FUNCTION SITE OF SUPPLEMENTAL ESCHERIACHIA COLI PHYTASE IN THE GASTROINTESTINAL TRACTS AND ITS ROLE IN BONE METABOLISM OF YOUNG PIGS

Abstract

This thesis consists of two studies on phytase. The objective of the first study was to determine the functional site of a supplemental Escherichia coli AppA2 phytase and its impact on digesta phosphorus and calcium concentrations in different segments of the gastrointestinal tract of pigs. In Exp. 1, 18 weanling pigs [8.3 ? 0.2 kg body weight (BW)] were allotted to three groups (n = 6) and fed a low-P (0.4%) corn-soy basal diet (BD), BD + phytase [500 units (U)/kg] or BD + inorganic P (0.1%) for 4 wk. In Exp. 2, 30 weanling pigs (14.5 ? 0.2 kg BW) were allotted to three groups (n = 10) and fed BD, BD + phytase (500 U/kg) or BD + phytase (2000 U/kg) for 2 wk. Five or six pigs out of each treatment group were slaughtered at the end of both trials to collect digesta from six segments of the digestive tract to assay for phytase activity, soluble P concentration, and(or) total P and Ca concentrations. Pigs fed BD + phytase had similar phytase activities in the stomach, duodenum, and upper jejunum digesta, and the detected activities were proportional to the supplemental levels of dietary AppA2. But, no phytase activity was detected in digesta of these three segments from pigs fed BD or BD + 0.1% iP or in digesta of lower jejunum and ileum from any of the treatment groups. Digesta soluble P decreased sharply from the stomach to duodenum of pigs fed BD + phytase or BD + iP, whereas it peaked in the upper jejunum of pigs fed BD. Colonic digesta phytase activity and soluble P were highest (P < 0.05) in pigs fed only BD and were inversely affected by dietary phytase supplementation. Pigs fed BD + phytase showed phytase-dose dependent reductions (P < 0.05) in total colonic P and (or) Ca concentrations, compared with those fed BD or BD + iP. In conclusion, supplemental dietary AppA2 mainly functioned in the stomach, but remained fairly active in the upper jejunum. Colonic microbial phytase activity was greatly reduced by the supplemental phytase-mediated phytate-phosphorus hydrolysis in the fore segments of the digestive tract via substrate limitation. The objective of the second study was to determine if high levels of supplemental dietary microbial phytase, in a phosphorus (P)-adequate diet, additionally improved bone strength of growing female pigs. For three experiments a total of 80 pigs (28-35 d old) were fed a low-P (0.4%) corn-soy basal diet (BD), or BD + 0.2% or 0.25% inorganic P and (or) 1,000 or 2,000 units E. coli AppA2 phytase/kg for 4-6 wk. At the end of the experiments, pigs were slaughtered to collect 3rd and 4th metacarpals from front legs to test for bone strength. In Exp. 3, metacarpals were analyzed for their contents of Ca, P and other minerals. Weekly growth performance, plasma alkaline phosphatase activity and plasma inorganic P, were improved by phytase and inorganic P in BD. While these measures were not further improved by supplementing phytase at 2,000 U/kg in the P-adequate diets, bone strength of pigs fed the additional phytase was 12% (P < 0.05) greater than that of those fed only the P-adequate diets. The additional 2,000 U/kg in the P-adequate diets did not affect bone concentrations of Ca, P, K, Na, S, Mg, Fe, Mn, Zn, B or Cr, but resulted in 7% (P < 0.05) increase in bone Sr concentration. In conclusion, supplementing phytase at 2,000 U/kg of the P-adequate diets produced additional benefit on bone strength of young female pigs, and the improvement was not directly associated with P accretion. In summary, the two studies reported in this thesis help in locating the functional site of E. coli AppA2 phytase in the digestive tract of young pigs, in unveiling a novel role of high levels of AppA2 activity in bone metabolism of young pigs.