Alkaline phosphatase from Atlantic cod (Gadus morhua). Kinetic and structural properties which indicate adaptation to low temperatures

Alkaline phosphatase was purified from the pyloric caeca of Atlantic cod in a procedure requiring column chromatography on phenyl-Sepharose, Q-Sepharose, Sephacryl S-300, Reactive Red and concanavalin-A-Sepharose. The enzyme is a dimeric glycoprotein of identical 70 kDa subunits and partly associated with membranes. It was inactivated by EDTA and could be reactivated with zinc and magnesium. The catalytic efficiency (k_cat/K_m) of the cod enzyme was found to be 2.5-fold greater in comparison with the calf intestinal enzyme. The cod enzyme was unstable towards heating above 40°C whereas the calf enzyme remained stable at temperatures up to 55°C. Differences were also found in the response to some commonly used inhibitors, where the cod intestinal alkaline phosphatase resembled most the liver/bone/kidney isozyme from humans. Notably, the sensitivity towards phosphate inhibition was lower with the cod enzyme, and the pH optimum for catalysis was different. We conclude that the observed kinetic differences are indicative of cold-adaptation. Fewer stabilizing interactions in the cod enzyme structure, as evidenced by lower thermal stability, would help to maintain necessary flexibility at the low level of thermal energy in the body of this ectothermic species.