Dietary flavonoid and isoflavone glycosides are hydrolysed by the lactase site of lactase phlorizin hydrolase

Lactase phlorizin hydrolase (LPH; EC 3.2.1.62) is a membrane-bound, family 1 beta-glycosidase found on the brush border of the mammalian small intestine. LPH, purified from sheep small intestine, was capable of hydrolysing a range of flavonol and isoflavone glycosides. The catalytic efficiency (k(cat)/K(m)) for the hydrolysis of quercetin-4'-glucoside, quercetin-3-glucoside, genistein-7-glucoside and daidzein-7-glucoside was 170, 137, 77 and 14 (mM(-1) s(-1)) respectively. The majority of the activity occurred at the lactase and not phlorizin hydrolase site. The ability of LPH to deglycosylate dietary (iso)flavonoid glycosides suggests a possible role for this enzyme in the metabolism of these biologically active compounds.

Maturation of human intestinal lactase-phlorizin hydrolase: generation of the brush border form of the enzyme involves at least two proteolytic cleavage steps

Human lactase-phlorizin hydrolase (LPH), a brush border membrane hydrolase of the small intestine, is synthesized as a precursor molecule that undergoes proteolytic cleavage to yield mature LPH (LPHbeta) by a trypsin-like protease (Naim et al., 1987, 1991). Arg868-Ala869 has been previously proposed to be the putative cleavage site for this processing step. Site-directed mutagenesis of this monobasic site does not lead to the generation of an uncleaved proLPH species, which strongly suggests the existence of an additional cleavage site. Further analyses of LPH synthesized in different cell lines lend support to this hypothesis. Biosynthetic labeling of human intestinal biopsy samples in the presence of trypsin reveals an LPHbeta species that is slightly smaller than the intracellularly cleaved molecule. When the proLPH molecule is screened for potential cleavage sites, two dibasic pairs are revealed upstream of the N-terminal end of brush border LPH at Lys851-Arg852 and Arg830-Lys831. Treatment of proLPH with trypsin for different periods of time supports the idea of at least two cleavage steps, whereby Arg868-Ala869 represents the final cleavage site that generates LPHbeta. We propose that the initial cleavage of proLPH takes place intracellularly at a site further away from Arg868-Ala869, to generate LPHbeta initial; LPHbeta is subsequently cleaved extracellularly in the gut lumen, presumably by trypsin, at Arg868-Ala869 to mature brush border LPH (LPHbeta initial).

Fetal endoderm primarily holds the temporal and positional information required for mammalian intestinal development

In rodents, the intestinal tract progressively acquires a functional regionalization during postnatal development. Using lactase-phlorizin hydrolase as a marker, we have analyzed in a xenograft model the ontogenic potencies of fetal rat intestinal segments taken prior to endoderm cytodifferentiation. Segments from the presumptive proximal jejunum and distal ileum grafted in nude mice developed correct spatial and temporal patterns of lactase protein and mRNA expression, which reproduced the normal pre- and post-weaning conditions. Segments from the fetal colon showed a faint lactase immunostaining 8-10 d after transplantation in chick embryos but not in mice; it is consistent with the transient expression of this enzyme in the colon of rat neonates. Heterotopic cross-associations comprising endoderm and mesenchyme from the presumptive proximal jejunum and distal ileum developed as xenografts in nude mice, and they exhibited lactase mRNA and protein expression patterns that were typical of the origin of the endodermal moiety. Endoderm from the distal ileum also expressed a normal lactase pattern when it was associated to fetal skin fibroblasts, while the fibroblasts differentiated into muscle layers containing alpha-smooth-muscle actin. Noteworthy, associations comprising colon endoderm and small intestinal mesenchyme showed a typical small intestinal morphology and expressed the digestive enzyme sucrase-isomaltase normally absent in the colon. However, in heterologous associations comprising lung or stomach endoderm and small intestinal mesenchyme, the epithelial compartment expressed markers in accordance to their tissue of origin but neither intestinal lactase nor sucrase-isomaltase. A thick intestinal muscle coat in which cells expressed alpha-smooth-muscle actin surrounded the grafts. The results demonstrate that: (a) the temporal and positional information needed for intestinal ontogeny up to the post-weaning stage results from an intrinsic program that is fixed in mammalian fetuses prior to endoderm cytodifferentiation; (b) this temporal and positional information is primarily carried by the endodermal moiety which is also able to change the fate of heterologous mesodermal cells to form intestinal mesenchyme; and (c) the small intestinal mesenchyme in turn may deliver instructive information as shown in association with colonic endoderm; yet this effect is not obvious with nonintestinal endoderms.

Transport, function, and sorting of lactase-phlorizin hydrolase in Madin-Darby canine kidney cells

Lactase-phlorizin hydrolase (LPH), a small intestinal brush-border glycoprotein, is synthesized as a single chain precursor (pro-LPH, M(r) = 215,000-230,000) that undergoes cleavage to the final mature form (LPHm, M(r) = 160,000 in the human). In the human and pig small intestine as well as in the colon carcinoma cell line Caco-2, this cleavage takes place intracellularly prior to insertion into the brush-border membrane. To assess the role of proteolytic cleavage on the transport, function, and sorting of LPH a stable Madin-Darby canine kidney cell line was generated which expresses LPH (denoted as MDCK-ML). Biosynthetic labeling experiments demonstrated that the transport kinetics and posttranslational processing pattern of LPH in this cell line are similar to those in intestinal cells. Moreover, the enzymatic activity was found to be indistinguishable from that of brush-border LPH (LPHm). The sorting of LPH was studied by biosynthetic labeling of cells grown on filters followed by cell surface immunoprecipitation. Here, we could demonstrate that the cleaved LPHm molecule was predominantly found at the apical membrane, whereas complex glycosylated uncleaved pro-LPH (pro-LPHc) was targeted to both domains, the apical as well as the basolateral. In pulse-chase experiments at 20 degrees C pro-LPH was arrested in the trans-Golgi network, and cleavage to LPHm did not take place. By contrast, when the chase temperature was raised to 37 degrees C transport of pro-LPHc resumed, and cleavage to LPHm occurred. We conclude that the proteolytic cleavage of pro-LPHc to LPHm is a post-trans-Golgi network event and is most likely not implicated in the sorting of LPH by exposition of otherwise masked sorting elements.

Impact of O-glycosylation on the function of human intestinal lactase-phlorizin hydrolase. Characterization of glycoforms varying in enzyme activity and localization of O-glycoside addition

Lactase-phlorizin hydrolase (LPH) is an integral intestinal brush border membrane glycoprotein responsible for the hydrolysis of lactose, the primary carbohydrate in mammalian milk. To assess the role of N- and O-glycosylation on the function of LPH, lectin-binding experiments combined with enzymatic and chemical deglycosylation of purified LPH molecules were performed. These investigations provided evidence for the existence of two forms of brush border LPH, an N-glycosylated molecule (LPHN) and an N- and O-glycosylated molecule (LPHN/O). These two forms could be discriminated on the basis of (i) their binding capacity to Helix pomatia lectin, which has high specificity toward O-linked oligosaccharides, and (ii) their deglycosylation patterns with endo-beta-N-acetylglucosaminidase F/GF, O-glycanase, and trifluoromethanesulfonic acid. Interestingly, both forms have identical Km values (approximately 14 mM) when assayed with lactose, but hydrolyze this substrate at different rates. Thus, the N- and O-glycosylated form exhibits almost a 4-fold higher Vmax than that of the N-glycosylated enzyme (3.28 nM/min versus 0.90 nM/min) and is therefore enzymatically more active than the latter. Sequential affinity chromatography of glycopeptides derived from [3H]mannose-labeled LPHN and LPHN/O on lectin columns revealed similar patterns of N-linked glycosylation of both forms indicating that the presence of O-linked oligosaccharides did not affect or alter the processing of N-linked oligosaccharides. O-Linked glycosylation of LPH appears to occur in the Golgi apparatus, since the earliest detectable forms of LPH, the mannose-rich precursor (pro-LPH) is not O-glycosylated. In view of the fact that differentiation of intestinal crypt cells to mature epithelial cells is accompanied by significant phenotypical, morphological, and structural alterations, including changes in the levels of several Golgi glycosyl, -sialyl, galactosyl-, and N-acetylgalactosaminyltransferases, and since O-glycosylation is a Golgi event, we suggest that the generation of LPHN and LPHN/O is strongly linked to differentiation of intestinal cells. Finally, the variations in the enzymatic activity of the two forms propose a role for O-glycosylation in posttranslational regulation of LPH activity.

Mechanism of maturational decline of rat intestinal lactase-phlorizin hydrolase

The maturational decline in lactase-phlorizin hydrolase (LPH) activity was studied in groups of young rats ranging from suckling to early post-weaned states. Associated maturational increases in sucrase-isomaltase (SI) and maltase-glucoamylase (MG) activities were also examined as a comparison. Over this time period changes in cellular concentrations of the three enzymes were observed, reflecting corresponding changes in enzyme activities. Synthesis patterns accompanying these maturational changes in concentration were examined using labelled leucine as a marker. Synthesis of LPH was found to be maintained at constant rates independent of the maturation-associated decline in its concentration, whereas the increases in cellular concentrations of SI and MG were due to accelerated synthesis of the enzyme. Turnover of LPH, based on both the fractional synthesis rate and the disappearance rate of labelled leucine from prelabelled LPH pools, was increased in a quantitatively similar way to the decline in LPH concentration. These findings are consistent with our earlier proposal that the maturational decline of LPH occurs because of accelerated turnover, without a decrease in its rate of synthesis.

Glycosylation of lactase-phlorizin hydrolase in rat small intestine during development

Age-specific changes in glycosylation of rat intestinal lactase-phlorizin hydrolase were analyzed using enzyme immunoprecipitated from microvillus membranes of suckling, weaning, and adult rats, and carbohydrate moieties were examined by lectin affinity binding, metabolic labeling, and neuraminidase treatment. Lectin binding indicated the presence of N-linked and O-linked oligosaccharide chains containing mannose and galactose throughout development. An age-dependent shift in sialic acid and fucose was seen during the period of weaning; no fucose was detectable in lactase-phlorizin hydrolase until after the rats were 20 days of age, whereas sialic acid was reduced in adult lactase-phlorizin hydrolase. The presence of sialic acid in suckling intestines and fucose in adult was confirmed by metabolic labeling with appropriate radioactive precursors. Sodium dodecyl phosphate-polyacrylamide gel electrophoresis analysis of immunoprecipitated lactase-phlorizin hydrolase from the proximal and mid small intestine showed two bands of approximately 220 and 130 kilodaltons in all age groups. In the distal part of the adult small intestine, lactase-phlorizin hydrolase appeared as two bands of similar size to those found in the proximal and mid portions. In contrast, during the suckling and weaning periods, these distal bands were approximately 225 and 135 kilodaltons. [35S]-methionine labeling and fluorography of neonatal intestines confirmed these observations. The size difference between proximal and distal small intestines was virtually eliminated by neuraminidase treatment. These data indicate that the core structure of microvillus membrane lactase-phlorizin hydrolase, consisting of both N-linked and O-linked oligosaccharides, remains constant during development, although terminal sugars shift from predominantly sialic acid during the suckling period to fucose in adulthood. This alteration in glycosylation of the protein occurs in a different pattern from the postweaning decline in lactase specific activity. Consequently, age-dependent changes in glycosylation cannot account for the decrease in lactase-phlorizin hydrolase-specific activity observed during development.

Human intestinal lactase-phlorizin hydrolase: isolation and preparation of a specific antiserum

Human intestinal lactase-phlorizin hydrolase (lactase) was selectively isolated with monospecific polyclonal antibodies to rat lactase. In addition to their immunologic similarities indicated by this isolation, human and rat lactase have similar kinetic characteristics but different subunit structure when analyzed by gel electrophoresis under reducing conditions. Rabbits immunized by injecting human lactase complexed with anti-rat lactase produced specific antibodies to human lactase that exhibited little cross-reactivity to the rat enzyme. The simple single-step procedure allows isolation of human lactase in high purity from small biologic samples and preparation of specific antisera to the human enzyme.

Evidence for biosynthesis of lactase-phlorizin hydrolase as a single-chain high-molecular weight precursor

Precursor forms of lactase-phlorizin hydrolase, sucrase-isomaltase and aminopeptidase N were studied by pulse-labelling of organ-cultured human intestinal biopsies. After labelling the biopsies were fractionated by the Ca2+-precipitation method and the enzymes isolated by immunoprecipitation. The results indicate that the lactase-phlorizin hydrolase is synthesized as a Mr 245 000 polypeptide, which is intracellularly cleaved into its mature Mr 160 000 form. Sucrase-isomaltase is shown to be synthesized as a single chain precursor (Mr 245 000 and 265 000) while the precursor of aminopeptidase N is shown to be of apparently the same size as the mature enzyme (Mr 140 000 and 160 000).

Further characterization of intestinal lactase/phlorizin hydrolase

Pig intestinal lactase/phlorizin hydrolase (EC 3.2.1.23/62) was purified in its amphiphilic form by immunoadsorbent chromatography. The purified enzyme was free of other known brush border enzymes and appeared homogeneous in immunoelectrophoresis and polyacrylamide gel electrophoresis in the presence of SDS. Pig lactase/phlorizin hydrolase was shown to have the same quaternary structure as the human enzyme, i.e., built up of two polypeptides of the same molecular weight (160000). In addition to hydrolyzing lactose, phlorizin and a number of synthetic substrates, both the human and the pig enzyme were shown to have a considerable activity against cellotriose and cellotetraose, and a low but significant activity against cellulose. The lactase/phlorizin hydrolase isolated from pigs in which the pancreatic ducts had been disconnected 3 days before death and from Ca2+-precipitated enterocyte membranes (basolateral and intracellular membranes) exhibited in SDS-polyacrylamide gel electrophoresis the same size of constituent polypeptides and the same catalytic and immunological properties as a normal brush border lactase/phlorizin hydrolase.

The glycosylceramidase in the murine intestine. Purification and substrate specificity

The intestinal glycosylceramidase of the mouse which we reported previously as a taurodeoxycholate-activated galactosylceramidase (Kobayashi, T., and Suzuki, K. (1981) J. Biol. Chem. 256, 1133-1137) has been purified to homogeneity. The enzyme gave a single band of a molecular weight of 130,000 in the sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight estimated by Sepharose 4B or Sephadex G-200 gel filtration under nondenaturing conditions was 290,000 to 300,000. In the double immunodiffusion test, rabbit antiserum raised against the purified enzyme gave a single precipitin band against the enzyme, but no cross-reactivity was observed against the brain or kidney galactosylceramidase (EC 3.2.1.46). The purified enzyme was active toward 4-methylumbelliferyl beta-D-galactoside, beta-D-glucoside, beta-D-xyloside, beta-D-fucoside, and alpha-L-arabinoside. Among potential glycolipid substrates, the enzyme was active, in the presence of sodium taurodeoxycholate, toward galactosylceramide, glucosylceramide, lactosylceramide, galactosylsphingosine, and glucosylsphingosine. It was inactive toward GM1 ganglioside, asialo-GM1 ganglioside, desialylated fetuin, and desialylated transferrin. Among disaccharides, the enzyme showed the highest catalytic activity toward lactose (18.9 mumol/min/mg of protein) and the lowest toward galactose beta (1 leads to 4)-N-acetylglucosamine (0.06 mumol/min/mg of protein). Galactose beta (1 leads to 6)-N-acetylglucosamine was not hydrolyzed. Phlorizin was also a substrate for the enzyme.

Purification and characterisation of amphiphilic lactase/phlorizin hydrolase from human small intestine

Human intestinal lactase/phlorizin hydrolase (EC 3.2.1.23/62) was purified in its amphiphilic form by immunoadsorbent chromatography. The purification factor was approximately 600 and the recovery 14%. The enzyme was essentially free from other known brush-border peptidases and disaccharidases and appeared homogeneous in crossed immunoelectrophoresis and polyacrylamide gel electrophoresis in sodium dodecylsulphate. The purified enzyme hydrolyzed lactose (pH optimum 5.8--6.0, Km 21 mM), phlorizin (Km 0.44mM) and other beta-galactosides and beta-glucosides. Tris inhibited the hydrolysis of lactose whereas phlorizin hydrolysis was almost unaffected. The activity against these two substrates also showed different thermal stability. It is suggested that the human enzyme has two different sites: one for lactose hydrolysis, inhibited by phlorizin and one for phlorizin hydrolysis. By gel filtration on Ultrogel AcA 34 the amphiphilic form of the enzyme had a molecular weight of 320000 while the hydrophilic form (papain-treated) had a molecular weight of 280000. This indicates that the anchoring segment(s) plus the bound detergent has a molecular weight of approximately 40000. In polyacrylamide gel electrophoresis in sodium dodecylsulphate the fully denatured enzyme had an apparent molecular weight of 160000. It is therefore suggested that the human lactase/phlorizin hydrolase is composed of two monomers each with a molecular weight of 160000. The electromicroscopic picture gives further evidence for this suggestion. In addition the possibility of a high molecular weight, one polypeptide chain is discussed.

Isolation and characterization of the proximal and distal forms of lactase-phlorizin hydrolase from the small intestine of the suckling rat

The complex between lactase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) and phlorizin hydrolase (glycosyl-N-acylsphingosine glycohydrolase, EC 3.2.1.62) has been purified from the proximal and distal regions of the small intestine of suckling rats. The two enzymes behaved differently on DEAE-cellulose ion-exchange chromatography and during electrophoresis in the presence and absence of sodium dodecyl sulphate (SDS), but they have very similar cyanoge bromide cleavage patterns. Kinetic studies on the proximal and distal enzymes showed the same pH optimum of 6.0 and the same heat stability at 45 degrees C, but a small difference in Km. Treatment of both enzymes with fucosidase, mannosidase or N-acetylhexosaminidase did not affect enzymic activity or electrophoretic mobility. Neuraminidase digestion abolished the electrophoretic differences and gave two active enzymes with similar isoelectric points.