Intestinal phytase II: A comparison of activity and in vivo phytate hydrolysis in three teleost species with differing digestive strategies

Phytates are the primary source of phosphorus in animal feeds of plant origin; however, this phytate phosphorus (PP) is only available to growing fish provided it is released from the phytate molecule as inorganic orthophosphate. Two experiments were conducted to compare phytase activity in the intestinal brush border membrane of carnivorous hybrid striped bass Morone chrysops × M. saxatilis, omnivorous tilapia Oreochromis niloticus × O. aureus and omnivorous koi Cyprinus carpio, and to evaluate the capability of these three species for PP digestibility. The results indicate that tilapia exhibit a significantly higher specific intestinal brush border membrane phytase activity of 5 nmol inorganic phosphate released mg−1 protein min−1 than hybrid bass or koi, which do not differ in their lower activities of 3 nmol inorganic phosphate released mg−1 protein min−1. There were no statistical differences among the three species in terms of total intestinal phytase activity, although the level of total intestinal phytase activity exhibited by the two omnivorous species tended to be higher than that of the carnivorous bass. The tilapia were shown to be capable of digesting substantial amounts of dietary PP, with observed apparent PP digestibility values of 50%. The hybrid bass and the koi could digest only 1–2% of the PP present in the diet. It appears that the only species of the three examined for which intestinal phytase activity is of physiologic significance from a practical standpoint of utilizing dietary PP is tilapia.