Small Intestinal Microvillus Membrane Research Articles

Congenital Sucrase‐Isomaltase Deficiency

Congenital Sucrase‐Isomaltase Deficiency

Simplified preparation of a refined milk formula comparable to rat's milk: influence of the formula on development of the gut and brain in artificially reared rat pups.

Milk formulas for artificially reared (AR) rat pups are mostly based on complex cow's milk products, prepared by laborious and time-consuming processes. The aim of this study was to develop a simplified procedure for preparing a refined formula and to examine its influences on gut and brain development. The formula comprised a combination of purified cow's casein and whey proteins, five kinds of edible oil, minerals, and vitamins. Detailed analyses showed that the composition of macro- and micro-nutrients, osmolarity, and pH of the new formula closely resembled those of rat's milk. Rat pups, each with an intragastric cannula implanted at age 5 days, were artificially reared for the following 10-15 days. The body weight gain of AR pups matched that of mother-reared (MR) pups. Histoplanimetrical analyses showed that the small intestine in AR pups was more developed in relation to area of a transverse section, number and length of villi, and thickness of tunica muscularis than that of MR pups. Fat components in the formula influenced the fatty acid composition and the cholesterol-to-phospholipid ratio in the small intestinal microvillus membrane (MVM) of AR pups, but not the MVM fluidity. Brain weight was not significantly different between the two groups at age 15-20 days. This formula is useful for artificial rearing of rats and for identifying dietary components contributing to metabolic adaptation during the suckling period.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). III. Food Protein Binding and MVM Proteins in Rats from Newborn to Young Adult Age

SummaryTo investigate postnatal maturational profiles of functional and biochemical properties of rat small intestinal microvillus membranes (MVM), we did a longitudinal study in rats from birth to the age of 12 weeks. In parallel, we studied binding of cow's milk proteins and of the wheat gliadin peptide B 3142, as well as MVM proteins (SDS‐PAGE). Changes in MVM fluidity and lipid composition exhibited early (0–4 weeks) and intermediate and late (6–12 weeks) patterns, as has been published earlier. Postnatal changes of food protein and peptide binding occurred early during the observation period, not related to weaning. There was not much further change in binding after 6–8 weeks. Developmental profiles of MVM protein and some lipid changes resembled, but did not equal, changes in food protein binding. We conclude that changes in MVM biochemical composition affect MVM binding characteristics. In particular, high molecular weight MVM proteins (susceptible to trypsin treatment) appear to play a role in postnatal maturational differences in MVM food protein binding.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). III. Food Protein Binding and MVM Proteins in Rats from Newborn to Young Adult Age

To investigate postnatal maturational profiles of functional and biochemical properties of rat small intestinal microvillus membranes (MVM), we did a longitudinal study in rats from birth to the age of 12 weeks. In parallel, we studied binding of cow's milk proteins and of the wheat gliadin peptide B 3142, as well as MVM proteins (SDS-PAGE). Changes in MVM fluidity and lipid composition exhibited early (0-4 weeks) and intermediate and late (6-12 weeks) patterns, as has been published earlier. Postnatal changes of food protein and peptide binding occurred early during the observation period, not related to weaning. There was not much further change in binding after 6-8 weeks. Developmental profiles of MVM protein and some lipid changes resembled, but did not equal, changes in food protein binding. We conclude that changes in MVM biochemical composition affect MVM binding characteristics. In particular, high molecular weight MVM proteins (susceptible to trypsin treatment) appear to play a role in postnatal maturational differences in MVM food protein binding.

Neonatal and Adult Patterns of Lectin Binding to Rat Small Intestinal Microvillus Membranes

To investigate postnatal developmental changes of rat small intestinal microvillus membrane (MVM) sugar components, binding studies were done using peanut agglutinin (PNA) and soybean agglutinin (SBA). MVM were prepared from rats of different ages (new-borns to 12 weeks). The Airfuge method was used for separation of unbound material. Characteristics of sugar ligand specificity were different for PNA and SBA, although D(+)galactose was used as an inhibitor for both lectins. Concentration curves and Scatchard plots showed partial saturation of total binding, and at least two groups of MVM ligands. Differences in lectin binding between newborn and adult MVM were opposite for PNA and SBA (newborns bound more PNA, but less SBA). During the first few postnatal weeks in rats, SBA binding changed early, and PNA binding exhibited an intermediate change, not related to weaning. Changes in MVM glycoproteins shown by lectin binding appear to be part of a complex postnatal maturational process in rats, involving different biochemical, functional, and biophysical membrane characteristics.

Quantitation of the rabbit intestinal glycolipid receptor for Shiga toxin: Further evidence for the developmental regulation of globotriaosylceramide in microvillus membranes

Shiga toxin, produced by Shigella dysenteriae 1, causes enterotoxic, cytotoxic, and neurotoxic effects, which may be mediated by a glycolipid receptor, globotriaosylceramide, Gb3. To study the relationship of this receptor and toxin effects, globotriaosylceramide was quantitated and further characterized in rabbit small intestinal microvillus membranes at various ages. Glycolipids were extracted from rabbit microvillus membranes, purified on Unisil columns, and quantitated by highperformance liquid chromatography. The major glycolipid peaks were hydroxylated fatty acid-containing glucosylceramide, lactosylceramide, and globotriaosylceramide. There was a marked increase of globotriaosylceramide levels with age, ranging from 0.02 to 16.2 pmol/μg microvillus membrane protein in neonates and adults, respectively. The globotriaosylceramide peak was susceptible to α-galactosidase treatment, which produced an elevation in the lactosylceramide peak, but markedly reduced globotriaosylceramide content in 34-day-old rabbits. Binding of iodinated Shiga toxin to globotriaosylceramide was documented on highperformance thin-layer chromatography plates by autoradiography. The glycolipid receptor for Shiga toxin in rabbit microvillus membranes is thus a hydroxylated fatty acid-containing globotriaosylceramide. This moiety is virtually absent in neonates and gradually increases with age. Quantitative differences in globotriaosylceramide may be the underlying basis for the age-specific differences in functional responsiveness of rabbit intestinal tissue to Shiga toxin.

Changes in Phospholipid and Cholesterol Concentrations of the Rat Microvillus Membrane During Maturation

Summary: We studied the cholesterol and phospholipid content and the cholesterol-to-phospholipid molar ratio of the small intestinal microvillus membrane in fetal, weanling, and adult rats. We also investigated the effect of glucocorticoid administration on these lipids and the ontogeny of 1,2-diglycerol-CDP choline phosphocholine transferase (PCT). Cholesterol and phospholipid concentrations decreased with maturation, phospholipids declining more than cholesterol. Thus, the cholesterol-to-phospholipid molar ratio rose with maturation. A similar decline was seen with PCT activity. Glucocorticoid treatment had no effect on cholesterol, phospholipids, or PCT. However, the maturational increase in the cholesterol-to-phospholipid molar ratio was blunted by glucocorticoid administration in the weanling and adult rats.

Changes in Phospholipid and Cholesterol Concentrations of the Rat Microvillus Membrane During Maturation

We studied the cholesterol and phospholipid content and the cholesterol-to-phospholipid molar ratio of the small intestinal microvillus membrane in fetal, weanling, and adult rats. We also investigated the effect of glucocorticoid administration on these lipids and the ontogeny of 1,2-diglycerol-CDP choline phosphocholine transferase (PCT). Cholesterol and phospholipid concentrations decreased with maturation, phospholipids declining more than cholesterol. Thus, the cholesterol-to-phospholipid molar ratio rose with maturation. A similar decline was seen with PCT activity. Glucocorticoid treatment had no effect on cholesterol, phospholipids, or PCT. However, the maturational increase in the cholesterol-to-phospholipid molar ratio was blunted by glucocorticoid administration in the weanling and adult rats.

Changes in Phospholipid and Cholesterol Concentrations of the Rat Microvillus Membrane During Maturation

Summary:We studied the cholesterol and phospholipid content and the cholesterol‐to‐phospholipid molar ratio of the small intestinal microvillus membrane in fetal, weanling, and adult rats. We also investigated the effect of glucocorticoid administration on these lipids and the ontogeny of 1,2‐diglycerol‐CDP choline phosphocholine transferase (PCT). Cholesterol and phospholipid concentrations decreased with maturation, phospholipids declining more than cholesterol. Thus, the cholesterol‐to‐phospholipid molar ratio rose with maturation. A similar decline was seen with PCT activity. Glucocorticoid treatment had no effect on cholesterol, phospholipids, or PCT. However, the maturational increase in the cholesterol‐to‐phospholipid molar ratio was blunted by glucocorticoid administration in the weanling and adult rats.

Glycoproteins and glycolipids of rat small intestinal microvillus and basolateral membranes.

Glycoprotein and glycolipid constituents were examined in purified microvillus and basolateral membranes isolated from rat small intestinal epithelial cells. SDS-polyacrylamide gel electrophoresis showed that the molecular weights of most of the major proteins from microvillus membranes were over 100 kD, whereas the majority of those from basolateral membranes tended to have lower molecular weights. Glycoprotein profiles were also examined using three labeling methods, and in each case marked differences were observed between microvillus and basolateral membranes. In both membranes, lectins with a specificity toward N-linked sugar chains bound to the majority of the glycoproteins, in contrast to those lectins which preferentially bind to O-linked sugar chains. Glycolipids were labeled in vivo and isolated from both membrane fractions. Some differences were observed in the fucolipids and neutral glycolipids suggesting a more complex pattern in microvillus membranes. These results indicate that there are differences in the glycoprotein and glycolipid compositions of microvillus and basolateral membranes that may reflect the functional polarity of intestinal epithelial cells.

Lipid peroxidation of rabbit small intestinal microvillus membrane vesicles by iron complexes

Fe(II)- and Fe(III)-induced lipid peroxidation of rabbit small intestinal microvillus membrane vesicles was studied. Ferrous ammonium sulphate, ferrous ascorbate at a molar ratio of 10:1, and ferric citrate, at molar ratios of 1:1 and 1:20, did not stimulate lipid peroxidation. Ferrous ascorbate, 1:1, induced low stimulation, while ferrous ascorbate, 1:20 gave higher stimulation of lipid peroxidation. These results show that in our experimental system, ascorbate is a promotor rather than an inhibitor of lipid peroxidation. Ferric nitrilotriacetate (at molar ratios of 1:2 and 1:10), at an iron concentration of 200 μM, was by far the most effective in inducing lipid peroxidation. Superoxide dismutase, mannitol and glutathione had no effect, while catalase, thiourea and vitamin E markedly decreased ferrous ascorbate l:20-induced lipid peroxidation. Ferric nitrilotriacetate-induced lipid peroxidation was slightly reduced by catalase and mannitol, significantly reduced by superoxide dismutase, and completely inhibited by thiourea. Glutathione caused a 100% increase in the ferric nitrilotriacetate-induced lipid peroxidation. These results suggest that Fe(II) in the presence of trace amounts of Fe(III), or an oxidizing agent and Fe(III) in the presence of Fe(II) or a reducing agent, are potent stimulators of lipid peroxidation of microvillus membrane vesicles. Addition of deferoxamine completely inhibited both ferrous ascorbate, 1:20 and ferric nitrilotriacetate-induced lipid peroxidation, demonstrating the requirement for iron for its stimulation. Iron-induced peroxidation of microvillus membrane may have physiological significance because it could already be demonstrated at 2 μM iron concentration.

Cholesterol modulates alkaline phosphatase activity of rat intestinal microvillus membranes.

Experiments were conducted, using a nonspecific lipid transfer protein, to vary the cholesterol/phospholipid molar ratio of rat proximal small intestinal microvillus membranes in order to assess the possible role of cholesterol in modulating enzymatic activities of this plasma membrane. Cholesterol/phospholipid molar ratios from 0.71 to 1.30 were produced from a normal value of 1.05 by incubation with the transfer protein and an excess of either phosphatidylcholine or cholesterol/phosphatidylcholine liposomes for 60 min at 37 degrees C. Cholesterol loading or depletion of the membranes was accompanied by a decrease or increase, respectively, in their lipid fluidity, as assessed by steady-state fluorescence polarization techniques using the lipid-soluble fluorophore 1,6-diphenyl-1,3,5-hexatriene. Increasing the cholesterol/phospholipid molar ratio also decreased alkaline phosphatase specific activity by approximately 20-30%, whereas decreasing this ratio increased this enzymatic activity by 20-30%. Sucrase, maltase, and lactase specific activities were not affected in these same preparations. Since the changes in alkaline phosphatase activity could be secondary to alterations in fluidity, cholesterol, or both, additional experiments were performed using benzyl alcohol, a known fluidizer. Benzyl alcohol (25 mM) restored the fluidity of cholesterol-enriched preparations to control levels, did not change the cholesterol/phospholipid molar ratio, and failed to alter alkaline phosphatase activity. These findings, therefore, indicate that alterations in the cholesterol content and cholesterol/phospholipid molar ratio of microvillus membranes can modulate alkaline phosphatase but not sucrase, maltase, or lactase activities. Moreover, membrane fluidity does not appear to be an important physiological regulator of these enzymatic activities.

Food proteins and maturation of small intestinal microvillus membranes (MVM). I. Binding characteristics of cow's milk proteins and concanavalin A to MVM from newborn and adult rats.

To study maturational changes of food protein and lectin binding to rat small intestinal microvillus membranes (MVM), MVM were prepared from newborn and adult animals by a modified CaCl2 precipitation technique. Radiolabeled cow's milk proteins [alpha-lactalbumin, alpha-casein, beta-lactoglobulin, bovine serum albumin (BSA)] and the lectin concanavalin A (Con A) were used for incubations. Binding assays were done using miniature ultracentrifugation for separation of unbound material. Binding of Con A to MVM from newborn and adult rats was strong, specific, and saturable. Binding of Con A was inhibited by cold Con A and by the sugar ligand polymer mannan. Adult MVM bound more Con A than newborn preparations. Unlike Con A, binding of cow's milk proteins by MVM was weak, nonspecific, and noninhibitable. Newborn MVM bound more cow's milk proteins than adult controls. This was true for all the proteins tested (p less than 0.001). Binding rose with decreased molecular weight of cow's milk proteins, but molecular weight was not the only determining factor for binding. Trypsin treatment of MVM caused a marked increase of BSA binding in adult but not in newborn preparations. This finding indicated the importance of protein components of MVM for cow's milk protein binding. Maturational changes in protein-lipid interactions and membrane fluidity possibly influence nonspecific cow's milk protein binding to MVM. Differences in binding between newborns and adults were not directly related to maturational shifts in membrane glycosylation that are indicated by differential Con A binding. Increased cow's milk protein binding in newborn individuals might increase the potential risk to develop an adverse reaction to food proteins.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). I. Binding Characteristics of Cow's Milk Proteins and Concanavalin A to MVM from Newborn and Adult Rats

SummaryTo study maturational changes of food protein and lectin binding to rat small intestinal microvillus membranes (MVM), MVM were prepared from newborn and adult animals by a modified CaCl2 precipitation technique. Radiolabeled cow's milk proteins [α‐lactalbumin, α‐casein, β‐lactoglobulin, bovine serum albumin (BSA)] and the lectin concanavalin A (Con A) were used for incubations. Binding assays were done using miniature ultracentrifugation for separation of unbound material. Binding of Con A to MVM from newborn and adult rats was strong, specific, and saturable. Binding of Con A was inhibited by cold Con A and by the sugar ligand polymer mannan. Adult MVM bound more Con A than newborn preparations. Unlike Con A, binding of cow's milk proteins by MVM was weak, nonspecific, and noninhibitable. Newborn MVM bound more cow's milk proteins than adult controls. This was true for all the proteins tested (p < 0.001). Binding rose with decreased molecular weight of cow's milk proteins, but molecular weight was not the only determining factor for binding. Trypsin treatment of MVM caused a marked increase of BSA binding in adult but not in newborn preparations. This finding indicated the importance of protein components of MVM for cow's milk protein binding. Maturational changes in protein‐lipid interactions and membrane fluidity possibly influence nonspecific cow's milk protein binding to MVM. Differences in binding between newborns and adults were not directly related to maturational shifts in membrane glycosylation that are indicated by differential Con A binding. Increased cow's milk protein binding in newborn individuals might increase the potential risk to develop an adverse reaction to food proteins.

Food proteins and maturation of small intestinal microvillus membranes (MVM). II. Binding of gliadin hydrolysate fractions and of the gliadin peptide B3142.

To investigate in vitro interactions between gliadin peptide fractions that have been shown to be toxic to celiac small intestinal mucosa in humans and small intestinal microvillus membranes (MVM) from rats during postnatal maturation, MVM were prepared from newborn, 18-day-old preweanling, and adult rats. Partially hydrolyzed gliadin peptide fractions B1-B4, and the pure gliadin peptide B3142 were radioiodinated and used for binding assays. Miniature ultracentrifugation was used for separation of unbound material. Binding of gliadin fractions to MVM was weak and nonspecific in terms of lacking saturation and inhibition. There was no inhibition of binding by mannan. Enzyme pretreatment of MVM (trypsin, neuraminidase, phospholipase C) did not result in any significant change of binding. Compared with peptides prepared from bovine serum albumin as a control, there was no significant difference in binding of gliadin peptide fractions to MVM. Thus, a lectin-like effect of gliadin peptides toward MVM, or the existence of a specific intestinal surface receptor for gliadin peptides appeared improbable. There were, however, consistent maturational changes in MVM binding in that newborn MVM bound more B1-B4 and B3142 compared with adult controls (p less than 0.001). Nonspecific binding of gliadin fractions to MVM might be related to the initiation of nonspecific in vitro effects of gliadin, particularly toward the immature small intestine. The MVM binding model in the rat clearly does not provide a system for studying celiac disease pathogenesis, but it might help clarify basic processes in the interaction between food-derived substances and elements of the gastrointestinal mucosal barrier.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). II. Binding of Gliadin Hydrolysate Fractions and of the Gliadin Peptide B3142

SummaryTo investigate in vitro interactions between gliadin peptide fractions that have been shown to be toxic to celiac small intestinal mucosa in humans and small intestinal microvillus membranes (MVM) from rats during postnatal maturation, MVM were prepared from newborn, 18‐day‐old preweanling, and adult rats. Partially hydrolyzed gliadin peptide fractions B1‐B4, and the pure gliadin peptide B3142 were radioiodinated and used for binding assays. Miniature ultracentrifugation was used for separation of unbound material. Binding of gliadin fractions to MVM was weak and nonspecific in terms of lacking saturation and inhibition. There was no inhibition of binding by mannan. Enzyme pretreatment of MVM (trypsin, neuraminidase, phospholipase C) did not result in any significant change of binding. Compared with peptides prepared from bovine serum albumin as a control, there was no significant difference in binding of gliadin peptide fractions to MVM. Thus, a lectin‐like effect of gliadin peptides toward MVM, or the existence of a specific intestinal surface receptor for gliadin peptides appeared improbable. There were, however, consistent maturational changes in MVM binding in that newborn MVM bound more B1‐B4 and B3142 compared with adult controls (p < 0.001). Nonspecific binding of gliadin fractions to MVM might be related to the initiation of nonspecific in vitro effects of gliadin, particularly toward the immature small intestine. The MVM binding model in the rat clearly does not provide a system for studying celiac disease pathogenesis, but it might help clarify basic processes in the interaction between food‐derived substances and elements of the gastrointestinal mucosal barrier.

Alterations in piglet small intestine after cholesterol deprivation.

Mammalian cells require cholesterol for normal cell function. This requirement can be fulfilled by endogenous biosynthesis or by extracellular supplementation. Infants fed with human milk receive greater quantities of cholesterol than those fed commercial formulas. Whether this lack of cholesterol in commercial formulas poses a threat to normal neonatal cell function is not known. We compared small intestinal microvillus membrane fluidity, hydrolase activities, protein concentration, permeability to nonabsorbable markers, and weight gain in neonatal piglets receiving restricted intake of isocaloric formulas containing either normal amounts of cholesterol (145 mg/dl) or very low levels of cholesterol (less than 2 mg/dl). Using the fluorescent probe, diphenylhexatriene, and fluorescence polarization, microvillus membranes from cholesterol deprived piglets demonstrated higher fluidities than did microvillus membranes from animals fed normal concentrations of cholesterol. Cholesterol-deprived animals, even though their caloric intake was similar to cholesterol-fed animals, demonstrated a net weight loss per animal whereas the cholesterol-fed animals demonstrated a weight gain. These results demonstrate that in a pig model on a restricted intake, cholesterol deprivation alters the biophysical properties of the microvillus membrane.

Effects of ethinyl estradiol on intestinal membrane structure and function in the rabbit

Structural and functional properties of the small intestinal microvillus membrane were evaluated in the rabbit after administration of ethinyl estradiol, a synthetic estrogen with a demonstrated propensity to alter hepatic membrane lipid fluidity, and promote cholestasis. In the jejunum, no estrogen-induced changes in microvillus membrane total lipid, cholesterol or phospholipid content were observed. However, the ileal microvillus membrane in estradiol-treated animals demonstrates significant reductions vs. controls (per mg protein) in total lipid (0.55 μg vs. 0.89 μg) and phospholipid (206.7 μg vs. 304.91 μg) ( p < 0.001) content, as well as modifications in specific phospholipid species. The increase in the ileal microvillus membrane cholesterol: phospholipid molar ratio (0.65 vs. 0.51, p < 0.05) was associated with a significant decrease in membrane lipid fluidity reflected by an increase in fluorescence anisotropy measurements utilizing diphenyl hexatriene as the fluorophore ( r at 25 °C = 0.306 vs. 0.282, p < 0.05). Thermotropic lipid phase transitions, assessed by Arrhenius plots of both fluorescence data and ileal microvillus membrane p-nitrophenyl-phosphatase activity demonstrate that phase changes occur between and 24 and 28°C in both treated and untreated groups. Within the temperature range studied (40−10°C) no differences from control were observed in microvillus membrane alkaline phosphatase activity following estrogen treatment. These data therefore indicate that ethinyl estradiol-induced effects on microvillus membrane lipid composition and physical properties occur predominantly in the ileum and appear to be related, in part, to specific alterations in the availability of phospholipid following estrogen treatment.

Structural and biochemical differentiation of the guinea-pig colon during foetal development.

We have studied some aspects of the morphological and biochemical differentiation of the foetal guinea-pig colonic epithelium. At day 40 the epithelium was organised in ridges and appeared pseudo-stratified. Folding of the epithelium, followed by villus formation, occurred between days 45 and 55, and by day 50 mucus-secreting goblet cells appeared at the bases of the colonic villi. By day 55 most epithelial cells, including goblet cells, possessed numerous microvilli which, by day 65, had become organised into well developed brush-borders. Between day 55 and term (day 65-68) mucosal depth increased markedly and the colon attained its final glandular morphology. Biochemical studies showed the specific activities of the microvillar hydrolases to be much lower in the washed colon than in either foetal meconium or small intestine at all times during development. Furthermore, a membrane fraction highly enriched in microvillus hydrolase activities was prepared from foetal colonic meconium using techniques originally devised to isolate the foetal small intestinal microvillus membrane. This meconial subfraction was almost identical in polypeptide composition to the highly-purified foetal small intestinal microvillus membrane. Identification of the colonic microvillus membrane was hampered by the absence of reliable membrane markers. Nevertheless, a fraction 14-fold enriched in aminopeptidase activity was prepared from day 40 foetal colon and its polypeptide composition compared by SDS-PAGE to that of the small intestinal microvillus membrane at the same age.

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

(1) 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. (2) 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, M r, 108 000, 116 000 and 127 000, as well as a 47 kDa protein (actin), as major constituents of the membrane. The proteins of M r 108 000 and 116 000 were strongly concanavalin A reactive. A detailed two-dimensional IEF/SDS-PAGE map of the membrane was constructed. (3) Sodium carbonate treatment showed the two concanavalin A-reactive glycoproteins, M r 108 000 and 116 000, comprising the sucrase-isomaltase complex, to be loosely-associated ‘extrinsic’ microvillus membrane proteins. Two proteins, M r 127 000 and 135 000, were tightly-associated ‘intrinsic’ microvillus proteins. (4) 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 M r 108 000 and 116 000 accompanied a 10-fold rise in sucrase-isomaltase activity, and loss of a major protein of M r 131 000 accompanied the complete loss of lactase activity from the membrane during postnatal development.