Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.2.1.108 (lactase)
 2,133 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The distribution of the mRNA encoding rat intestinal lactase-phlorizin hydrolase (LPH) was analyzed by ultrastructural in situ hybridization, and compared to the distribution of the cognate protein. Different pictures were obtained depending on the epithelial cell position along the crypt-villus axis: i) cells localized at the crypt base were devoid of LPH mRNA and protein; ii) cells of the crypt-villus junction and of the villus base showed a high level of mRNA but a low amount of enzyme; and iii) enterocytes of the middle and upper part of the villi exhibited an intense protein immunolabeling, but a low content of LPH mRNA. The analysis of the intracellular distribution of the LPH mRNA revealed a gradient of concentration along the cellular axis, as the transcripts were the most abundant in the apical and subapical domains of the enterocytes. In addition, LPH mRNA labeling was often paired or arranged in a circular shape, suggesting that the transcripts may be located in closed vicinity to rounded structures within the cytoplasm. The results indicate that distinct patterns of mRNA and protein occur along the villus height during the continuous process of cell differentiation of the intestinal epithelium. In addition, the emergence of the functional polarity of the enterocytes comes together with the establishment of an intracellular gradient of concentration of the mRNA encoding a brush border enzyme. A link between the intracellular distribution of LPH mRNA molecules and the membranous flow is proposed.
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PMID:Ultrastructural study of intestinal lactase gene expression. 754 16

Enterocytes are the major epithelial cell type of the small intestine. Their capacity to secret, absorb and digest specific ions and nutrients is dependent on their position along the length of the small intestine as well as their stage of development as they migrate and differentiate along the crypt-villus axis. In order to further understand the molecular processes that regulate enterocyte differentiation and function, this study has compared the levels of six mRNA species produced by genes expressed in rabbit enterocytes; specifically, the multidrug resistance (MDR1) gene encoding the 170-kDa P-glycoprotein, CaBP 9k, which encodes a putative intracellular calcium buffer, calbindin, LPH, APN, and AP which encode the brush-border hydrolases lactase-phlorizin hydrolase, aminopeptidase N and alkaline phosphatase, respectively, and SGLT1, encoding the brush border Na(+)-glucose cotransporter. The level of each mRNA species has been mapped along the small intestine using quantitative in situ hybridisation. This has revealed characteristic regional variations in the abundance of each of the mRNAs, supporting the opinion that there is a strong genetic component to the maintenance of gradients in epithelial function along the length of the small intestine. Analysis of the cellular accumulation of mRNA during enterocyte migration along the crypt-villus axis, over gut-associated lymphoid tissue, and at epithelial boundaries, has, by contrast, established a clear correlation in the expression of these genes. These data illustrate the dynamics of enterocyte gene expression, thereby providing an insight into the molecular mechanisms which co-ordinate the events of cell transformation that underlie functional differences between the epithelial populations of the small intestine.
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PMID:Parallel patterns of cell-specific gene expression during enterocyte differentiation and maturation in the small intestine of the rabbit. 758 2

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.
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PMID:Furin, PC1/3, and/or PC6A process rabbit, but not human, pro-lactase-phlorizin hydrolase to the 180-kDa intermediate. 759 52

To explore the mechanisms by which jejunal lactase activity is modified by carbohydrate and/or fat intake, mRNA levels and the absolute synthesis rate of lactase-phlorizin hydrolase (LPH) were determined in 6-wk-old rats that were fed either low-starch diets containing long-chain triacylglycerol (LCT, 73% energy as corn oil) or medium-chain triacylglycerol (MCT, 66% energy as MCT, 7% energy as corn oil), or a high-starch diet (70% energy as cornstarch) for 7 days. LPH mRNA levels in the jejunum were similar between LCT-fed and MCT-fed rats, but animals fed the high-starch diet exhibited a greater (2x) LPH mRNA level than other groups. The absolute synthesis rate of LPH, estimated by the flooding dose technique using [3H]phenylalanine, was greater (2.4x) in rats fed the high-starch diet than in other groups. A short-term force-feeding experiment revealed that sucrose was able to evoke LPH mRNA levels within 12 h but that a nonmetabolizable sugar (alpha-methylglucoside) was unable to enhance it. By contrast, animals fed the high-LCT diet showed a lower (by 30%) lactase activity than rats fed the low-starch, high-MCT diet, which was accompanied by not only a reduction of immunoreactive LPH in brush-border membranes but also a reduction in lactase activity per unit weight of immunoreactive LPH. These results suggest that both gene expression and posttranslational events of LPH might be influenced by dietary manipulations; carbohydrate intake primarily increases LPH mRNA levels, and LCT accelerates inactivation and/or degradation of lactase.
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PMID:Diet-induced changes in gene expression of lactase in rat jejunum. 761 7

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.
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PMID:The pro-region of human intestinal lactase-phlorizin hydrolase is enzymatically inactive towards lactose. 762 35

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.
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PMID:Disposition of the carboxy-terminus tail of rabbit lactase-phlorizin hydrolase elucidated by phosphorylation with protein kinase A in vitro and in tissue culture. 763 22

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.
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PMID:Substrate specificity of small-intestinal lactase: study of the steric effects and hydrogen bonds involved in enzyme-substrate interaction. 764 81

The development of brush-border enzymes and the possible regulatory role of cortisol were investigated in the small intestine of the fetal and neonatal pig. With the sows under pentobarbitone anesthesia, osmotic minipumps containing either saline or cortisol were inserted s.c. into 25 fetuses from 10 pregnant sows (82-96 d gestation). Six d later, the infused fetuses were removed by cesarean section and samples of the proximal, middle, and distal intestine taken for analysis. Samples were also obtained from 48 piglets that did not undergo an operation (controls) and that were removed at intervals from 82 d gestation until term (114 +/- 2 d). In the proximal and middle intestine, the mean levels of lactase-phlorizin hydrolase (EC 3.2.1.23-62), maltaseglucoamylase (EC 3.2.1.20), aminopeptidase N (EC 3.4.11.2), and aminopeptidase A (EC 3.4.11.7) increased during the last 10-15 d before term, correlated positively with log10 plasma cortisol values, and were higher in cortisol-infused than in saline-infused fetuses (p < 0.05). Activity of sucrase-isomaltase (EC 3.2.1.48-10) was low in fetal pigs, and this enzyme and dipeptidyl peptidase IV (EC 3.4.14.5) were not significantly affected by fetal age or exogenous cortisol. Maltase (EC 3.2.1.48-10 and EC 3.2.1.20) activity was significantly decreased in the middle and distal intestine of cortisol-infused fetuses. The results suggest that the prepartum rise in endogenous cortisol secretion stimulates the prenatal expression of certain brush-border enzymes in the pig small intestine at this critical time. However, the effects of cortisol on the developing intestine were highly idiosyncratic for particular enzymes and intestinal regions.
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PMID:The prenatal development and glucocorticoid control of brush-border hydrolases in the pig small intestine. 773 59

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.
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PMID:Do pancreatic proteases play a role in processing prolactase and/or in the postweaning decline of lactase? 784 Feb 6

A number of transmembrane digestive enzymes of the porcine small intestinal brush border membrane were found to be partially Triton X-100-insoluble at 0 degree C and colocalized in gradient centrifugation experiments with the GPI-anchored alkaline phosphatase in low-density, detergent-insoluble complexes commonly known as glycolipid "rafts". Thus, aminopeptidase N (EC 3.4.11.2), aminopeptidase A (EC 3.4.11.7), dipeptidyl peptidase IV (EC 3.4.14.5), and sucrase-isomaltase (EC 3.2.1.48-10) were 34-48% detergent-insoluble. Maltase-glucoamylase (EC 3.2.1.20) was markedly less detergent-insoluble (20%), and lactase-phlorizin hydrolase (EC 3.2.1.23-62) was essentially fully soluble in detergent. In radioactively labeled, mucosal explants, the newly synthesized brush border enzymes began to associate with detergent-insoluble complexes while still in their transient, high mannose-glycosylated form, and their insolubility increased to that of the steady-state level soon after they achieved their mature, complex glycosylation, i.e., after passage through the Golgi complex. Detergent-insoluble complexes isolated by density gradient centrifugation were highly enriched in brush border enzymes, and the enrichment was apparent after only 1 h of labeling, where aminopeptidase N, sucrase-isomaltase, and alkaline phosphatase together comprised 25-30% of the total labeled, detergent-insoluble proteins, showing that sorting of newly made brush border membrane proteins into the glycolipid "rafts" does take place intracellularly. I therefore propose that, in the enterocyte, the brush border enzymes are targeted directly from the trans-Golgi network toward the apical cell surface.
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PMID:Involvement of detergent-insoluble complexes in the intracellular transport of intestinal brush border enzymes. 784 19


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