The origin of plasma alkaline phosphatase activity in mice and rats.

Swiss albino mice displayed the highest activity of alkaline phosphatase at 4-6 weeks with a precipitous decline by 18 weeks of age to a value seen in the mature animal. Circulating activity of alkaline phosphatase was significantly higher in the rat than the mouse in the fed state. With fasting, enzyme activity declined in the rat yet increased in the mouse. The net result was significantly higher plasma alkaline phosphatase activity in the mouse than the rat after the 48 hr fast. L-Phenylalanine inhibition of plasma alkaline phosphatase was greater in plasma from the rat than the mouse in the fed state. Yet in the fed condition, L-homoarginine and L-p-bromotetramisole inhibited alkaline phosphatase activity in plasma from mice to a greater extent than in rats. Heat inactivation as well as urea denaturation of alkaline phosphatase was significantly faster with plasma of the mouse than the rat in the fed state. Thus, it appears that the alkaline phosphatase isoenzyme of skeletal origin contributes a greater proportion of the circulating activity in the fed Swiss albino mouse than occurs in the Sprague-Dawley rat in which the intestinal isoenzyme plays a greater role in the fed condition.