Phytases are phosphohydrolytic enzymes that initiate stepwise removal of phosphate from phytate. Simple-stomached species such as swine, poultry, and fish require extrinsic phytase to digest phytate, the major form of phosphorus in plant-based feeds. Consequently, this enzyme is supplemented in these species' diets to decrease their phosphorus excretion, and it has emerged as one of the most effective and lucrative feed additives. This chapter provides a comprehensive review of the evolving course of phytase science and technology. It gives realistic estimates of the versatile roles of phytase in animal feeding, environmental protection, rock phosphorus preservation, human nutrition and health, and industrial applications. It elaborates on new biotechnology and existing issues related to developing novel microbial phytases as well as phytase-transgenic plants and animals. And it targets critical and integrated analyses on the global impact, novel application, and future demand of phytase in promoting animal agriculture, human health, and societal sustainability.
Whereas selenium was found to act as an insulin-mimic and to be anti-diabetic in earlier studies, recent animal experiments and human trials have shown unexpected risk of prolonged high Se intake in potentiating insulin resistance and type 2 diabetes. Elevating dietary Se intakes (0.4 to 3.0 mg/kg of diet) above the nutrient requirements, similar to overproduction of selenoproteins, led to insulin resistance and(or) diabetes-like phenotypes in mice, rats, and pigs. Although its diabetogenic mechanism remains unclear, the high Se intake elevated activity or production of selenoproteins including GPx1, MsrB1, SelS, and SelP. This up-regulation diminished intracellular reactive oxygen species (ROS) and then dys-regulated key regulators of β cells and insulin synthesis and secretion, leading to chronic hyperinsulinaemia. Over-scavenging intracellular H2O2 also attenuated oxidative inhibition of protein tyrosine phosphatases and suppressed insulin signaling. High Se intake might affect expression and(or) function of key regulators for glycolysis, gluconeogenesis, and lipogenesis. Future research is needed to find out if certain forms of Se metabolites in addition to selenoproteins and if mechanisms other than intracellular redox control mediate the diabetogenic effect of high Se intakes. Furthermore, a potential interactive role of high Se intakes in the interphase of carcinogenesis and diabetogenesis should be explored to make the optimal use of Se in human nutrition and health.
Inulin, a linear b fructan, is present in a variety of plants including chicory root and wheat. It exhibits prebiotic properties and has been shown to enhance mineral absorption and increase beneficial bacteria in the colon. The aim of the present study was to assess the effect of dietary inulin on the gene expression of selected intestinal Fe transporters and binding proteins. Anaemic piglets at age 5 weeks were allocated to a standard maizesoya diet (control) or the same diet supplemented with inulin at a level of 4 %. After 6 weeks, the animals were killed and caecum contents and sections of the duodenum and colon were removed. Segments of the genes encoding for the pig divalent metal transporter 1 (DMT1) and duodenal cytochrome-b reductase (Dcytb) were isolated and sequenced. Semi-quantitative RT-PCR analyses were performed to evaluate the expression of DMT1, Dcytb, ferroportin, ferritin, transferrin receptor (TfR) and mucin genes. DMT1, Dcytb, ferroportin, ferritin and TfR mRNA levels in duodenal samples were significantly higher in the inulin group (P# 0·05) compared with the control. In colon, DMT1, TfR and ferritin mRNA levels significantly increased in the inulin group. Additionally, the caecal content microflora was examined using 16S rDNA targeted probes from bacterial DNA. The Lactobacillus and Bifidobacterium populations were significantly increased in the inulin group (P#0·05) compared with the control group. These results indicate that dietary inulin might trigger an up regulation of genes encoding for Fe transporters in the enterocyte. The specific mechanism for this effect remains to be elucidated.
Pancreatic islets contain low activities of catalase, selenium-dependent glutathione peroxidase 1 (GPX1), and Cu,Zn-superoxide dismutase 1 (SOD1). Thus, enhancing expression of these enzymes in islets has been unquestionably favored. However, such an attempt has produced variable metabolic outcomes. While β cell-specific overexpression of Sod1 enhanced mouse resistance to streptozotocin-induced diabetes, the same manipulation of catalase aggravated onset of type 1 diabetes in nonobese diabetic mice. Global overexpression of Gpx1 in mice induced type 2 diabetes-like phenotypes. Although knockouts of Gpx1 and Sod1 each alone or together decreased pancreatic β cell mass and plasma insulin concentrations, these knockouts improved body insulin sensitivity to different extents. Pancreatic duodenal homeobox 1, forkhead box A2, and uncoupling protein 2 are three key regulators of β cell mass, insulin synthesis, and glucose-stimulated insulin secretion. Phenotypes resulted from altering GPX1 and/or SOD1 were partly mediated through these factors, along with protein kinase B and c-jun terminal kinase. A shifted reactive oxygen species inhibition of protein tyrosine phosphatases in insulin signaling might be attributed to altered insulin sensitivity. Overall, metabolic roles of antioxidant enzymes in β cells and diabetes depend on body oxidative status and target functions. Revealing regulatory mechanisms for this type of dual role will help prevent potential pro-diabetic risk of antioxidant over-supplementation to humans.
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