Isozymes of Bighead Shrimp Alkaline Phosphatase

Three alkaline phosphatase isozymes, named APase-I, II, and III, were isolated from the cephalothoraxes of bighead shrimp (Solenocera melantho). The molecular weights of APase-I, II, and III estimated by SDS-PAGE were 88.6, 53, and 20 kDa, respectively. They were all glycosylated monomeric enzymes. The optimum pH of APase-I and II were 10 and 8, and the pH ranges for maximum stability were 8.0-12.5 and 5.0-8.0, respectively. Both enzymes showed optimum temperature around 37°C. Arrehnius plots for enzyme thermal inactivation showed that APase II was more stable than APase I above 18°C but less stable below this temperature. Both enzymes showed the highest activities toward aryl phosphate esters among several phosphoesters tested. The Km for p-nitrophenyl phosphate were 0.14 and 4.44 mm for APase I and II, respectively. Na(+) stimulated both enzyme activities in physiological concentration range. APase I was less susceptible to denaturating agents and inhibitors than Apase II.