Protein–phytate interactions in pig and poultry nutrition: a reappraisal

Abstract

<jats:p>Protein–phytate interactions are fundamental to the detrimental impact of phytate on protein/amino acid availability. The inclusion of exogenous phytase in pig and poultry diets degrades phytate to more innocuous esters and attenuates these negative influences. The objective of the present review is to reappraise the underlying mechanisms of these interactions and reassess their implications in pig and poultry nutrition. Protein digestion appears to be impeded by phytate in the following manner. Binary protein–phytate complexes are formed at pH levels less than the isoelectric point of proteins and complexed proteins are refractory to pepsin digestion. Once the protein isoelectric points are exceeded binary complexes dissociate; however, the isoelectric point of proteins in cereal grains may be sufficiently high to permit these complexes to persist in the small intestine. Ternary protein–phytate complexes are formed at pH levels above the isoelectric point of proteins where a cationic bridge links the protein and phytate moieties. The molecular weights of protein and polypeptides in small-intestinal digesta may be sufficient to allow phytate to bind nutritionally important amounts of protein in ternary complexes. Thus binary and ternary complexes may impede protein digestion and amino acid absorption in the small intestine. Alternatively, phytate may interact with protein indirectly.<jats:italic>Myo-</jats:italic>inositol hexaphosphate possesses six phosphate anionic moieties (HPO<jats:sub>4</jats:sub><jats:sup>2–</jats:sup>) that have strong kosmotropic effects and can stabilise proteins by interacting with the surrounding water medium. Phytate increases mucin secretions into the gut, which increases endogenous amino acid flows as the protein component of mucin remains largely undigested. Phytate promotes the transition of Na<jats:sup>+</jats:sup>into the small-intestinal lumen and this suggests that phytate may interfere with glucose and amino acid absorption by compromising Na<jats:sup>+</jats:sup>-dependent transport systems and the activity of the Na pump (Na<jats:sup>+</jats:sup>-K<jats:sup>+</jats:sup>-ATPase). Starch digestion may be depressed by phytate interacting with proteins that are closely associated with starch in the endosperm of cereal grains. While elucidation is required, the impacts of dietary phytate and exogenous phytase on the site, rate and synchrony of glucose and amino acid intestinal uptakes may be of importance to efficient protein deposition. Somewhat paradoxically, the responses to phytase in the majority of amino acid digestibility assays in pigs and poultry are equivocal. A brief consideration of the probable reasons for these inconclusive outcomes is included in this reappraisal.</jats:p>

Separate feeding of calcium improves performance and ileal nutrient digestibility in broiler chicks

“…Binary protein-phytate complexes involve the interaction between basic amino acid residues in dietary or endogenous protein and phytate by salt-like linkages (Cosgrove 1966). Ternary protein-phytate complexes are formed in the small intestine with the major components linked by a cationic bridge, usually Ca 2+ , which is the most abundant divalent cation in digesta (Selle et al 2012). Therefore, reducing the amount of dietary Ca in broiler diets would likely promote greater phytate destruction by endogenous phytases due to a reduction in ternary complex formation.…”

Nutritional geometry of calcium and phosphorus nutrition in broiler chicks. The effect of different dietary calcium and phosphorus concentrations and ratios on nutrient digestibility

“…Protein-phytate interactions are not completely understood, however, the benefits to protein digestibility from phytase are speculated to be derived from numerous factors (Selle et al, 2000). This is likely to be a combination of the release of protein from endogenous plant protein-phytate complexes, prevention of the formation of binary and ternary protein-phytate complexes within the intestine as well as the alleviation of negative phytate impacts on digestive enzyme and endogenous amino acids losses (Selle et al, 2012). Upon ingestion of phytate and passage into the duodenum there is increased secretion of mucin, increasing the presence of endogenous amino acids in the lumen ).…”

An isothermal titration calorimetry study of phytate binding to lysozyme

“…The core body temperature of pigs and poultry is in the range of 310-340 K, and based on the binding stoichiometry presented herein, this may promote phytate-protein interactions. However, putatively, phytateprotein interactions are more likely to occur in the gastric region of the intestine where pH is more commonly below the isoelectric point of dietary protein [41], and in this context, the relevance of drinking water temperature should be considered. There have been several studies that explored the effect of drinking water temperature on broiler chicken performance, but most are associated with alleviation of stress at high ambient temperature.…”