Detergent-induced proteolysis of rabbit intestinal brush border vesicles

Characterization of lipid exchange proteins isolated from small intestinal brush border membrane

Subjecting rabbit small intestinal brush border membrane vesicles (BBMV) to freeze-thaw cycles releases water-soluble lipid exchange (transfer) proteins into the supernatant. They differ widely in apparent molecular weight and catalyze cholesterol, phosphatidylcholine, and phosphatidylinositol exchange between two populations of small unilamellar lipid vesicles. In order to determine their interrelations, the smallest water-soluble lipid exchange protein was purified to homogeneity by gel filtration on Sephadex G-75 and cation exchange chromatography on Mono S. It is a basic protein of apparent molecular mass of 13 +/- 0.5 kDa. The purified protein was used to raise polyclonal antibodies. Polyclonal antibodies were also produced against a lipid exchange protein of apparent molecular mass of 100-120 kDa. By comparing lipid exchange, lipid binding, and immunological properties of the water-soluble lipid exchange proteins it can be shown that the 13-kDa (peak 3) protein is related to the 100-120 kDa (peak 1) protein; the properties of these two proteins are different from those of the peak 2 lipid exchange protein of apparent molecular mass of 22 kDa. Based on the immunological cross-reactivity observed between the 13 and 100-120 kDa and the lipid binding properties of these two proteins, a working hypothesis is proposed: both proteins are probably part of an integral membrane protein of the brush border membrane that facilitates cholesterol and phosphatidylcholine absorption in this membrane. Evidence derived from immunogold labeling of BBMV supports the notion that this protein is located on the external (luminal) side of the brush border membrane. The analogous behavior of rabbit and human small intestinal brush border membrane in terms of lipid absorption and the release of water-soluble lipid exchange proteins is discussed.

Membrane proteins exposed on the external side of the intestinal brush-border membrane have fusogenic properties

The intestinal brush-border membrane contains one or several membrane proteins that mediate fusion and/or aggregation of small unilamellar egg phosphatidylcholine vesicles. The fusion is accompanied by a partial loss of vesicle contents. Proteolytic treatment of the brush-border membrane with proteinase K abolishes the fusogenic property. This finding suggests that the fusogenic activity is associated with a membrane protein exposed on the external or luminal side of the brush-border membrane. Activation of intrinsic proteinases of the brush-border membrane liberates water-soluble proteins (supernate proteins). These proteins behave in an analogous way to intact brush-border membrane vesicles; they induce fusion of egg phosphatidylcholine vesicles and render the egg phosphatidylcholine bilayer permeable to ions and small molecules (Mr less than or equal to 5000). Furthermore, supernate proteins mediate phosphatidylcholine and cholesterol exchange between two populations of small, unilamellar phospholipid vesicles. Supernate proteins are fractionated on Sephadex G-75 SF yielding three protein peaks of apparent Mr greater than or equal to 70,000, Mr = 22,000 and Mr = 11,500. All three protein fractions show similar phosphatidylcholine-exchange activity, but they differ in their effects on the stability of egg phosphatidylcholine vesicles. The protein fraction with an apparent Mr greater than or equal to 70,000 has the highest fusogenic activity while the protein fraction of apparent Mr = 11,500 appears to be most effective in rendering the egg phosphatidylcholine bilayer permeable.

The uptake of phosphatidylcholine by small intestinal brush border membrane is protein-mediated

Brush border membrane vesicles prepared from rabbit small intestine are essentially free of basolateral membranes and nuclear, mitochondrial, microsomal and cytosolic contaminants. The resulting brush border membrane is unstable due to intrinsic lipases and proteinases. The PC transfer between small unilamellar lipid vesicles or mixed lipid micelles as the donor and the brush border membrane vesicles as the acceptor is protein-mediated. After proteolytic treatment of brush border membrane with papain or proteinase K the PC transfer activity is lost and the kinetics of PC uptake are similar to those measured with erythrocytes under comparable conditions. Evidence is presented to show that the PC transfer activity resides in the apical membrane of the enterocyte and not in the basolateral part of the plasma membrane. Furthermore, the activity is localized on the external surface of the brush border membrane exposed to the aqueous medium with its active centre probably not in direct contact with the lipid bilayer of the membrane. Proteins released from brush border membrane by proteolytic treatment catalyze PC exchange between different populations of small unilamellar vesicles. Furthermore, these protein(s) bind(s) PC forming a PC-protein complex.

Early biochemical responses of the small intestine of coeliac patients to wheat gluten

The pathogenesis of coeliac disease has been investigated by studying the response of small intestinal hydrolases in patients with coeliac disease subject to gluten challenge. Small intestinal biopsies taken before and two and a half hours after a gluten challenge in five patients with coeliac disease who had been maintained on a gluten free diet were examined by a combination of electron and light microscopy, organ culture, pulse chase biosynthetic labelling, SDS-PAGE and autoradiography. Before the challenge, the small intestinal biopsies showed nearly normal morphology. Two and a half hours after the challenge there was deterioration in villus architecture, distortion of microvillus structure, disorganisation of the intermicrovillus pit region, an increase in lysosome like bodies in the apical cytoplasm of the luminal enterocytes and pronounced hypertrophy of the rough endoplasmic reticulum of these cells. SDS-PAGE of small intestinal biopsies from four treated coeliac patients before gluten challenge revealed normal microvillus membrane and hydrolase composition. There was a generalised reduction but no specific alteration in the pattern of polypeptide synthesis in the mucosa of the small intestine in these subjects two and a half hours after the gluten challenge. These results suggest that the generalised reduction in small intestinal brush border enzymes in coeliac patients is not the primary pathogenetic mechanism and represents a secondary effect.

Direct photoaffinity labelling of binding proteins for beta-lactam antibiotics in rabbit intestinal brush border membranes with [3H]benzylpenicillin

Brush border membrane vesicles from rabbit small intestine were used to study the intestinal uptake system for beta-lactam antibiotics. Benzylpenicillin inhibited the H+-dependent uptake of alpha-aminocephalosporins in a concentration-dependent manner suggesting a common transport system for alpha-aminocephalosporins and benzylpenicillin. Benzylpenicillin is therefore a suitable probe to characterize this transport system. Irradiation of [3H]benzylpenicillin using light sources having their maximum of radiation at 300 or 254 nm resulted in a covalent incorporation of radioactivity into penicillin binding proteins as was shown with serum albumin. Hence [3H]benzylpenicillin can be used for direct photoaffinity labeling of penicillin binding proteins in different cells and tissues. In brush border membrane vesicles from rabbit small intestine predominantly a membrane polypeptide with an apparent molecular weight of 127,000 was labeled by [3H]benzylpenicillin. Competition labeling experiments demonstrated that beta-lactam antibiotics--penicillins and cephalosporins--specifically interact with this protein, whereas amino acids, sugars or bile acids had no effect on the labeling pattern. Compounds which decreased the labeling of the 127,000 molecular weight membrane polypeptide also inhibited the H+-dependent uptake of the alpha-aminocephalosporin cephalexin into intestinal brush border membrane vesicles. These results suggest that a polypeptide of molecular weight 127,000 in the brush border membrane from rabbit small intestine is a constituent of a common transport system responsible for the uptake of orally effective beta-lactam antibiotics and dipeptides. beta-Lactam antibiotics which are not absorbed from the small intestine also bind from the luminal site to this transport system, but are not transported across the brush border membrane.

Identification of identical binding polypeptides for cephalosporins and dipeptides in intestinal brush-border membrane vesicles by photoaffinity labeling

The uptake of a photolabile derivative of the orally effective cephalosporin cephalexin, N-(4-azidobenzoyl)cephalexin, was investigated in brush-border membrane vesicles. The compound was taken up into the intravesicular space and inhibited the active uptake of cephalexin in a concentration-dependent manner. Therefore, this probe interacts with the transport system shared by alpha-aminocephalosporins and dipeptides. Photoaffinity labeling of brush-border membrane vesicles from rat small intestine with N-(4-azido[3,5-3H]benzoyl) derivatives of the cephalosporin cephalexin and the dipeptide glycyl-L-proline resulted in the covalent incorporation of radioactivity into membrane polypeptides with apparent molecular weights of 127,000, 100,000, 94,000 and 86,000, the polypeptide of molecular weight 127,000 being predominantly labeled. The specificity of labeling was demonstrated by a decrease in the labeling of the polypeptide of apparent molecular weight 127,000 in the presence of beta-lactam antibiotics and dipeptides, whereas glucose, taurocholate or amino acids had no effect on the labeling pattern. These data demonstrate an interaction of cephalosporins and dipeptides with a common membrane protein of molecular weight 127,000, which could be a component of the intestinal transport system(s) responsible for the uptake of orally effective cephalosporins and dipeptides.

Structural features of absorptive cell and microvillus membrane preparations from rat small intestine

Absorptive cells of the small intestine are highly polarized cells with distinct microvillus membrane (MVM) and basolateral plasma membrane domains. We compared membrane structure in the following preparations of rat small intestine commonly used for in vitro study of MVM function: epithelial sheets, isolated epithelial cells, and four different MVM vesicle preparations, using electron microscopy of thin sections and freeze fracture replicas. We also quantitated mean vesicle diameter of the four MVM preparations by quasielastic light scattering and determined their actin content. Epithelial sheets maintained their plasma membrane polarity as judged by intramembrane particle (IMP) distribution for at least 30 min after isolation. In contrast, the plasma membrane of isolated cells showed redistribution of IMPs, indicating considerable loss of polarity in the few minutes required for cell recovery. The P-face IMPs in MVM prepared by Ca++ precipitation were randomly distributed but became aggregated after exposure to potassium thiocyanate, which removed approximately 50% of core actin. The P-face IMPs in Mg++ precipitated MVM were aggregated whether or not core actin was depleted with potassium thiocyanate. The shape and size of MVM vesicles differed considerably with different preparative techniques. The extremely rapid loss of plasma membrane polarity of isolated intestinal epithelial cells and the striking structural heterogeneity of MVM vesicles prepared by commonly used techniques should be considered in the interpretation of functional studies with these preparations.

Vitamin D status and brush border membrane vesicles: 1,25-dihydroxyvitamin D3 induced destabilization

Purified chick duodenal brush border membrane vesicles (BBMV) were used to assess the effect of vitamin D on intestinal Ca2+ transport and membrane stability. BBMV preparations are right-side-out as judged by a nine-fold increase in accessibility of lactoperoxidase to core material actin in the presence of Triton X-100. Freshly prepared BBMV from vitamin D-deficient chicks support both sodium-dependent glucose transport and Ca2+ uptake. In vivo treatment with 1,25(OH)2D3 results in an 85% increase in the Vmax of Ca2+-uptake from 2.2 to 3.9 nmol/min/mg protein. The Km of Ca2+-uptake (0.9 mM) is independent of the vitamin D status of the chick. The majority of BBMV derived from vitamin D-replete chicks were destabilized and rendered incapable of supporting either sodium-dependent glucose uptake or Ca2+ uptake if they were held at 0-4 degrees C for 2 to 24 h. In 40 separate experiments, 80% of membranes derived from vitamin D-replete chicks showed characteristics of destabilization, whereas only 24% of all control membranes exhibited a lack of viability.

Partial purification and characterization of a serine endopeptidase from rat liver plasma membranes

A serine endopeptidase was partially purified from rat liver plasma membranes by using a four-step procedure: solubilization with N-lauroylsarcosine; Ultrogel AcA-34 chromatography; CM Affi-Gel blue chromatography; agarose-soybean trypsin inhibitor chromatography. This enzyme was found to hydrolyze casein and various chromogenic peptide substrates; highest activity occurred with H-D-Val-Leu-Arg-p-nitroanilide, reported to be a specific substrate for human glandular kallikreins. The enzyme was heat-sensitive, showed a pH optimum between 8.0 and 9.0 and was inhibited by D-Phe-L-Phe-L-Arg-CH2Cl, aprotinin, diisopropyl fluorophosphate (DFP), soybean trypsin inhibitor, phenylmethylsulphonyl fluoride, leupeptin, antipain and dithiothreitol. This liver plasma membrane proteinase has an apparent molecular weight of about 30 000 as determined by Ultrogel AcA-34 chromatography and by autoradiography of [3H]DFP-labelled protein electrophoresis.

Cholesterol oxidase as a structural probe of biological membranes: its application to brush-border membrane

Cholesterol present in intact brush-border membrane vesicles made from rabbit small intestine is a poor substrate for cholesterol oxidase (EC 1.1.3.6, from Nocardia sp. and Nocardia erythropolis). It becomes susceptible to oxidation by the enzyme only after the addition of detergent, e.g., Triton X-100, in quantities sufficient to disrupt the membrane. This is also true for cholesterol present in bilayers of small unilamellar phosphatidylcholine or phosphatidylserine vesicles made by ultrasonication. The data presented here on intestinal brush-border membrane are in good agreement with results reported on other biological membranes, e.g., from erythrocytes and vesicular stomatitis virus, but are somewhat different from those on rat intestinal brush-border membrane. Our results on phospholipid bilayers agree well with published work on model membranes. From the work presented we conclude that, with our present understanding, cholesterol oxidase can hardly be used to probe the distribution of cholesterol in biological membranes. A prerequisite for using the enzyme successfully as such a probe would be the understanding of the factors controlling the interaction of the enzyme with its substrate cholesterol. The question under which conditions cholesterol oxidase could be useful for probing the distribution and preferred location of cholesterol in biological membranes is discussed.

Two-dimensional isoelectric focussing/sodium dodecyl sulphate polyacrylamide gel electrophoretic mapping and some molecular characteristics of the proteins of the adult guinea-pig small intestinal microvillus membrane

The adult guinea-pig small intestinal microvillus membrane was purified approximately 25-fold by both cation-precipitation and differential centrifugation methods. Comparison by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed no substantial differences in polypeptide composition between the two preparations. One-dimensional SDS-PAGE and two-dimensional isoelectric focussing (IEF)/SDS-PAGE, together with Coomassie-blue, silver and lectin-staining, showed three major high molecular weight polypeptides, Mr 108 000, 116 000 and 127 000, as well as a 47 kDa protein (actin), as major constituents of the membrane. The proteins of Mr 108 000 and 116 000 were strongly concanavalin A reactive. A detailed two-dimensional IEF/SDS-PAGE map of the membrane was constructed. Sodium carbonate treatment showed the two concanavalin A-reactive glycoproteins, Mr 108 000 and 116 000, comprising the sucrase-isomaltase complex, to be loosely-associated 'extrinsic' microvillus membrane proteins. Two proteins, Mr 127 000 and 135 000, were tightly-associated 'intrinsic' microvillus proteins. Despite regional differences in specific activity of some small intestinal microvillar enzymes, most noticeably enterokinase (EC 3.4.21.9) and dipeptidyl peptidase IV (EC 3.4.14.x), no substantial regional differences were seen in microvillus membrane polypeptide composition. In contrast, a substantial increase in the major high molecular weight proteins of Mr 108 000 and 116 000 accompanied a 10-fold rise in sucrase-isomaltase activity, and loss of a major protein of Mr 131 000 accompanied the complete loss of lactase activity from the membrane during postnatal development.

Effect of micellar lipids on rabbit intestinal brush-border membrane phospholipid bilayer integrity studied by 31P NMR

The effect of biliary salts and fatty acids on the bilayer structure of rabbit intestinal brush-border membranes was studied using the nonperturbing probe 31P NMR. The broad, asymmetric lineshape of the 31P NMR spectrum of isolated brush-border vesicles demonstrates that their component phospholipids are organized in extended bilayers. These membranes are not significantly perturbed by incubation with physiological concentrations of biliary salts (3, 9, 18 mM), demonstrating that the vesicles are highly stable, corresponding to their biological function. However, the emergence of a narrow peak superimposed on the broad lineshape indicates that a small proportion of the membrane phospholipids has reached isotropic motion, which may correspond to external or internal micellar structures. Incubation with mixed micelles of fatty acids and taurochlorate show that long-chain fatty acids enhance the membrane-perturbing effect of taurocholate while short-chain, water-soluble fatty acids do not, suggesting a difference in the absorption mechanisms.

Leakiness of brush-border vesicles

From the water content of pelleted brush-border vesicles and from a comparison of the aqueous volume within the pellet that is available to [3H]inulin (58%), inulin [14C]carboxylic acid (34%, both approx. 5000 daltons), [3H]raffinose (97%, 540 daltons) and [3H]glucose (94%, 180 daltons) it is concluded that only 1 in 4 to 6 of the brush-border vesicles is sealed. The implication of this finding for labelling and transport studies and for vesicle formation is discussed.

Immunolocalization of the 33 kD protein in the microvilli of rabbit small-intestinal epithelial cells

Rabbit small-intestinal microvilli isolated by a Ca2+ precipitation method contain a 33 kD protein, which has not been observed in microvilli isolated in the presence of Ca2+-chelators. The intracellular localization of this protein in rabbit intestinal epithelial cells was studied by immunofluorescence and immunoperoxidase microscopy, and was compared with that of aminopeptidase M, a well-known microvillus membrane-bound enzyme. The results obtained show that the 33 kD protein is located in the inside of the microvillus, but not in the terminal web of the epithelial cell. The protein may also be located on the basolateral surface of the cell.

A 31P- and 2H-NMR study on lecithins in liquid crystalline polyoxyethylene detergents

Phosphatidylcholines were incorporated into hexagonal liquid crystalline mixtures of the non-ionic detergents Triton X-100 and octaethyleneglycoldodecylether with D2O. It is shown by nuclear magnetic resonance (NMR) that the phospholipids adopt the hexagonal liquid crystalline structure of the detergent host lattice. The anisotropic motion of the phospholipid headgroups seems to be unaffected, whereas the acyl chains are disordered. Increasing phospholipid concentration leads to separation of a lamellar phase. The lamellar structure is also preferred at elevated temperatures. Phosphatidylcholines with saturated acyl chains undergo a transition from the hexagonal liquid crystalline to an ordered lamellar state. The shape of the 31P-NMR signals suggests that pure gel phase phospholipid separates out. The headgroup region of this gel phase phospholipid becomes immobilized after a few weeks of storage below the transition temperature as judged from 31P-NMR. At the same time 2H-NMR exhibits a new signal from D2O undergoing slow isotropic motion. This behavior bears resemblance to the formation of a coagel in fatty acid-water systems.