Enzymatic synthesis of a galactose-containing trehalose analogue disaccharide by Pyrococcus horikoshii trehalose-synthesizing glycosyltransferase: Inhibitory effects on several disaccharidase activities

Pyrococcus horikoshii trehalose-synthesizing glycosyltransferase employed a galactose as an acceptor in the glucosyl transfer reaction with an NDP-Glc donor. The reaction produced a non-reducing transfer product in a yield of more than 30% based on the molar concentration of donor used. The transfer product was purified by paper chromatography and preparative HPLC, and its glycosidic structure was confirmed by ¹³C nuclear magnetic resonance to be -d-glucopyranosyl -d-galactopyranoside. Interestingly, this trehalose analogue disaccharide inhibited the action of several disaccharidases, including a trehalase. The analogue competitively inhibited porcine kidney and rat intestinal trehalases with K_i values of 0.68 and 3.7 mM, respectively. It also competitively inhibited other intestinal disaccharidases such as sucrase, maltase, and isomaltase with respective K_i values of approximately 0.66, 3.0, and 2.1 mM. Accordingly, this trehalose analogue would be a potentially indigestible disaccharide, effectively inhibiting intestinal brush border disaccharidases.     

猪胃肠道黏膜二糖酶的性质

研究了猪小肠中麦芽糖酶,蔗糖酶和乳糖酶3种二糖酶的生化性质及活性分布。试验以3头“杜加”生长猪为对象,屠宰并刮取胃底部,十二指肠,空肠上段和回肠黏膜。其中空肠黏膜用于3种二糖酶生化性质的研究,包括酶的最适温度,热稳定性,最适pH ,pH稳定范围和金属离子对酶活性的影响;胃底部,十二指肠,空肠上段和回肠黏膜用于测定3种二糖酶活性以揭示其在胃扬中的分布规律。试验结果表明：麦芽糖酶,蔗糖酶和乳糖酶的最适反应温度分别为50,45和55℃,最适pH分别为6.8,6.5和6．0;乳糖酶的耐热温度（70℃）高于麦芽糖酶和蔗糖酶（50℃）;不同pH对3种二糖酶活性影响不大;金属离子Cu^2 和Fe^2 对3种二糖酶均有激活作用;而Mn^2 有抑制作用。此外,Zn^2 能抑制麦芽糖酶活性,提高蔗糖酶活性,而不影响乳糖酶活性。3种二糖酶活性在肠道中由高到低的分布为：空肠,回肠,十二指肠和胃;其中麦芽糖酶在空肠和回肠中活性相近。回肠和空肠黏膜中的麦芽糖酶活性均显著高于十二指肠和胃中的酶活性,十二指肠酶活性显著高于胃;空肠中蔗糖酶和乳糖酶活性显著高于回肠,十二指肠和胃底部。从3种二糖酶活性大小看,胃底部和十二指肠中的麦芽糖酶活性显著高于蔗糖酶和乳糖酶;空肠上段和回肠中的麦芽糖酶活性显著高于蔗糖酶活性,蔗糖酶活性又显著高于乳糖酶。上述结果表明,3种二糖酶的生化性质具有一定的差异,在胃肠道中的活性分布规律相似。     

A hydrolase-related transport system is not required to explain the intestinal uptake of glucose liberated from phlorizin

The fate of [³H]glucose released from a wide range of [³H]phlorizin concentrations by phlorizin hydrolase has been studied under conditions where the Na⁺-dependent glucose transport system in hamster intestine is profoundly inhibited by the glucoside. At 0.2–2.0 mM phlorizin, the [³H]glucose uptake was a constant 11–12% of that generated by the enzyme and at the highest level, it was reduced to that of passive diffusion. Glucose liberated from 0.2 mM [³H]phlorizin is accumulated at a rate nearly equal to that found for 0.2 mM [¹⁴C]glucose when this free sugar uptake is measured in a medium containing 0.2 mM unlabeled phlorizin. Furthermore, without sodium, the accumulation rates of hydrolase-derived or exogenous glucose are both reduced to the rate of [¹⁴C]mannitol. Our results indicate that the glucose released from phlorizin enters the tissue via the small fraction of the Na⁺-dependent glucose carriers which escape phlorizin blockade together with a mannitol-like passive diffusion. It enjoys a kinetic advantage for tissue entry over free glucose in the medium by virtue of the position of the site where it is formed, i.e. inside the unstirred water layer and near normal entry portals. No special hydrolase-related transport system, like the one proposed for disaccharides, needs to be considered to account for our findings.     

Further evidence that epidermal growth factor enhances the intestinal adaptation following small bowel transplantation

Recent reports indicate that epidermal growth factor (EGF) plays a crucial role for graft adaptation in rat model of small bowel transplantation (SBT). The administration of EGF enhances intestinal cell proliferating rate and the recovery of mucosal structure. However, the effect of EGF on biological functions including glucose absorption in intestinal graft remains to be elucidated. SBT was performed in the two-step procedure. On the first step, intestinal graft (30-cm jejunum) from Brown Norway rats was exteriorized through abdominal wall as a Thiry-Vella loop in recipient Lewis rats for one week. On the second surgery (POD 7), recipient jejunum was replaced orthotopically by the graft, and transplanted rats were treated intraperitoneally with EGF or its vehicle for 3 days. Analyses of histology and biological functions in the graft were done at POD 14. EGF increased both levels of villus height and crypt depth in the graft of transplanted groups. EGF enhanced the glucose absorption as well as the induction of sodium glucose cotransporter 2- to 3-fold in transplanted groups. Further, EGF stimulated the activities of disaccharidase (maltase and sucrase) and the induction of dipeptide cotransporter. These results demonstrate that EGF enhances the structural and functional adaptation of intestinal grafts after SBT. EGF may be useful therapy for patients following intestinal transplantation.