Characterization of paraoxonase activity in pericardial fluid: usefulness as a marker of coronary disease

Assay of serum arylesterase activity by fitting to the reaction curve with an integrated rate equation

BACKGROUND

Conventional enzyme activities make use of the initial reaction rate at high substrate concentrations. Because this is not always practical, alternative enzyme assays have been sought.

METHODS

Reaction curve fitting with an integrated rate equation was investigated to assay serum arylesterase (ArE) activity using phenyl acetate (PA) and p-nitrophenol acetate (PNPA) as substrates. At a much lower initial concentration of substrate (S(0)), the simplified integrated rate equation for the ArE reaction was ln(S(0)/S(i))=(V(m)/K(m)+K(d))t(i). Treating S(0) as a parameter, the enzyme activity as V(m)/K(m) was estimated through nonlinear least square fitting to reaction curve, and the multiplication of V(m)/K(m) by K(m) produced V(m). Spontaneous hydrolysis of the substrate with a rate constant, K(d), served as the background for the estimation of V(m)/K(m).

RESULTS

Substrate concentration at 8% of K(m) was well suited for the estimation of V(m)/K(m). With either substrate, the V(m)/K(m) showed a close relation to the percentage of substrate consumed, and was not affected by common systematic errors. With either substrate, the between-run precision for V(m)/K(m) was 6% (n>7), V(m)/K(m) was proportional to the amount of ArE and closely correlated with its initial rate. The upper limit of linearity by this integrated method was much higher than the initial rate method, while the detection limit was comparable. By using either V(m)/K(m) or the initial rate, there was negligible interference with ArE activity assay from triglycerides, bilirubin, and hemoglobin.

CONCLUSIONS

These results indicate the feasibility of the integrated method for routine assay of serum enzyme activity.

Serum paraoxonase activity and phenotype distribution in Turkish subjects with coronary heart disease and its relationship to serum lipids and lipoproteins

Recently, biochemical studies of paraoxonase in the serum of humans have shown that much of this enzymes' activity is associated with high-density lipoprotein (HDL) and paraoxonase may play a role in lipid metabolism preventing the accumulation of the lipoperoxides. In this study, a possible relationship between coronary heart disease (CHD) and paraoxonase activity were investigated. Serum triglycerides, total cholesterol, HDL-cholesterol and paraoxonase activity were measured in unrelated healthy donors and CHD patients. It was found that paraoxonase activity was trimodally distributed in both groups but no statistically significant difference was found between phenotype distributions of controls and CHD patients (gene frequencies; 0.632 and 0.382 of controls, 0.702 and 0.298 of patients for the A and B alleles, respectively). However, in CHD group, a high possibility was found to be phenotype A compared with the control group. A relative risk of 1.48 (95% confidence intervals (CI), 0.986-2.227) was found for the relation between CHD and the paraoxonase activity. Patients' HDL-cholesterol values were lower and triglycerides were higher than controls (P<0.001). It may be concluded from the present study that although no statistically significant difference was found between paraoxonase phenotype distributions of controls and CHD patients, a decrease in paraoxonase activity could become a risk factor for this disease.

Inhibition of paraoxonase activity in human liver microsomes by exposure to EDTA, metals and mercurials

Inhibition of paraoxon hydrolase (paraoxonase) activity by 'in vitro' exposure to EDTA, Mg2+, Co2+, Ba2+, La3+, Zn2+, Cu2+, Hg2+, p-hydroxymercuribenzoate (p-OH-MB) and phenyl mercuric acetate (PMA) was investigated in human liver microsomes. Enzyme activity was totally inhibited by 1 mM EDTA in a time-dependent manner, in contrast to previous data obtained in rat liver where an EDTA-resistant fraction was detected. The possible influence of postmortem changes in these results was checked in a parallel experiment using rat livers with different postmortem intervals. From our results the existence in human liver of an EDTA-resistant fraction cannot be discarded. Ba, La and PMA showed immediate inhibition. By contrast the other compounds tested were time-dependent inhibitors. Ba and Zn showed the highest IC50 values. Cu and mercurials (Hg, p-OH-MB, PMA) were the most potent inhibitors of human liver paraoxonase. Kinetic analysis (Lineweaver-Burk and Dixon plots) indicated that different inhibitors exhibit different inhibition patterns: competitive (EDTA, Ba, La, Cu, p-OH-MB and PMA), non competitive (Zn) and mixed (Hg). The pretreatment of sample with dithiothreitol (DTT) protects against the inhibitory effect of mercurials. Furthermore after inhibition by mercurials the activity was restored by DTT. These results confirmed the essential role of the -SH groups to maintain the catalytic activity of paraoxonase and suggest the existence of two types of -SH groups that could differ in their localization.