A rapid isolation procedure of plasma membranes from human neutrophils using self-generating Percoll gradients. Importance of pH in avoiding contamination by intracellular membranes

In this study we report an overall procedure for the isolation of both human polymorphonuclear neutrophils and their plasma membrane, by means of self-generating Percoll gradients. After efficient purification (40% yield), neutrophils were lysed by nitrogen cavitation and cellular structures quickly isolated in a one-step procedure. Plasma membrane recovery was monitored by [3H]concanavalin A and 5'-nucleotidase (EC 3.1.3.5) activity. We showed the latter activity is indeed present in human neutrophils. The procedure resulted in a good yield of plasma membrane, since 45% and 55% of total 5'-nucleotidase and [3H]concanavalin A activity, respectively, were recovered within two gradient fractions. Depending on the final pH of the Percoll gradient medium, endoplasmic reticulum markers contaminated either the plasma membrane or the granule fractions. At pH 9.05, NADH-ferricyanide reductase activity clearly separated from plasma membrane markers and displayed the same profile as CDPcholine:diacylglycerolcholine phosphotransferase (EC 2.7.8.2), a typical enzyme of endoplasmic reticulum. These results emphasize the need for strict monitoring of the pH of the gradient medium in subcellular fractionation of neutrophils.     

Characterization of plasma membranes from A431 cells, isolated by self-generating Percoll gradient: a rapid isolation procedure to obtain plasma membranes with functional epidermal growth factor receptors

Plasma membranes have been isolated from the human epidermoid carcinoma cell line A431 by a rapid fractionation of lysate on Percoll density gradient at pH 9.6. Endoplasmic reticulum, lysosomes and mitochondria sedimented at the bottom of gradient whereas plasma membranes focused at low density, as shown with specific markers. Plasma membranes displayed a 4.5- and 4.4-fold enrichment in [3H]concanavalin A and 5'-nucleotidase, respectively. This proteic fraction was further characterized by its lipid composition and phospholipid analysis. The cholesterol/phospholipid molar ratio was 0.45 in plasma membranes against 0.19 in lysate. Sphingomyelin increased from 7.5% of total phospholipids in lysate to 16.2% in plasma membranes, as well as phosphatidylserine which displayed a 1.5-fold enrichment in the plasma membrane fraction. This was at the expense of phosphatidylcholine (45.2% in lysate, against 35% in plasma membranes). Electron microscopy of the isolated material showed vesicles essentially free from endoplasmic reticulum and organelles. These plasma membranes retained the ability to bind 125I-labelled epidermal growth factor (125I-EGF) with a Kd = 4.7 nM and Bmax = 63 pmol/mg protein. EGF binding resulted in a stimulation of the phosphorylation protein reaction in the presence of [gamma-32P]ATP and sodium dodecyl sulfate polyacrylamide gels of phosphorylated proteins indicated that the radioactivity of the major band of molecular weight 170,000 was clearly enhanced by EGF binding. These results indicate that the EGF receptor and its intrinsic protein kinase activity were preserved during our plasma membrane isolation procedure.     

Isolation and characterization of Neurospora crassa plasma membranes.

The isolation and characterization of plasma membranes from a cell wall-less mutant of Neurospora crassa are described. The plasma membranes are stabilized against fragmentation and vesiculation by treatment of intact cells with concanavalin A just prior to lysis. After lysis, the concanavalin A-stabilized plasma membrane ghosts are isolated by low speed centrifugation techniques and the purified ghosts subsequently converted to vesicles by removal of the bulk of the concanavalin A. The yield of ghosts is about 50% whereas the yield of vesicles is about 20%. The isolated plasma membrane vesicles have a characteristically high sterol to phospholipid ratio, Mg2+-dependent ATPase activity and (Na+ plus K+)-stimulated Mg2+ATPase activity. Only traces of succinate dehydrogenase and 5'-nucleotidase are present in the plasma membrane preparations.     

Biochemical and enzymatic characterization of thymic and splenic lymphocyte plasma membranes from inbred rats.

Purified splenic and thymic lymphocytes from the ACI and F344 strains of inbred rats were disrupted by controlled hypotonic treatment, and their plasma membranes were prepared by sucrose density gradient centrifugation. The plasma membrane preparations were highly purified as judged by the structural appearance of the smooth membrane vesicles, by the 10- to 15-fold enrichment of 5'-nucleotidase, which cytochemically localized exclusively in the plasma membranes of intact lymphocytes, by the high cholesterol to phospholipid molar ratio (0.7-1.0), and by the very low specific activities of the enzymes associated predominantly with mitochondria, lysosomes, and endoplasmic reticulum. The protein and the lipid contents of the membranes were 48-55 and 37-48%, respectively. The total lipid content of plasma membranes was characteristically higher in thymic than splenic lymphocytes from both ACI and F344 strains. The specific activity of 5'-nucleotidase was similar in splenic lymphocyte membranes of the ACI strain, and in both the thymic and splenic lymphocyte membranes of the F344 strain. In contrast, the thymic lymphocyte membranes in the ACI strain showed half as much 5'-nucleotidase specific activity. Cytochemical results indicated that the 5'-nucleotidase is located on the outside surface of the lymphocyte plasma membranes.     

Comparison of plasma membranes and endoplasmic reticulum fractions obtained from whole white adipose tissue and isolated adipocytes.

The influence of the mode of preparation upon some of the characteristics of white adipose tissue plasma membranes and microsomes has been reported. Plasma membrane fractions prepared from mitochondrial pellet were shown to have higher specific activities of (Mg2+ + Na+ + K+)-ATPase than plasma membranes originating in crude microsomes. Isolation of fat cells by collagenase treatment was found to result in a decrease in specific activity of the plasma membrane enzymes; in plasma membranes prepared from isolated fat cells, the specific activity values obtained for (Mg2+ + Na+ +k+)-ATPase and 5'-nucleotidase were only 42% and 6.3% respectively of those obtained in plasma membranes prepared from whole adipose tissue. Purification of whole adipose tissue crude microsomes by hypotonic treatment caused extensive solubilization of the endoplasmic reticulum marker enzymes, NADH oxidase and NADPH cytochrome c reductase. The lability of endoplasmic reticulum marker enzymes, however, was found to be greatly diminished in the preparations from isolated fat cells. The possibility that NADH oxidase and NADPH cytochrome c reductase activities found in the plasma membranes are microsomal enzymes adsorbed by the plasma membranes is discussed. The peptide patterns as well as the NADH oxidase and NADPH cytochrome c reductase activity patterns of plasma membranes and purified microsomes were compared by means of sodium dodecyl sulfate or Triton X-100 polyacrylamide gel electrophoresis.     

Ultrastructural localization of nucleoside phosphatase activity in the thymocytes of rats

A comparative electron histochemical investigation was made of ATPase and 5'-nucleotidase activities in isolated cells and in cryostate sections of the rat thymus after various pretreatment. A most optimal demonstration of intracellular ATPase and 5'-nucleotidase activities was possible in non-fixed isolated cells whose cytoplasm was partially or completely destroyed in the process of homogenization. ATPase and 5'-nucleotidase activities were revealed in the nuclear chromatin and in interchromatin ribonucleoproteins, perinuclear space, endoplasmic reticulum and Golgi complex. ATPase activity on the plasma membrane was revealed in the best way in isolated cells after glutaraldehyde prefixation.     

Gonadotropin and prostaglandins binding sites in nuclei of bovine corpora lutea.

Highly purified nuclei isolated from bovine corpora lutea showed marked enrichment of NAD pyrophosphorylase, a marker for this organelle. Rough endoplasmic reticulum and lysosomal markers were undetectable, whereas plasma membrane and Golgi markers were detectable but not enriched in nuclei. These highly puridied nuclei exhibited specific binding with 125I-labeled human choriogonadotropin, [3H]prostaglandin E1 and [3H]prostaglandin F2 alpha. However, these bindings were only 15.4% (human choriogonadotropin), 7.9% (prostaglandin E1) and 8.9% (prostaglandin F2 alpha) of the plasma membrane binding observed under the same conditions. Washing of nuclei and plasma membranes twice with buffer containing 0.1% Triton X-100 resulted in gonadotropin and prostaglandin F2 alpha binding site and 5'-nucleotidase (EC 3.1.3.5) losses from nuclei that were different from those observed for plasma membranes. More importantly, the washed nuclei exhibited 44% (human choriogonadotropin), 21--26% (prostaglandins) of original specific binding despite virtual disappearance of 5'-nucleotidase activity. The nuclear membranes isolated from nuclei, specifically bound 125I-labeled human choriogonadotropin and [3H]prostaglandin F2 alpha to the same extent or significantly more ([3H]prostaglandin E1, P less than 0.05) than nuclei themselves, despite the marked losses of chromatin. In summary, our data suggest that gonadotropin and prostaglandins bind to nuclei and that this binding was intrinsic and was primarily associated with the nuclear membrane.     

Identification and subcellular distribution of adipocyte peptides and phosphopeptides.

Subcellular fractions of high purity (including plasma membrane, endoplasmic reticulum, mitochondria, nuclei, and cytoplasm) were prepared from isolated adipocytes, and the peptide components were examined by detergent gel electrophoresis. Each fraction except the endoplasmic reticulum exhibited a unique and reproducible complement of major peptides. Although the endoplasmic reticulum was distinctive in its enzymic markers, its peptide components showed striking homologies with certain species in the plasma membrane and cytoplasm. The two major adipocyte glycopeptides appear to be contained in the plasma membrane, inasmuch as they followed the distribution of 5'-nucleotidase. Incubation of adipocytes with extracellular 32Pi led to a uniform rate of incorporation of 32P into cellular peptides, with steady-state incorporation reached by 2 hours. Plasma membrane, mitochondria, nuclei, and cytoplasm all contained a distinctive complement of from two to five major phosphopeptides of different molecular weights. The majority of endoplasmic reticulum phosphopeptides exhibited molecular weights closely similar to those of certain species in the plasma membrane and cytoplasm. The phosphopeptides of the plasma membrane exhibited the highest absolute 32P incorporation of all phosphopeptides, next was the single major mitochondiral phosphopeptide. All fractions except the mitochondria contained, in addition to the few major phosphopeptides, numerous minor 32P-labeled phosphopeptides.     

Lipids of rabies virus and BHK-21 cell membranes.

The lipid composition of highly purified Flury strain of rabies virus (HEP) propagated in BHK-21 cells in a chemically defined medium was observed to be 6.7% neutral lipids, 15.8% phospholipids, and 1.5% glycolipids. In the virion, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelin were the most abundant phospholipids, accounting for 90% of the total, and the molar ratio of cholesterol to phospholipid was 0.48. Uninfected BHK-21 cell membranes were obtained by nitrogen cavitation techniques and separated by density gradient centrifugation, and the membranes were assayed for purity using 5'-nucleotidase, cytochrome oxidase, and reduced nicotinamide adenine dinucleotide phosphate diaphorase activities. Lipids of the plasma membrane were enriched in cholesterol, phosphatidylcholine, and phosphatidylethanolamine. In contrast, membranes of the endoplasmic reticulum were enriched in phosphatidylcholine, but contained smaller amounts of phosphatidylethanolamine and sphingomyelin. Comparison of the fatty acyl chains of virus and membranes from uninfected cells revealed the virion to have the lowest ratio of C18:1 to C18:0 (1.771), compared with values of about 3.0 for the plasma membrane and endoplasmic reticulum. Total polyenoic fatty acids were enriched in the plasma membrane, whereas the virus contained higher amounts of total saturates than either of the two membrane preparations. Analysis of the polar and neutral lipid fractions as well as the acyl chain analysis suggests the virion has a lipid composition that is intermiediate to that of the plasma membrane and endoplasmic reticulum and is consistent with the view that numerous viral particles are synthesized de novo by not utilizing a preexisting membrane template. From the ratio of cholesterol to phospholipid of 0.48, we calculated that 1.92 X 10(5) molecules of lipid would cover 4.14 X 10(4) nm2 in the form of a bilayer. Considerations of the molecular dimensions of the rabies envelope (total surface area, 5 X 10(4) nm2) as a bilayer suggest that some penetration of lipids by envelope proteins (M and G) is necessary.     

Sub-cellular fractionation of Trypanosoma brucei. Isolation and characterization of plasma membranes

A procedure is described for the isolation of sub-cellular fractions from bloodstream forms of Trypanosoma brucei. The method leaves intact most of the nuclei, mitochondria and microbodies. All the fractions have been chemically characterized and tested for 10 enzymatic markers. About 5% of total cell protein was isolated as a microsomal fraction containing mostly plasma membranes and endoplasmic reticulum vesicles. Plasma membranes were purified by high-speed centrifugation on magnesium-containing Dextran, and on linear sucrose-density gradients. The yield of membranes was approximately 0.3% of the total cell protein. The purified material had a sucrose density of 1.14 g/cm3 and consisted of smooth vesicles. Specific activity of the membrane markers Na+, K+, ouabain-sensitive ATPase and adenylate cyclase were 26- and 20-fold higher, respectively, than in total cells. Neither DNA nor RNA was detected. The sum of the cholesterol and phospholipid content was 0.99 mg/mg protein. The cholesterol/phospholipid molar ratio was 1:2.     

Similarities of the Golgi apparatus membrane and the plasma membrane in rat liver cells.

A Golgi apparatus-rich fraction and a plasma membrane-rich fraction were isolated from a common homogenate of rat liver. Their respective buovant densities, appearances in the electron microscope and 5'-nucleotidase and UDP-galactose ovalbumin galactosyltransferase activities were in accord with published data on separately isolated Golgi apparatus-rich and plasma membrane-rich fractions. Contamination by endoplasmic reticulum and mitochondria was low. Gel electrophoresis of the membrane proteins of the Golgi apparatus-rich and plasma membrane-rich fractions (separately and mixed) showed a close similarity. After Neville's demonstration that electrophoretic patterns of membrane protein subunits from different subcellular fractions are easily distinguishable, the present work demonstrates an unusually close relationship between the Golgi apparatus membrane and the cell membrane. It is possible that membrane similarity may be mediated by the transfer of membrane-bound vesicles from the Golgi apparatus to the cell membrane.     

Identification of specific proteins and glycoproteins associated with membrane fractions isolated from Zea mays L. coleoptiles

The aim of this work was to identify proteins specific for plant cell membranes which could then be used as unique markers. A crude membrane fraction was isolated from corn coleoptiles and separated on non-linear sucrose density gradients. Separation of endoplasmic reticulum (NADH-cytochrome c reductase), mitochondria (cytochrome c oxidase), golgi (inosine diphosphatase), and plasma membranes (N-1-naphthylphthalamic acid-binding) was achieved. The membrane proteins from the gradient fractions were separated using sodium dodecyl sulphate-poly-acrylamide gel electrophoresis and the gels stained with coomassie blue or with concanavalin A/peroxidase to detect glycoproteins. Proteins specific for the various membranes were identified. Five proteins including two glycoproteins were plasma membrane markers. Protoplasts were isolated and iodinated using lactoperoxidase/glucose oxidase covalently attached to beads. Eleven iodinated proteins were found and three of these corresponded to proteins specifically associated with plasma membranes in the density gradients. Two methods for detecting Ca²⁺-binding proteins following sodium dodecylsulphate polyacrylamide gel electrophoresis were employed. The majority of such proteins were found in the endoplasmatic reticulum and one was specific for plasma membranes. In vitro and in vivo phosphorylation of membrane proteins was examined and the majority of proteins phosphorylated were glycoproteins. Two of the phosphorylated proteins (M_r=110,000 and 20,000) were also iodinated on protoplasts and may be part of the plasma membrane ATPases.Abbreviations ER endoplasmic reticulum - IDP inosine diphosphate - NPA N-1-naphthylphthalamic acid     

Properties of 5'-nucleotidase in rat heart sarcolemma

The activity of 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) was examined in membrane fractions isolated by hypotonic shock-LiBr treatment (fraction HL) and sucrose gradient separation (fraction S) of rat ventricle homogenate. The enzyme activity in these two fractions differed significantly in several respects. In fraction HL, 5'-nucleotidase had a high affinity for AMP (Km 35 microM), and ATP was a potent competitive inhibitor. In contrast, the 5'-nucleotidase displayed by fraction S showed a low substrate affinity (Km 130 microM) and less sensitivity to ATP. Treatment of membranes with trypsin and neuraminidase markedly stimulated 5'-nucleotidase in fraction HL, whereas only a modest effect was observed in fraction S. Exposure of the membranes to Triton X-100 resulted in a 60% and 10% increase in the enzyme activity in fractions HL and S, respectively. The characteristic activity ratios of 5'-nucleotidase at 200 microM relative to 50 microM AMP in fractions HL and S were modified by alamethicin in an opposite way and became identical. Although concanavalin A almost completely inhibited the 5'-nucleotidase activity in both membrane preparations at a concentration of 2 microM, Hill plots of the data on concanavalin A inhibition revealed a coefficient of 2.2 for fraction S and 1.1 for fraction HL. The differences in 5'-nucleotidase activity of the two membrane fractions are considered to be due to differences in the orientation of the vesicles of the sarcolemmal preparations. These results suggest that two distinct catalytic sites for 5'-nucleotidase are present at the intra- and extracellular surface of the rat heart sarcolemma.     

ISOLATION OF A GOLGI APPARATUS-RICH FRACTION FROM RAT LIVER : II. Enzymatic Characterization and Comparison with Other Cell Fractions

Enzymatic activities associated with Golgi apparatus-, endoplasmic reticulum-, plasma membrane-, mitochondria-, and microbody-rich cell fractions isolated from rat liver were determined and used as a basis for estimating fraction purity. Succinic dehydrogenase and cytochrome oxidase (mitochondria) activities were low in the Golgi apparatus-rich fraction. On the basis of glucose-6-phosphatase (endoplasmic reticulum) and 5'-nucleotidase (plasma membrane) activities, the Golgi apparatus-rich fraction obtained directly from sucrose gradients was estimated to contain no more than 10% endoplasmic reticulum- and 11% plasma membrane-derived material. Total protein contribution of endoplasmic reticulum, mitochondria, plasma membrane, microbodies (uric acid oxidase), and lysosomes (acid phosphatase) to the Golgi apparatus-rich fraction was estimated to be no more than 20–30% and decreased to less than 10% with further washing. The results show that purified Golgi apparatus fractions isolated routinely may exceed 80% Golgi apparatus-derived material. Nucleoside di- and triphosphatase activities were enriched 2–3-fold in the Golgi apparatus fraction relative to the total homogenate, and of a total of more than 25 enzyme-substrate combinations reported, only thiamine pyrophosphatase showed a significantly greater enrichment.     

Separation of visual pigments on columns of controlled-pore glass beads

Plasma membranes have been isolated from 3T3 and SV-3T3 cells grown in culture. Cells were harvested mechanically and disrupted in simple isotonic buffered salt solutions without resorting to hypotonic swelling or chemical membrane “hardeners.” The method of storing collected cells, the cell concentration during disruption, and the method of mechanical disruption were found to be significant variables affecting the yield of plasma membranes. The plasma membranes were separated from mitochondria and other cellular organelles by a single centrifugation through a step sucrose gradient containing a viscosity barrier of Dextran T-500 (modified fromA. S. Sun and B. Poole (1975)Anal. Biochem.68, 260). The isolated plasma membranes were located by assay for the “marker” enzyme, alkaline phosphatase (EC 3.1.3.1). The isolated plasma membrane fraction was free of mitochondrial and essentially free of lysozymal and endoplasmic reticulum contamination, which were assayed by measuring cytochrome c reductase, arylsulfatase, and hydrolysis of α-naphthol acetate, respectively. Of the enzymes tested, the phosphodiesterase activity was found to be the most specific assay for the plasma membrane from culture mouse fibroblast cells. The 5′-nucleotidase (EC 3.1.3.5) activity, the other plasma membrane marker, was extremely low in activity and gave an additional peak of activity when 5′-adenilic acid was used as substrate as compared to the expected single peak obtained with 5′-cytidilic acid as substrate. Overall recovery of isolated plasma membranes was greater than 75% as measured by the final recovery of phosphodiesterase activity.     

Uridine phosphorylase activity of isolated plasma membranes of rat liver.

Plasma membranes were isolated from rat liver homogenates either by differential centrifugation or by fractionation in discontinuous sucrose density gradients. Both membrane preparations contained about 17% of the total uridine phosphorylase (EC 2.4.2.3) activity and 44% of the total 5'-nucleotidase (EC 3.1.3.5). The enrichment factor for uridine phosphorylase in the fractions prepared by differential centrifugation was about 2.8 and by the gradient method, as much as 11.0; the respective enrichment factors for 5'-nucleotidase were 1.8 and 9.5. Uridine phosphorylase activity of isolated plasma membrane fractions was stimulated 2.5-fold by 0.1% Triton X-100. Unlike the cytosol enzyme, uridine phosphorylase of plasma membranes showed little or no deoxyuridine-cleaving activity. Contamination of the membrane fractions by thymidine phosphorylase (EC 2.4.2.4) of the cytosol was negligible. The other subcellular organelles obtained by either procedure and characterized by marker enzyme activities were found not to contain significant uridine phosphorylase activity; the cytosol fractions contained just over 70% of the total uridine phosphorylase activity with an enrichment of only about 2.8-fold. The activity of the cytosol enzyme was not stimulated by Triton X-100.     

Location of rat liver iodothyronine deiodinating enzymes in the endoplasmic reticulum.

The iodothyronine-deiodinating enzymes (iodothyronine-5- and 5'-deiodinase) of rat liver were found to be located in the parenchymal cells. Differential centrifugation of rat liver homogenate revealed that the deiodinases resided mainly in the microsomal fraction. The subcellular distribution pattern of these enzymes correlated best with glucose-6-phosphatase, a marker enzyme of the endoplasmic reticulum. Plasma membranes, prepared by discontinuous sucrose gradient centrifugation, were found to contain very little deiodinating activity. Analysis of fractions obtained during the course of plasma membrane isolation showed that the deiodinases correlated positively with glucose-6-phosphatase (r larger than or equal to 0.98) and negatively with the plasma membrane marker 5'-nucleotidase (r ranging between -0.88 and -0.97). It is concluded that the iodothyronine-deiodinating enzymes of rat liver are associated with the endoplasmic reticulum.     

The isolation of endosome-derived vesicles from rat hepatocytes.

Intracellular 5'-nucleotidase involved in membrane circulation in rat hepatocytes is latent, and is protected from inhibition when whole cells are incubated with inhibiting antiserum at 2 degrees C [Stanley, Edwards & Luzio (1980) Biochem. J. 186, 59-69]. These two criteria were used to identify intracellular membrane vesicles containing 5'-nucleotidase on Ficoll density gradients. A sharply defined turbid band containing intracellular 5'-nucleotidase isolated on density gradients was further fractionated by immunoadsorption of plasma-membrane fragments derived from the cell surface of surface-inhibited cells on to an anti-(immunoglobulin G) immunoadsorbent. The resulting non-adsorbed membrane fraction consisted of vesicles of uniform size (approx. 65 nm diam.), but was not identifiable as any known organelle. This fraction could account for approx. 5% of the total cell 5'-nucleotidase activity, and the enzyme activity measured was 55% latent. The fraction had a restricted polypeptide composition but similar phospholipid composition compared with plasma membrane. We suggest that the vesicles observed in this fraction were derived from the endocytic pathway.     

Isolation of plasma membrane from human blood monocytes. Subcellular fractionation and marker distribution

The isolation of plasma membrane from human peripheral blood monocytes is described. Monocytes were isolated by centrifugal elutriation, to eliminate an adherence step, thus minimizing functional and surface antigenic alterations to the cells. Monocytes were surface-labelled with a radiolabelled monoclonal antibody, 125I-WVH-1, and then disrupted by nitrogen cavitation. Membranes were separated according to equilibrium buoyant density by isopycnic centrifugation on a sucrose gradient. The subcellular membranes were localized using marker enzymes for the plasma membrane, 5'-nucleotidase and leucine 2-naphthylamidase (leucine aminopeptidase), and for intracellular membranes: galactosyltransferase (Golgi), arylsulfatase C (endoplasmic reticulum), monoamine oxidase (mitochondria), catalase (peroxisomes), beta-hexosaminidase and beta-glucuronidase (lysosomal vesicles) and lactate dehydrogenase (cytosol). The monoclonal antibody 125I-WVH-1 was shown to label the plasma membrane, as judged by known markers, and represents a highly specific trace label, applicable to the use of plasma membrane as an immunogen for monoclonal antibody production. The NAD-splitting enzyme, NAD+ nucleosidase, was detected and its presence on the plasma membrane was demonstrated. The subcellular localization of non-specific esterase in human mononuclear phagocytes is controversial. No evidence was found for alpha-naphthyl acetate esterase activity on the plasma membrane or in lysosomal vesicles. However, a membrane-bound esterase in fractions with properties similar to the smooth endoplasmic reticulum was detected.