PT - JOURNAL ARTICLE AU - Chung-Liang Kuo AU - Bert N. La Du TI - Calcium Binding by Human and Rabbit Serum Paraoxonases DP - 1998 Jul 01 TA - Drug Metabolism and Disposition PG - 653--660 VI - 26 IP - 7 4099 - http://dmd.aspetjournals.org/content/26/7/653.short 4100 - http://dmd.aspetjournals.org/content/26/7/653.full SO - Drug Metab Dispos1998 Jul 01; 26 AB - 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. The American Society for Pharmacology and Experimental Therapeutics