Total intestinal lactase and sucrase activities are reduced in aged rats

Lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) are intestinal microvillus membrane hydrolases that play important roles in carbohydrate digestion. Although the expression of these enzymes during postnatal development has been characterized, the effect of old age on disaccharidase activity is poorly understood. In the present investigation, we examined the effect of aging on lactase and sucrase activities and their mRNA levels in the small intestines of 3-, 12- and 24- mo-old rats by sampling from nine equidistant segments of small intestine. Total intestinal disaccharidase activity or mRNA abundance was determined from areas under the proximal-to-distal curves. Rats 24 mo of age had total intestinal lactase and sucrase activities that were 12 and 38% lower, respectively, than the 3-mo-old animals (P < 0.05). In contrast, total LPH and SI mRNA abundance did not change significantly. Thus, total intestinal lactase and sucrase activities decrease with age in a manner that likely involves a posttranscriptional process. The age-related decline in disaccharidase activity, if extrapolated to humans, may have important implications for the digestion of carbohydrate contained in the diet of the elderly.

Human lactase-phlorizin hydrolase expressed in COS-1 cells is proteolytically processed by the lysosomal pathway

Lactase-phlorizin hydrolase (LPH) (EC 3.2.1.23/62), a major glycoprotein of the microvillus membrane of human small intestinal epithelial cells (enterocytes), is vital for the digestion of lactose during early infancy. The enzyme is synthesized in enterocytes as a single-chain precursor and subsequently proteolytically processed to the mature microvillus membrane-bound form. Because it has been reported that COS-1 cells were not able to proteolytically process LPH to the mature protein, these cells have been used as a model system to study potential roles of different proteases. COS-1 cells transfected with a full-length cDNA for human LPH synthesized enzymatically active enzyme. Immunoprecipitation of the expressed glycoproteins and their subsequent analysis by SDS-PAGE showed synthesis of two polypeptide species having apparent molecular masses of 210 and 220 kDa, respectively, corresponding to the high-mannose (pro-LPHh) form and the complex glycosylated (pro-LPHc) form of the LPH precursor. Surprisingly, an additional polypeptide species corresponding in size to the mature LPH found in human intestinal cells was also detected after longer chase periods. The source of this species was clearly pro-LPH, as its formation was inhibited by Brefeldin A. The cleaved form of LPH was not found on the cell surface; furthermore, its formation was prevented by an inhibitor of lysosomal function. We conclude from these data that in transfected COS-1 cells pro-LPH is transported to the cell surface, from which it is internalised and enters the lysosomal pathway, where proteolytic cleavage leads to a molecule not unlike mature LPH.

The apical sorting of lactase-phlorizin hydrolase implicates sorting sequences found in the mature domain

Polarized transport of proteins is contingent on the presence of specific protein structures or motifs that function as sorting signals. Our model protein to analyze and to identify such signals is that of lactase-phlorizin hydrolase (LPH), a strictly polarized brush border membrane protein of small intestinal epithelial cells. It is synthesized as a large pro-LPH precursor molecule, which is proteolytically processed to yield the mature brush border enzyme (LPHbeta). Pro-LPH as well as LPHbeta are correctly sorted to the brush border membrane. In this paper we examine the location of putative sorting signals in the pro-LPH molecule. Expression of a cDNA encoding the LPHbeta mature form in the absence of the LPHalpha species in Madin-Darby canine kidney (MDCK) cells reveal an LPHbeta molecule that is not as transport-competent as wild type pro-LPH. The proportion of complex glycosylated LPHbeta constitutes not more than 10% of the total synthesized protein. This form displays a similar trypsin sensitive pattern as wild type intestinal LPHbeta suggesting comparable folding patterns of the two species. Complex glycosylated LPHbeta is sorted to the apical membrane more efficiently than wild type pro-LPH. We conclude that the apical sorting signals for pro-LPH are exclusively found in the LPHbeta mature domain.

Lactase phlorhizin hydrolase turnover in vivo in water-fed and colostrum-fed newborn pigs

We have estimated the synthesis rates in vivo of precursor and brush-border (BB) polypeptides of lactase phlorhizin hydrolase (LPH) in newborn pigs fed with water or colostrum for 24h post partum. At the end of the feeding period, piglets were anaesthetized and infused intravenously for 3h with L-[4-3H]- phenylalanine. Blood and jejunal samples were collected at timed intervals. The precursor and BB forms of LPH were isolated from jejunal mucosa by immunoprecipitation followed by SDS/PAGE, and their specific radioactivity in Phe determined. The kinetics of precursor and BB LPH labelling were analysed by using a linear compartmental model. Immunoisolated LPH protein consisted of five polypeptides [high-mannose LPH precursor (proLPHh), complex glycosylated LPH precursor (proLPHe), intermediate complex glycosylated LPH precursor (proLPH1i) and two forms of BB LPH]. The fractional synthesis rate (Ks) of proLPHh and proLPHc (approx. 5%/min) were the same in the two groups but the absolute synthesis rate (in arbitrary units, min-1) of proLPHh in the colostrum-fed animals was twice that of the water-fed animals. The Ks values of proLPHi polypeptides were significantly different (water-fed, 3.89%/min; colostrum-fed, 1.6%/min), but the absolute synthesis rates did not differ. The Ks of BB LPH was not different between experimental treatment groups (on average 0.037%/min). However, the proportion of newly synthesized proLPHh processed to BB LPH was 48% lower in colostrum-fed than in water-fed animals. We conclude that in neonatal pigs, the ingestion of colostrum stimulates the synthesis of proLPHh but, at least temporarily, disrupts the processing of proLPH polypeptides to the BB enzyme.

Proteolytic processing of human lactase-phlorizin hydrolase is a two-step event: identification of the cleavage sites

Human lactase-phlorizin hydrolase (EC 3.2.1.23/62) is a major disaccharidase in the microvillus membrane of small intestinal epithelial cells. The enzyme is synthesized as a single-chain precursor protein and undergoes proteolytic processing during maturation. We studied proteolytic processing of human lactase-phlorizin hydrolase in transfected COS-1, Caco-2, and MDCK cells using metabolic labeling, surface immunoprecipitation, protease sensitivity assays, and microsequencing. Furthermore, we generated mutated forms of the enzyme to alter potential proteolytic cleavage sites and expressed these in Caco-2 and COS-1 cells. Since the N-terminal amino acid of microvillus lactase-phlorizin hydrolase corresponds to Ala869 in the precursor protein, it has been speculated that processing occurs at position Arg868-Ala869. Substitution of Arg868 with isoleucine, lysine, or glutamic acid had no effect on the proteolytic processing of pro-LPH in Caco-2 cells. As in wild-type enzyme a processed 160-kDa form was generated. These data are not consistent with a primary proteolytic processing at position Arg868-Ala869. Using amino-terminal amino acid sequencing of this processed form isolated from stable transfected MDCK cells we identified the cleavage site at Arg734-Leu735. Treatment of pro-lactase-phlorizin hydrolase expressed in COS-1 and MDCK cells by trypsin yielded a 145-kDa form with an identical amino terminal as the mature microvillus enzyme isolated from intestinal mucosa (Ala869). These data provide unambiguous evidence of a two-step processing of human lactase-phlorizin hydrolase. The first cleavage occurs intracellularly after a dibasic site (Arg734-Leu735) and yields the 160-kDa intermediate form. In a second step the intermediate form inserted into the microvillus membrane is trimmed to the mature enzyme by luminal trypsin.

Regulation of sucrase and lactase in Caco-2 cells: relationship to nuclear factors SIF-1 and NF-LPH-1

Recent studies suggest the importance of two transcription factors, Cdx-2 and NF-LPH-1, in the regulation of sucrase-isomaltase (SI) and lactase-phlorizin hydrolase (LPH) gene expression, respectively. Cdx-2 accounts for the tissue specificity of sucrase expression (16), and NF-LPH-1 varies with postnatal changes in lactase activity, suggesting a role in its developmental regulation (22). We used electrophoretic mobility shift assays to study the relationship of Cdx-2 and NF-LPH-1 to SI and LPH gene expression in Caco-2 cells to provide evidence regarding the role of these factors in the development of sucrase and lactase with cellular differentiation. We found that Cdx-2 levels correlated with SI expression and that NF-LPH-1 did not correlate with LPH expression. These studies suggest a role for Cdx-2 but not for NF-LPH-1 in the development of carbohydrase expression in these cells.

Dimerization of lactase-phlorizin hydrolase occurs in the endoplasmic reticulum, involves the putative membrane spanning domain and is required for an efficient transport of the enzyme to the cell surface

Analysis of the quaternary structure of human intestinal lactase-phlorizin hydrolase (LPH) by chemical cross-linking and sucrose-gradient centrifugation reveals that the brush border form of LPH (LPH beta; 160-kDa) is a homodimeric molecule. Dimerization ensures in the ER when LPH is still exclusively found as an uncleaved mannoserich precursor (pro-LPHb; 215-kDa). This is supported by the following observations. (i) Biosynthetically labeled intestinal biopsy specimens as well as transfected COS-1 cells expressing pro-LPH contain monomeric and dimeric forms of pro-LPHb; the complex glycosylated pro-LPH (pro-LPHc; 230-kDa) as well as the cleaved mature LPH beta species in intestinal biopsy samples are discerned exclusively as dimers. (ii) Dimeric forms of pro-LPHh could be also detected when cells were biosynthetically labeled at 15 degrees C, at which temperature the egress of pro-LPH from the ER is blocked. Dimerization is essential for the transport competence of pro-LPH and is strongly associated with the presence of an intact transmembrane domain. Mutant pro-LPH-mact lacking the complete transmembrane domain persists as a monomeric, mannose-rich and transport-incompetent molecule that is not secreted into the exterior milieu, accumulates most likely in the ER and is ultimately degraded. Further, deletion of the cytoplasmic tail in the pro-LPH-ct mutant leads to marked reduction in the proportion of dimeric as well as complex glycosylated pro-LPH-ct. Finally, dimerization is linked to the acquisition of LPH to its biological function, since only dimers of wild type pro-LPH or pro-LPH-ct are enzymatically active, while their monomeric counterparts as well as pro-LPH-mact are not.

Human lactase-phlorizin hydrolase is not processed by furin, PC1/PC3, PC2, PACE4 and PC5/PC6A of the family of subtilisin-like proprotein processing proteases

Human lactase-phlorizin hydrolase (LPH, EC 3.2.1.23/62) is synthesized as a single-chain precursor glycoprotein (pro-LPH) with a relative molecular mass of just over 200 kDa. Maturation to the mature enzyme (m-LPH, 160 kDa) occurs after passage of pro-LPH through the Golgi complex and involves the proteolytic removal of a 849 amino acid propeptide. The role of this propeptide as well as its removal is not fully understood and the proteolytic enzyme or enzymes involved are unknown. We studied the potential role of five different members of the family of subtilisin-like proprotein processing proteases in the maturation process of human LPH using a vaccinia virus based coexpression system in pig kidney PK(15) cells. Infected/transfected PK(15) cells expressed full-length pro-LPH but no maturation to m-LPH was observed. Coexpression of human pro-LPH with human furin, human PC1/PC3, human PC2, human PACE4 and mouse PC6A in PK(15) cells did not result in maturation of the enzyme. Cleavage and secretion of von Willebrand factor precursor (pro-vWF) was used as a positive control. None of the five proprotein processing proteases tested were capable of cleaving human pro-LPH, strongly suggesting that they are not involved in the maturation of this enzyme.

Quantitative analysis of lactase-phlorizin hydrolase expression in the absorptive enterocytes of newborn rat small intestine

At birth, the mammalian small intestine displays regional differences in morphology as well as complex proximal-to-distal (horizontal) patterns of protein distribution. Lactase-phlorizin hydrolase (LPH), an enterocyte-specific disaccharidase crucial for the digestion of lactose in milk, reveals a characteristic horizontal pattern of expression at birth. However, it is not certain whether this topographic pattern is due to variations in epithelial structure along the length of the small intestine or to regional differences in the transcription of the LPH gene. In order to understand the mechanisms that regulate the regionalization of LPH at birth, we characterized the epithelial structure along the horizontal axis using stereologic techniques and correlated these data with the patterns of lactase activity and LPH mRNA abundance in the small intestine of unsuckled, newborn rats. Epithelial volume and microvillar surface area per unit of intestinal length decreased three-to fourfold from duodenum to distal ileum. In contrast, lactase activity and LPH mRNA abundance were highest in proximal jejunum and lowest in the most proximal and distal ends of the small intestine. Mean lactase activity per cell in proximal duodenum, proximal jejunum, and distal ileum was estimated at 12.0, 26.7, and 5.6 nU/absorptive enterocyte, respectively, and paralleled the concentration of LPH mRNA in the same segments: 20, 45, and 15 molecules of LPH mRNA/absorptive enterocyte. Our data indicate that horizontal gradients of lactase activity in the newborn rat intestine do not depend on epithelial organization or on enteral factors, since the horizontal gradient is established before suckling. Each absorptive enterocyte along the small intestine expresses lactase activity in a position-dependent manner which is controlled at the level of mRNA abundance.

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).

Furin, PC1/3, and/or PC6A process rabbit, but not human, pro-lactase-phlorizin hydrolase to the 180-kDa intermediate

Small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a large precursor (prepro-LPH) of 1926 amino acids. In the endoplasmic reticulum, prepro-LPH is split by signal protease. The resulting pro-LPH is cut to mature LPH directly (human) or via a 180-kDa intermediate (rabbit), most likely in the trans-Golgi network or in a later compartment. Antibodies directed against different regions of rabbit pro-LPH locate the cleavage site resulting in the 180-kDa intermediate between amino acid residues 79 and 286. This stretch contains the two sequences -Arg-Cys-Tyr-Arg114 approximately -Arg-Ala-Ser-Arg191 approximately, which are potential cleavage sites for subtilisin-like proprotein convertases. These sites are not conserved in human pro-LPH. By coexpression in COS 7 cells of rabbit prepro-LPH and proprotein convertases (PC 1/3, PC2, PC6A, PC6B, furin), we show that furin, PC 1/3, and PC6A generate a processing intermediate that is immunologically indistinguishable from the one observed in vivo. Furin, PC 1/3, and PC6A are all expressed in the small intestine as shown by a polymerase chain reaction-based approach and, more specifically, in enterocytes, as shown by in situ hybridization. These results suggest that furin, PC 1/3, and/or PC6A are responsible for the in vivo processing of rabbit pro-LPH to the 180-kDa intermediate.

Verification of the lactase site of rat lactase-phlorizin hydrolase by site-directed mutagenesis

BACKGROUND & AIMS

Lactase-phlorizin hydrolase (LPH) is an intestinal microvillus membrane glycoprotein that hydrolyzes lactose and phlorizin. These enzymatic activities have been assigned to glutamic acid (E) residues 1271 and 1747 in rabbit LPH. The aim of this study was to determine directly if this assignment was correct and if these two amino acids are the only nucleophiles required for LPH enzyme activity.

METHODS

Site-directed mutagenesis of a full-length rat LPH complementary DNA was used to convert the rat homologues E1274 and E1750 to aspartic acid or glycine. Mutants were analyzed by enzyme activity assays.

RESULTS

All tested activities of E1274D and E1274G were virtually unaffected. In contrast, mutations E1750D and E1750G resulted in total loss of lactase and cellobiose activities, leaving only low ONP-glc and ONP-gal hydrolase activities detectable. A double mutant containing both E1274G and E1750G had no activity.

CONCLUSIONS

These studies directly confirm that the two previously identified glutamic acids are essential to the enzymatic activity of rat LPH. Rat lactase activity is not associated with the E1274 site. This study provides the first evidence that rat LPH has its major catalytic site at E1750, representing all of the lactase and the majority of the phlorizin hydrolase activity.

Folding of human intestinal lactase-phlorizin hydrolase

The folding of human intestinal prolactase-phlorizin hydrolase (pro-LPH) has been analyzed in a cell-free transcription/translation system. In the presence of the thiol oxidant GSSG, disulfide bond formation in pro-LPH can be promoted concomitant with the binding of the molecule to a conformation-specific monoclonal anti-LPH antibody. Under these conditions, pro-LPH does not bind to the molecular chaperone BiP. In the absence of GSSG, on the other hand, pro-LPH does not bind to the monoclonal anti-LPH antibody, but can be immunoprecipitated with a polyclonal antibody that is directed against a denatured form of the enzyme. In this case, interaction of pro-LPH with immunoglobulin heavy chain binding protein can be discerned. The results demonstrate the existence of intramolecular disulfide bonds that are essential for the promotion of pro-LPH to a native conformation. Furthermore, BiP is involved in the folding events of pro-LPH.

Disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase elucidated by phosphorylation with protein kinase A in vitro and in tissue culture

The intracellular disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase (LPH) is demonstrated, using a specific phosphorylation of Ser1916 by protein kinase A (PKA). This phosphorylation is shown to occur not only in vitro (with pure LPH and pure catalytic subunit of PKA), but also in an organ culture of the small intestine. Cholera toxin, which is known to act in vivo on the membranes of the small intestine, with severe clinical consequences, and to elevate the intracellular cyclic AMP of enterocytes, is shown to enhance significantly the phosphorylation of LPH in intact cells grown as an organ culture. These findings establish the cytosolic orientation of the carboxy-terminus tail of LPH in situ, and raise the possibility that the tail itself and its phosphorylation by PKA may have a physiological or physiopathological significance.

Substrate specificity of small-intestinal lactase: study of the steric effects and hydrogen bonds involved in enzyme-substrate interaction

Milk lactose is hydrolysed to D-galactose and D-glucose in the small intestine of mammals by the lactase-phlorizin hydrolase complex (LPH, EC 3.2.1.23-62). Lactase activity has broad substrate selectivity and several glycosides are substrates. Recently, using the monodeoxy derivatives of methyl beta-lactoside (1), we have shown the importance of each hydroxyl group in the substrate molecule concerning the interaction with the enzyme. Now we have studied the corresponding O-methyl derivatives, as well as some of the halo derivatives of 1. We have found that the enzyme presents steric restrictions to the recognition of substrates modified in the galactose moiety. In contrast, the binding site for the aglycon part of the substrate is looser. On the other hand, we have previously shown that HO-3' and HO-6 were important for the recognition of the substrate by the enzyme. Now we have found that the corresponding fluorine derivatives are not, or very poorly, recognized. This suggests that the HO-3' and HO-6 participate, as donors, in hydrogen bonds in the interaction with the enzyme.

The pro-region of human intestinal lactase-phlorizin hydrolase is enzymatically inactive towards lactose

The pro-region of intestinal lactase-phlorizin hydrolase (LPH alpha) has been proposed to be important for the correct folding of pro-LPH and mature LPH (LPH beta). In this communication, analysis of the catalytic function of the LPH alpha pro-region is presented. Expression of a cDNA encoding LPH alpha in COS-1 cells reveals a polypeptide that does not hydrolyse lactose. Likewise, no lactase activity is detected in LPH alpha purified from trypsin-treated pro-LPH. Mixing of LPH alpha and LPH beta does not lead to the activation of the latter. We conclude that LPH alpha does not contribute to the lactase activity despite the strong homologies with mature LPH beta. LPH alpha may play an important role as an intra-molecular chaperone.

Do pancreatic proteases play a role in processing prolactase and/or in the postweaning decline of lactase?

To assess the role of pancreatic proteases in the proteolytic processing and in the postweaning decline of lactase-phlorizin hydrolase (LPH), we have determined lactase activity and the different LPH forms in postweaned rats in which a jejunal loop was excluded from contact with pancreatic secretions by a jejunal bypass procedure. As a control for the absence of pancreatic proteases, pro-sucrase-isomaltase (proSI), which is known to be split by pancreatic proteases into heterodimeric SI, was used. Nearly all proLPH was processed to mature LPH, indistinguishable from LPH isolated from control animals. SI was found only in the unsplit pro form, whereas it was normally processed to the heterodimeric SI in the control tissues. There were no significant differences in lactase and sucrase activities in operated and in sham-operated control animals. We conclude that pancreatic secretions are not essential for the processing of proLPH to LPH or in the postweaning decline of LPH.

The pro region of human intestinal lactase-phlorizin hydrolase

Human small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a single-chain polypeptide precursor, prepro-LPH, that undergoes two sequential cleavage steps: the first in the endoplasmic reticulum to pro-LPH (215-kDa) and the second, following terminal glycosylation in the Golgi apparatus, to mature 160-kDa LPH (denoted LPH beta). The LPH beta molecule is subsequently targetted to the brush-border membrane. Characterization of the N-terminal profragment (denoted LPH alpha) of pro-LPH using an epitope-specific, anti-peptide polyclonal antibody reveals that LPH alpha (i) has an apparent molecular weight of approximately 100,000, (ii) is not associated with LPH beta after cleavage of pro-LPH has occurred, and (iii) is not transported to the cell surface or secreted into the extracellular medium. In biosynthetic labeling experiments, a clear precursor/product relationship could be demonstrated between pro-LPH and the LPH alpha and LPH beta polypeptides. Further, LPH alpha has a significantly shorter half-life than LPH beta. LPH alpha is neither N- nor O-glycosylated, despite the presence of 5 potential N-glycosylation sites. LPH alpha, which is rich in cysteine and hydrophobic amino acid residues, may fold rapidly into a tight and rigid globular domain in which carbohydrate attachment sites are no longer accessible to glycosyltransferases. When expressed independently in COS-1 cells, the LPH beta polypeptide forms a misfolded, transport-incompetent molecule. We propose a role for the LPH alpha domain within the pro-LPH molecule as an intramolecular chaperone during folding in the ER.

Jejunal brush border hydrolase activity is higher in tallow-fed pigs than in corn oil-fed pigs

We tested the effect of dietary fat on the lipid composition and hydrolase activity of jejunal brush border membranes in piglets. Eighteen 5-wk-old piglets were divided into three groups and for 4 wk fed either an unsaturated low fat diet (3.2% corn oil), an unsaturated high fat diet (17.2% corn oil) or a saturated high fat diet (2.2% corn oil + 15% tallow). Brush border membranes were prepared from the jejunal mucosa and analyzed for cholesterol, phospholipid and fatty acids. The activities of sucrase-isomaltase, lactase-phlorizin hydrolase, maltase-glucoamylase, aminopeptidase and alkaline phosphatase were measured. Lactase-phlorizin hydrolase isoforms were immunopurified and separated by SDS-PAGE, and their relative proportions were measured by densitometry. The activities of the disaccharidases and alkaline phosphatase, but not aminopeptidase, were greater in animals fed the saturated high fat diet than in animals fed the unsaturated high fat diet. The fatty acid composition of the membranes generally reflected the composition of the diet. Correlation analysis demonstrated that the phospholipid, fatty acid and cholesterol compositions of the membranes were associated with the differences in brush border hydrolase activity.

Processing and transport of human small intestinal lactase-phlorizin hydrolase (LPH). Role of N-linked oligosaccharide modification

The effect of glycosylation on the intracellular transport of human intestinal lactase-phlorizin hydrolase (LPH) was investigated by biosynthetic labeling of biopsy samples in the presence or absence of glycosidase inhibitors. In the presence of deoxynojirimycin (dNM) and deoxymannojirimycin (dMM), endo H sensitive LPH glycoforms of M(r) = 135,000 in both cases were produced (LPHdNM and LPHdMM). The LPH glycoform generated in the presence of swainsonine had an apparent molecular mass of 141,000 (LPHSwa) and was partially sensitive to endo H. By contrast to unmodified mature LPH (LPHm, M(r) = 160,000), these glycoforms are either not O-glycosylated (LPHdNM and LPHdMM) or partially O-glycosylated (LPHSwa) indicating that processing of N-linked carbohydrates has direct effects on the O-glycosylation of pro-LPH. Analysis of transport kinetics of the various glycoforms strongly suggested that carbohydrate modification does not affect the transport of pro-LPH from the cis-Golgi to the cell surface, but could be rate limiting at the level of the ER.

Differential effects of lipid and carbohydrate on enterocyte lactase activity in newborn piglets

The influence of enteral feeding in the neonate on lactase-phlorizin hydrolase activity in the small intestine has been determined in newborn piglets fed a series of modified colostra, with a controlled metabolizable energy intake, during the first 31.5 h of life. Striking differences were observed between lactase specific activity in mucosal homogenates and enterocyte lactase activity along the villus axis; compared with newborns, the former decreased after feeding colostrum, whereas the latter increased significantly. When lipid was present in adequate amount, the increase in enterocyte lactase activity occurred when carbohydrate was present as either lactose or galactose. However, when the lipid content of the diet was low, there was a specific effect of carbohydrate composition which was dependent on position along the villus axis: in the lower villus, colostra high in lactose or glucose stimulated an increase in lactase, but there was no such effect with a high galactose intake. It is concluded that colostrum increases enterocyte lactase activity during the first day of life, and that this is dependent on both the nutrient composition of the diet and the position of the enterocytes along the villus.

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.

Analysis of lactase processing in rabbit

The proteolytic processing of rabbit intestinal lactase-phlorizin-hydrolase (LPH) was studied by pulse-chase and continuous labeling experiments in organ culture from 15-day-old rabbits in the presence of glycosylation and processing inhibitors. Monensin and brefeldin A inhibited the two proteolytic cleavages of the precursor indicating that they are post-Golgi events as previously reported for the unique cleavage of LPH in man. The inhibition was not related to a concomitant alteration glycosylation; in fact, if trimming was blocked by MDNM the abnormal glycosylated precursor was proteolytically processed normally. Finally the use of the anti-microtubular drug colchicine strongly inhibited both cleavages and caused accumulation of the complex-glycosylated precursor form the brush border fraction indicating that proteolytic events depend on intact microtubule (transport).