Phytase activity in the small intestinal brush border membrane of the chicken

The kinetics, mineral dependency, and pH dependency of phytate hydrolysis by preparations of chicken small intestinal brush border membrane vesicles were determined. Substantial phytate hydrolysis occurred over the pH range from 5 to 6.5 with a maximum hydrolysis at pH of 6. Inclusion of 25 mM MgCl2 in the media doubled the rate of phytate hydrolysis. The brush border was shown to have no nonspecific acid phosphatase activity and excess phytate had no effect on alkaline phosphatase activity at pH 11. Under optimal conditions of pH 6 plus 25 mM MgCl2, a kinetic model of a single Michaelis-Menten type of enzymatic activity with a Km of 0.160 +/- 0.008 mM and a Vmax of 42.5 +/- 1.0 nmol/mg vesicle protein per min plus a small unsaturable component converged to the data (P < 0.05). The specific and total activities of intestinal brush border phytase were highest in the duodenum (P < 0.05) and decreased progressively down the length of the gut. Intestinal brush border vesicles prepared from broiler chicks and mature laying hens had comparable specific phytase activity. However, the total activity of brush border phytase was 35% higher in the small intestine of laying hens (P < 0.05). Intestinal brush border phytase could contribute to phytate-phosphorus digestibility and may be subject to regulation in response to the dietary phosphorus and vitamin D status of the chicken.