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Vol. 26, Issue 7, 653-660, July 1998

Calcium Binding by Human and Rabbit Serum Paraoxonases
Structural Stability and Enzymatic Activity

Chung-Liang Kuo and Bert N. La Du

Department of Environmental and Industrial Health, Toxicology Program, School of Public Health (C.-L. K.) and Departments of Pharmacology and Anesthesiology, Medical School (B. N. L. D.), the University of Michigan

Equilibrium dialysis and Scatchard plots were used to establish that human and rabbit paraoxonases both have two calcium binding sites. Independent-site and stepwise constant analyses were used to calculate a higher affinity site (Kd1) of 3.6 ± 0.9 x 10-7 M for human A paraoxonase, and 1.4 ± 0.5 x 10-8 M for rabbit paraoxonase, and a lower affinity site (Kd2) of 6.6 ± 1.2 x 10-6 M for human A paraoxonase, and 5.3 ± 0.94 x 10-6 M for rabbit paraoxonase. In both species, the higher affinity sites were found to be essential to maintain hydrolytic activity; complete removal of calcium led to irreversible inactivation. The lower affinity sites were required for catalytic activity, and their binding of calcium was reversible. Experimentally estimated values of Kd2 based on the concentration of calcium required to obtain half the maximum enzymatic activity were 3 µM for human A and B paraoxonases, and also in the order of 3 µM for rabbit paraoxonase, using three different substrates. Calcium was the only metal found that protects against denaturation and also confers hydrolytic activity with these two mammalian paraoxonases.

Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics


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