Structural Stability and Enzymatic Activity
Abstract
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.
Footnotes
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Send reprint requests to: Bert N. La Du, M.D., Ph.D., 1150 West Medical Center Drive, Dept. of Anesthesiology, 7433 Medical Science Building I, University of Michigan Medical School, Ann Arbor, MI 48109-0615.
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This work was supported by National Institutes of Health Grant GM 46979 (to B.N.L.).
- Abbreviation used is::
- PON
- paraoxonase
- Received December 18, 1997.
- Accepted March 11, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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