Thierry Dantoine

Paraoxonase 1 192/55 Gene Polymorphisms in Alzheimer's Disease

Abstract: An esterase, paraoxonase 1 (PON1), protects against organophosphate neurotoxicity and decreases lipoprotein oxidation. Two polymorphisms of PON1 [192 (R or Q) and 55 (M or L)] exist and are associated with coronary artery disease. We have previously shown that serum PON1 activity (PON1a) is lower in vascular dementia (VaD) than in Alzheimers disease (AD), suggesting that PON1a may distinguish VaD from AD. As PON1 polymorphism modifies PON1a, we determined 192 and 55 PON1 polymorphisms by sequence‐specific primer PCR in 64 healthy subjects (HS; mean age: 79.5 ± 6.3 years; 38 women) and in 72 patients (mean age: 80.2 ± 6.8 years; 51 women) undergoing cognitive evaluations. According to DSM‐IV/NINCDS/ADRDA/NINDS/AIREN criteria, 45 patients (mean age: 80.0 ± 7.2 years, 34 women) had AD and 27 patients (mean age: 79.8 ± 6.6 years, 16 women) had VaD. We also measured serum PON1a by spectrophotometry. No significant differences in phenotype distributions among the three study groups were detected by χ2 test. Among the variables, age, sex, and phenotypes 192 and 55, logistic regression selected only polymorphism 192, but not 55, as a discriminating factor between AD and VaD (p < 0.05). Substitution of serum PON1a for genotype yielded a similar result. PON1 polymorphism 192 appears to be a reliable marker to distinguish patients with AD from patients with VaD and from healthy subjects. Changes in 192 polymorphism distributions in AD and in VaD may at least partially explain the significant difference in PON1a in these two types of dementia.

Paraoxonase 1 Activity: A New Vascular Marker of Dementia?

Abstract: Paraoxonase 1 (PON1), an A‐esterase with peroxidase‐like activity present on the surface of HDL, decreases the peroxidation of LDL. Serum PON1 activity (PON1a) decreases with aging and in disorders associated with a high risk of adverse cardiovascular events (acute myocardial infarction, diabetes mellitus, and chronic renal failure). The implication of vascular factors in Alzheimer‐type dementia (ATD) is strongly suspected. We measured PON1a by spectrophotometry using the paraoxon substrate in 180 healthy subjects (controls; mean age: 75.3 ± 8.9 years; 98 women) and 154 patients admitted for cognitive testing. According to criteria, 45 patients had mild cognitive impairments (MCI; mean age: 75.6 ± 9.3 years; 28 women), 60 had ATD (mean age: 75.6 ± 8.3 years; 47 women), and 49 had vascular dementia (VaD; mean age: 77.5 ± 7.2 years; 33 women). Mean PON1a was lower in VaD (0.25 ± 0.1 U/mL) than in controls or ATD (both 0.41 ± 0.2 U/mL, p < 0.01). Mean PON1a values in MCI (0.34 ± 0.2 U/mL) and ATD (0.41 ± 0.2 U/mL) were not significantly different. In multiple linear regression, PON1a was negatively correlated with male sex, age, and VaD, and positively correlated with ATD (each correlation p < 0.001). As shown in other high‐risk cardiovascular disorders, PON1a seems to be a reliable marker of VaD. Its modification in Alzheimers disease supports the implication of vascular risk factors in this type of dementia.

Inhibition of human serum arylesterase by metal chlorides.

The inhibition of arylesterase (paraoxonase, EC 3.1.8.1) by metal chlorides was studied with both pooled human serum (A phenotype) and purified enzyme, using phenyl acetate as substrate. Inhibition data were analysed with the Hill equation. Results obtained with whole serum and purified enzyme were very similar. On the basis of the Hill coefficient, n(H), three groups of inhibitors were distinguished: (1) Cu(2+) and Hg(2+) for which n(H)=1, suggesting a single binding site (probably the free cysteine at position 283); these metals were mixed inhibitors, with more affinity for the free enzyme than for the enzyme-substrate complex; (2) Mn(2+), Co(2+), Ni(2+), Zn(2+), and Cd(2+) for which n(H)>1, suggesting several cooperative binding sites; (3) La(3+), for which n(H)<1. Within groups (1) and (2) the inhibiting potency followed the order of the periodic table. For the 3d elements the inhibiting order followed the Irving-Williams series, with the classical exception of Cu(2+). Only Zn(2+) was inhibitory at its physiological concentration.

Inhibition of arylesterase by aliphatic alcohols

The inhibition of arylesterase (EC 3.1.8.1) by 11 aliphatic alcohols (one to seven carbon atoms) was studied in blood serum from healthy donors. Inhibition curves were described by the Hill equation, with a Hill coefficient (n) close to unity, except for some alcohols, mainly the lowest. The inhibiting activity of the alcohols was highly dependent on their structure, since the C50 values covered about three orders of magnitude. The least active compound was methanol (C50 approximately 1 M) and the most active was heptanol (C50 approximately 7.4 x 10(-4) M). The A and B isozymes (differing by the amino acid at position 191) had similar inhibition parameters with the alcohols tested. Quantitative structure-activity relationships were computed with either the experimental solvation parameters of Abraham [6] or the theoretical parameters of Wilson and Famini [11]. Both methods gave similar results, with a slight advantage to the empirical parameters in terms of simplicity and statistical significance. The two main determinants of inhibition were identified as molecular volume and lack of polarity. The effect of volume was non-linear, tending to a maximum when the length of the alcohol increased. For a given number of carbon atoms, the best inhibitor was the least polar compound. These results point to a binding site consisting mainly of nonpolar aliphatic amino acids, and located in the depth of the protein molecule.

De l’intoxication par les composés organophosphorés à l’athérosclérose : rôles de la paraoxonase 1

Resume Propos. – La paraoxonase 1 est une esterase serique et tissulaire ubiquitaire dans l’organisme humain. Elle est connue en toxicologie pour jouer un role protecteur contre les intoxications par les composes organophosphores mais semble aussi impliquee dans de nombreuses pathologies vasculaires. Actualites et points forts. – La localisation recente de la paraoxonase 1 a la surface des lipoproteines de hautes densites a oriente la recherche vers l’etude de son role physiologique dans l’atherosclerose. Sa structure riche en acides amines reducteurs (cysteine) a suggere un effet antioxydant qui a pu etre confirme sur les lipoproteines de basses densites (LDL) in vitro puis in vivo. La paraoxonase 1 hydrolyse un derive de l’acide arachidonique present a la surface des LDL oxydees qui sont desormais reconnues pour participer a l’etape initiale essentielle de l’atherogenese. Au plan clinique, l’activite paraoxonase 1 serique est abaissee dans des situations ou le vieillissement vasculaire pathologique est precoce (infarctus du myocarde) et dans les situations a risque cardiovasculaire eleve (diabete, insuffisance renale chronique, analphalipoproteinemies). Perspectives et projets. – Le polymorphisme genetique de cette enzyme explique en partie les variations des niveaux d’activites paraoxonase 1 seriques. Ceci peut se traduire par une plus ou moins grande sensibilite aux intoxications chroniques par les composes organophosphores et representer un facteur de risque vasculaire. Une diminution de cette activite enzymatique avec le vieillissement pourrait participer a l’accroissement de la prevalence des maladies cardiovasculaires avec l’âge.