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Isolation and complete covalent structure of liver microsomal paraoxonase.

ABSTRACT: Paraoxonase (PON1) is a serum esterase exclusively associated with high-density lipoproteins; it might confer protection against coronary artery disease by destroying pro-inflammatory oxidized lipids in oxidized low-density lipoproteins. Here I show that rabbit liver microsomes contain a PON analogue (MsPON) and report the isolation and complete covalent structure of MsPON. In detergent-solubilized microsomes, MsPON co-purifies with the microsomal triacylglycerol transfer protein (MTP) complex. MsPON was separated from the complex and purified to homogeneity under non-denaturing conditions. Automated sequence analysis of intact MsPON and peptides obtained from enzymic and chemical cleavages led to the elucidation of the complete covalent structure of MsPON. The protein is a single polypeptide consisting of 350 residues. The sequence of rabbit liver microsomal MsPON is 60% identical with that of rabbit serum PON1, and 84% identical with the sequence predicted by a human cDNA of unknown function, designated PON3. MsPON has a hydrophobic segment at the N-terminus that might serve to anchor the protein to the microsomal membrane or to the MTP complex. Unlike in the serum enzyme, two potential N-glycan acceptor sites in MsPON are not glycosylated. An absence of N-glycans was also indicated in the rabbit liver MTP. MsPON has a single free cysteine residue at position 38 and a disulphide bond between Cys-279 and Cys-348. The microsomal enzyme lacks three residues at the N-terminus that are present in the serum protein. MsPON lacks four residues at the C-terminus that are present in the rabbit serum protein but absent from human serum PON1. On the basis of the observation that MsPON displays a high degree of similarity with serum PON1, it is proposed that MsPON might have a function related to that of PON1 in serum high-density lipoprotein complexes.

SUBMITTER: Ozols J

PROVIDER: S-EPMC1220051 | biostudies-other | 1999 Mar

REPOSITORIES: biostudies-other

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Project description:Atorvastatin ?-lactone, a major, pharmacologically inactive metabolite, has been associated with toxicity. In a previous study we showed that polymorphisms of UGT1A3 influence atorvastatin ?-lactone formation. Here we investigated the reverse reaction, atorvastatin ?-lactone hydrolysis, in a human liver bank. Screening of microarray data revealed paraoxonases PON1 and PON3 among 17 candidate esterases. Microsomal ?-lactone hydrolysis was significantly correlated to PON1 and PON3 protein (r(s)?=?0.60; r(s)?=?0.62, respectively; P?<?0.0001). PON1 and PON3 were strongly correlated to each other (r(s)?=?0.60) but PON1 was shown to be more extensively glycosylated than PON3. In addition a novel splice-variant of PON3 was identified. Genotyping of 40 polymorphisms within the PON-locus identified PON1 promoter polymorphisms (-108T?>?C, -832G?>?A, -1741G?>?A) and a tightly linked group of PON3 polymorphisms (-4984A?>?G, -4105G?>?A, -1091A?>?G, -746C?>?T, and F21F) to be associated with changes in atorvastatin ?-lactone hydrolysis and expression of PON1 but not PON3. However, carriers of the common PON1 polymorphisms L55M or Q192R showed no difference in ?-lactone hydrolysis or PON expression. Haplotype analysis revealed decreased ?-lactone hydrolysis in carriers of the most common haplotype *1 compared to carriers of haplotypes *2, *3, *4, and *7. Analysis of non-genetic factors showed association of hepatocellular and cholangiocellular carcinoma with decreased PON1 and PON3 expression, respectively. Increased C-reactive protein and ?-glutamyl transferase levels were associated with decreased protein expression of both enzymes, and increased bilirubin levels, cholestasis, and presurgical exposure to omeprazole or pantoprazole were related to decreased PON3 protein. In conclusion, PON-locus polymorphisms affect PON1 expression whereas non-genetic factors have an effect on PON1 and PON3 expression. This may influence response to therapy or adverse events in statin treatment.

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Isolation and complete covalent structure of liver microsomal paraoxonase.

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