pathway, when palmitic acid is the substrate.     

Studies on acyl-coenzyme A: cholesterol acyltransferase activity in human liver microsomes

The aim of the present study was to characterize the acyl-coenzyme A: cholesterol acyltransferase (ACAT) activity in human liver microsomes. Liver biopsies were obtained from patients undergoing elective cholecystectomy under highly standardized conditions. In 34 patients the enzyme activity of the microsomal fraction averaged 6.6 +/- 0.7 (mean +/- SEM) pmol.min-1.mg protein-1 in the absence of exogenous cholesterol. Freezing of the liver biopsy in liquid nitrogen increased the enzyme activity five- to sixfold. Similarly, freezing of the microsomal fraction prepared from unfrozen liver tissue increased the enzyme activity about twofold. These results may help to explain previous disparate results reported in the literature. The enhanced ACAT activity obtained by freezing was at least partly explained by a transfer of unesterified cholesterol to the microsomal fraction and possibly also by making the substrate(s) more available to the enzyme. Preincubation of the microsomal fraction, prepared from unfrozen liver tissue, with unlabeled cholesterol increased the enzyme activity about fivefold. This finding indicates that hepatic ACAT in humans can also utilize exogenous cholesterol as substrate. Addition of cholesterol to frozen microsomes prepared from unfrozen liver tissue increased the ACAT activity two- to threefold, whereas addition of cholesterol to microsomes prepared from frozen liver tissue did not further increase the enzyme activity. No evidence supporting the concept that ACAT is activated-inactivated by phosphorylation-dephosphorylation could be obtained by assaying the enzyme under conditions similar to those during which the human HMG-CoA reductase is inactivated-activated.     

Cell-specific subcellular localization of soluble epoxide hydrolase in human tissues.

Soluble epoxide hydrolase (sEH) is a phase-I xenobiotic metabolizing enzyme having both an N-terminal phosphatase activity and a C-terminal epoxide hydrolase activity. Endogenous hydrolase substrates include arachidonic acid epoxides, which have been involved in regulating blood pressure and inflammation. The subcellular localization of sEH has been controversial. Earlier studies using mouse and rat liver suggested that sEH may be cytosolic and/or peroxisomal. In this study we applied immunofluorescence and confocal microscopy using markers for different subcellular compartments to evaluate sEH colocalization in an array of human tissues. Results showed that sEH is both cytosolic and peroxisomal in human hepatocytes and renal proximal tubules and exclusively cytosolic in other sEH-containing tissues such as pancreatic islet cells, intestinal epithelium, anterior pituitary cells, adrenal gland, endometrium, lymphoid follicles, prostate ductal epithelium, alveolar wall, and blood vessels. sEH was not exclusively peroxisomal in any of the tissues evaluated. Our data suggest that human sEH subcellular localization is tissue dependent, and that sEH may have tissue- or cell-type-specific functionality. To our knowledge, this is the first report showing the subcellular localization of sEH in a wide array of human tissues.     

Intracellular localization of long-chain acyl-coenzyme A hydrolase and acyl-L-carnitine hydrolase in brown adipose tissue from guinea pigs.

The activities of long-chain acyl-CoA hydrolase (palmitoyl-CoA hydrolase, EC 3.1.2.2) and long-chain acyl-L-carnitine hydrolase, EC 3.1.1.28) in brown adipose tissue from cold-exposed and control guinea pigs were studied. Mitochondria from cold-exposed animals hydrolysed 21 nmol of palmitoyl-CoA/min per mg of protein and 1.3 nmol of palmitoyl-L-carnitine/min per mg of protein, and the specific activities were respectively 2 and 5 times as high in cold-exposed as in control animals. The subcellular-localization studies showed that both the long-chain acyl-CoA hydrolase and long-chain acyl-L-carnitine hydrolase were localized in the mitochondria. A location also in the soluble fraction cannot be excluded. The long-chain acyl-CoA hydrolase activity was doubled when the mitochondria were disrupted; this indicates that the enzyme is localized in the matrix compartment.     

Regulation of hepatic cholesterol ester hydrolase and acyl-coenzyme A:cholesterol acyltransferase in the rat

Cholesterol exists within the hepatocyte as free cholesterol and cholesteryl ester. The proportion of intrahepatic cholesterol in the free or ester forms is governed in part by the rate of cholesteryl ester formation by acyl-coenzyme A:cholesterol acyltransferase (ACAT) and cholesteryl ester hydrolysis by neutral cholesterol ester (CE) hydrolase. In other cell types both ACAT and CE hydrolase activities are regulated in response to changes in the need for cellular free cholesterol. In rats, we performed a variety of experimental manipulations in order to vary the need for hepatic free cholesterol and to examine what effect, if any, this had on the enzymes that govern cholesteryl ester metabolism. Administration of a 20-mg bolus of lipoprotein cholesterol or a diet supplemented with 2% cholesterol resulted in an increase in microsomal cholesteryl ester content with little change in microsomal free cholesterol. This was accomplished by an increase in cholesteryl esterification as measured by ACAT but no change in CE hydrolase activity. An increased need for hepatic free cholesterol was experimentally induced by intravenous bile salt infusion or cholestyramine (3%) added to the diet. ACAT activity was decreased with both experimental manipulations compared to controls, while CE hydrolase activity did not change. Microsomal cholesteryl ester content decreased significantly with little change in microsomal free cholesterol content. Addition of exogenous liposomal cholesterol to liver microsomes from cholestyramine-fed and control rats resulted in a 784 +/- 38% increase in ACAT activity. Nevertheless, the decrease in ACAT activity with cholestyramine feeding was maintained. These studies allowed us to conclude that changes in hepatic free cholesterol needs are met in part by regulation of the rate of cholesterol esterification by ACAT without a change in the rate of cholesteryl ester hydrolysis by CE hydrolase.     

Some properties and subcellular distribution of acyl-coenzyme A: retinol acyltransferase activity in rat testes

The present study was conducted to examine esterification of retinol by testicular microsomes. The microsomes were isolated from rat testes and were incubated under varying assay conditions with [3H]retinol. [3H]Retinylpalmitate was identified by reversed-phase high-performance liquid chromatography as an esterified product. The rate of esterification was increased by the addition of a fatty acyl-CoA. Coenzyme A esters of oleic, palmitic and stearic acids were equally effective substrates for retinol esterification. A 17-fold increase was observed in the presence of palmitoyl-CoA when microsomes had been pretreated with hydroxylamine, a reagent that reacts with coenzyme A thioesters to form hydroxamic acids. The esterifying activity was stimulated by the addition of dithiothreitol (4 mM) and fatty acid-free bovine serum albumin (1 mg/ml). The optimal concentrations for retinol and palmitoyl-CoA were 40 microM and 30-40 microM, respectively. The enzyme activity was inhibited by p-hydroxymercuribenzoate, sodium taurocholate and 5,5'-dithiobis-(2-nitrobenzoic acid), but not by EDTA. The enzyme activity was highest in microsomes (36%). However, some activity was present in mitochondria (29%). These results clearly show the presence of a fatty acyl-CoA: retinol acyltransferase that catalyzes the esterification of retinol in rat testes.     

Dihydroxyacetone phosphate acyltransferase and alkyldihydroxyacetone phosphate synthase activities in rat liver subcellular fractions and human skin fibroblasts

Dihydroxyacetone phosphate acyltransferase (DHAP-AT) and alkyldihydroxyacetone phosphate synthase (DHAP-synthase) activities were examined in subcellular fractions of rat liver. The results indicate that at least 80% of DHAP-AT (assays carried out at pH 5.4) activity in rat liver is in peroxisomes, and the remaining activity is mitochondrial. In contrast to DHAP-AT, DHAP-synthase was detected in all subcellular fractions analyzed but the activity in peroxisomes was 208-fold and 42-fold greater compared to mitochondria and microsomes, respectively. We estimate that at least 70% of the DHAP-synthase activity in rat liver is in peroxisomes. DHAP-AT and DHAP-synthase activities were also examined in homogenates of skin fibroblasts from patients with inherited defects in peroxisomal structure and/or function. Both the enzyme activities were deficient in Zellweger syndrome whereas the activities were only partially deficient in infantile Refsum's disease. Greater reduction in DHAP-synthase activity, but only a partial reduction in DHAP-AT activity was observed in rhizomelic chondrodysplasia punctata. However, both DHAP-AT and DHAP-synthase activities were either normal or near normal in Refsum's disease or X-linked adrenoleukodystrophy. The results reported suggest that various peroxisomal disease states can be identified based on DHAP-AT and DHAP-synthase activities in skin fibroblasts of patients.     

Properties and subcellular localization of CMP-N-acetylneuraminic acid hydrolase of calf kidney.

The properties and subcellular distribution of CMP-N-acetylneuraminic acid (CMP-NAcNeu) hydrolase were studied in the cortex of calf kidney. The pH optimum was 9.0 in both Tris - HCl and glycine/NaOH buffer. The apparent Km was 0.47 mM and the apparent V 15.3 mumol/h/g wet wt of calf kidney cortex. A stimulation by divalent metal ions (Ca2+ and Mg2+) was demonstrated for the hydrolase. In the presence of Triton X-100 an increase in enzyme activity was observed. CMP-NAcNeu hydrolase was inhibited by EDTA, beta-mercaptoethanol, nucleoside phosphates and nucleotide-sugars. The inhibition was more pronounced when a sub-optimal CMP-NAcNeu concentration was used. The enzyme appeared to be localized in the plasma membranes. In the plasma membrane preparation of calf kidney cortex, which was derived mainly from the proximal tubule cells, the yield of CMP-NAcNeu hydrolase (13%) and its increase in specific activity (9-fold) was as high as for the plasma membrane marker enzymes. From subcellular distribution studies it appeared that the enzyme was localized mainly at the bursh border side of the plasma membrane of the proximal tubule cell.     

Esterolytic activities of rat intestinal mucosa. 1. Characterization, cellular distribution and subcellular localization of a glycerol-ester hydrolase.

The preferential cellular distribution in the villus tip and the subcellular localization in the endoplasmic reticulum of an intestinal glycerol-ester hydrolase from rat mucosa are described. The enzyme is shown not to be from either pancreatic or bacterial origin; it catalyzes the hydrolysis of short- and medium chain triglycerides and of p-nitrophenylacetate. Contrarily to the specificity found for the pig intestinal lipase (Serrero, Négrel and Ailhaud, 1975), no activity is detectable against acylCoA; a thiolester hydrolase different from the glycerol-ester hydrolase was demonstrated after differential solubilization and chromatographic separation. A high proportion of glycerol-ester hydrolase is present in the intestinal lumen; its possible complementary role in lipid degradation is discussed.     

The functional size of acyl-coenzyme A (CoA):cholesterol acyltransferase and acyl-CoA hydrolase as determined by radiation inactivation

Frozen rat liver microsomes and rough endoplasmic reticulum were irradiated with high energy electrons. The surviving enzymatic activity of acyl-CoA:cholesterol acyltransferase and activity for esterification of 25-hydroxycholesterol decreased as a simple exponential function of radiation exposure, leading to a target size of 170-180 kDa. The loss of acyl-CoA hydrolase activity with a radiation dose was complex and resolved as a 45-kDa enzyme associated with a large inhibitor. It is interpreted that acyl-CoA hydrolase is the acyl-CoA-binding component and the inhibitor is the cholesterol-binding component of acyl-CoA:cholesterol acyltransferase.     

Carnitine acyltransferase activities in rat liver and heart measured with palmitoyl-CoA and octanoyl-CoA. Latency, effects of K+, bivalent metal ions and malonyl-CoA.

1. Liver carnitine acyltransferase activities with palmitoyl-CoA and octanoyl-CoA as substrates and heart carnitine palmitoyltransferase were measured as overt activities in whole mitochondria or in mitochondria disrupted by sonication or detergent treatment. All measurements were made in sucrose/KCl-based media of 300 mosmol/litre. 2. In liver mitochondria, acyltransferase measured with octanoyl-CoA, like carnitine palmitoyltransferase, was found to have latent and overt activities. 3. Liver acyltransferase activities measured with octanoyl-CoA and palmitoyl-CoA differed in their response to changes in [K+], Triton X-100 treatment and, in particular, in their response to Mg2+. Mg2+ stimulated activity with octanoyl-CoA, but inhibited carnitine palmitoyltransferase. 4. The effects of K+ and Mg2+ on liver overt carnitine palmitoyltransferase activity were abolished by Triton X-100 treatment. 5. Heart overt carnitine palmitoyltransferase activity differed from the corresponding activity in liver in that it was more sensitive to changes in [K+] and was stimulated by Mg2+. Heart had less latent carnitine palmitoyltransferase activity than did liver. 6. Overt carnitine palmitoyltransferase in heart mitochondria was extremely sensitive to inhibition by malonyl-CoA. Triton X-100 abolished the effect of low concentrations of malonyl-CoA on this activity. 7. The inhibitory effect of malonyl-CoA on heart carnitine palmitoyltransferase could be overcome by increasing the concentration of palmitoyl-CoA.     

Characteristics and subcellular localization of pristanoyl-CoA synthetase in rat liver.

We have investigated the activation of pristanic acid to its CoA-ester in rat liver. The results show that peroxisomes, mitochondria as well as microsomes contain pristanoyl-CoA synthetase activity. On the basis of competition experiments and immunoprecipitation studies using antibodies raised against rat liver microsomal long-chain fatty acyl-CoA synthetase (EC 6.2.1.3) we conclude that pristanic acid is activated by the same enzyme which activates long-chain fatty acids, i.e., long-chain fatty acyl-CoA synthetase.     

The subcellular localization of neutral sphingomyelinase in rat liver.

The subcellular distribution of neutral sphingomyelinase activity has been determined in rat liver. Neutral sphingomyelinase is present in the plasma membrane. This enzyme requires either Mg2+ or Mn2+ for full activity; these cations cannot be replaced by Co2+ or Ca2+. The plasma membrane sphingomyelinase is strongly inhibited by Hg2+. A small amount of neutral spingomyelinase activity appears to be present in microsomes. No neutral sphingomyelinase activity is present in liver mitochondria or bytosol. Lysosomal sphingomyelinase is fully active at pH 4.4--4.8 without added divalent cations. However, between pH 5.0 and 7.5 lysosomal sphingomyelinase activity is stimulated by Mg2+, Mn2+, Co2+, and Ca2+. Below pH 4.8, Mg2+ inhibits the reaction. In contrast to the results obtained with the neutral sphingomyelinase activity of plasma membranes and microsomes, lysosomal sphingomyelinase is unaffected by sulfhydryl inhibitors.     

The subcellular localization of ubiquinone in human neutrophils.

Copper and zinc K-edge-extended X-ray-absorption fine structures were measured for the metal sites of freeze-dried bovine superoxide dismutase and the model compounds tetrakis(imidazole)cupric nitrate and tetrakis(imidazole)zinc perchlorate. Detailed simulation of the spectra indicates that the copper site of the enzyme is best fit by co-ordination of four imidazole groups with Cu-N(alpha) distances of 0.198 nm (1.98 A). The zinc site is best fit by three imidazole groups at 0.201 nm (2.01 A) and an oxygen (from aspartate) at 0.203 nm (2.03 A).     

Characterization of acyl-coenzyme A:diacylglycerol acyltransferase (DGAT) enzyme of human small intestine

Acyl-coenzyme A:diacylglycerol acyltransferase (DGAT) enzyme plays a significant role in dietary triacylglycerol (TAG) absorption in the small intestine. However, the characteristics of human intestinal DGAT enzyme have not been examined in detail. The aim of our study was to characterize the human intestinal DGAT enzyme by examining acyl-CoA specificity, temperature dependency, and selectivity for 1,2-diacylglycerol (DAG) or 1,3-DAG. We detected DGAT activity of human intestinal microsome and found that the acyl-CoA specificity and temperature dependency of intestinal DGAT coincided with those of recombinant human DGAT1. To elucidate the selectivity of human intestinal DGAT to 1,2-DAG or 1,3-DAG, we conducted acyl-coenzyme A:monoacylglycerol acyltransferase assays using 1- or 2-monoacylglycerol (MAG) as substrates. When 2-MAG was used as acyl acceptor, both 1,2-DAG and TAG were generated; however, when 1-MAG was used, 1,3-DAG was predominantly observed and little TAG was detected. These findings suggest that human small intestinal DGAT, which is mainly encoded by DGAT1, utilizes 1,2-DAG as the substrate to form TAG. This study will contribute to understand the lipid absorption profile in the small intestine.     

Synthesis of S-adenosyl-L-homocysteine hydrolase inhibitors and their biological activities.

Several nucleosides have been prepared as a possible inhibitor of human S-adenosyl-L-homocysteine (SAH) hydrolase for the development of anti-viral agents. Recently, SAH hydrolase has been considered as an attractive target for parasite chemotherapy for malaria. We report synthesis of several nucleosides including carbocyclic nucleosides and their inhibitory activities against recombinant malaria and human SAH hydrolases.