It is now widely accepted that the various pharmacologic and addictive consequences of alcohol consumption are related to the tissue concentration of ethanol or its metabolic products. The oxidative metabolism of ethanol in liver is principally catalyzed by alcohol dehydrogenase and aldehyde dehydrogenase. Both of these enzymes exist in multiple molecular forms, and genetic models have been proposed to account for the multiplicity of isoenzymes. Alcohol dehydrogenase subunits are encoded at five different gene loci, and genetic polymorphism occurs at two alcohol dehydrogenase loci. Variant isoenzymes produced at the two polymorphic alcohol dehydrogenase loci account for the differences in enzyme electrophoretic patterns observed among individuals. Some of these variant isoenzymes exhibit widely different kinetic properties, and this may account for the 2- to 3-fold variation in alcohol elimination rate among individuals. Since the protein sequence of several of the alcohol dehydrogenase subunits has been determined and several of the alcohol dehydrogenase genes has been cloned, some of the structural changes which give rise to differences in catalytic and electrophoretic properties are now known. Genetic polymorphism also occurs at the aldehyde dehydrogenase gene locus which encodes the mitochondrial low Km for acetaldehyde aldehyde dehydrogenase isoenzyme. The variant isoenzyme exhibits little or no catalytic activity. Individuals with this "null" variant have higher than normal blood acetaldehyde levels and exhibit an alcohol-flush reaction which appears to be a deterrent to heavy drinking and alcoholism.(ABSTRACT TRUNCATED AT 250 WORDS)

Many Orientals lack the mitochondrial aldehyde dehydrogenase (ALDH2) activity responsible for the oxidation of acetaldehyde produced during ethanol metabolism. These individuals suffer the alcohol-flush reaction when they drink alcoholic beverages. The alcohol-flush reaction is the result of excessive acetaldehyde accumulation, and the unpleasant symptoms tend to reduce alcohol consumption. The subunit of this homotetrameric enzyme was sequenced and the abnormality in the inactive enzyme shown to be a substitution of lysine for glutamate at position 487. We have used the polymerase chain reaction to determine the genotypes of 24 livers from Japanese individuals. Correlating genotype with phenotype leads to the conclusion that the allele (ALDH22) encoding the abnormal subunit is dominant.

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A human liver carboxylesterase (hCE-2) that catalyzes the hydrolysis of the benzoyl group of cocaine and the acetyl groups of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine was purified from human liver. The purified enzyme exhibited a single band on SDS-polyacrylamide gel electrophoresis with a subunit mass of approximately 60 kDa. The native enzyme was monomeric. The isoelectric point of hCE-2 was approximately 4.9. Treatment with endoglycosidase H caused an increase in electrophoretic mobility indicating that the liver carboxylesterase was a glycoprotein of the high mannose type. The complete cDNA nucleotide sequence was determined. The authenticity of the cDNA was confirmed by a perfect sequence match of 78 amino acids derived from the hCE-2 purified from human liver. The mature 533-amino acid enzyme encoded by this cDNA shared highest sequence identity with the rabbit liver carboxylesterase form 2 (73%) and the hamster liver carboxylesterase AT51p (67%). Carboxylesterases with high sequence identity to hCE-2 have not been reported in mouse and rat liver. hCE-2 exhibited different drug ester substrate specificity from the human liver carboxylesterase called hCE-1, which hydrolyzes the methyl ester of cocaine. hCE-2 had higher catalytic efficiencies for hydrolysis of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine and greater inhibition by eserine than hCE-1. hCE-2 may play an important role in the degradation of cocaine and heroin in human tissues.Carboxylesterases hydrolyze ester groups of drugs and toxins, and thus play an important role in their metabolism and detoxication. In humans, cocaine and heroin are metabolized in serum and liver tissue to de-esterified metabolites that are excreted in urine. Two different human liver carboxylesterases (here called hCE-1 and hCE-2) 1 were identified and shown to hydrolyze cocaine (1). hCE-1 catalyzes the hydrolysis of the methyl ester of cocaine producing benzoylecgonine and methanol, and the ethyl transesterification of cocaine with ethanol to form cocaethylene and methanol. Benzoyl ester hydrolysis of cocaine is catalyzed by liver hCE-2 (1, 2) and serum cholinesterase (3, 4) and produces ecgonine methyl ester and benzoic acid. Hydrolysis of cocaine by serum cholinesterase and nonenzymatic hydrolysis of the methyl ester of cocaine (1) have complicated the assessment of the relative roles of tissue carboxylesterases involved in cocaine metabolism. Heroin is metabolized by liver hCE-2 and hCE-1 carboxylesterases and serum cholinesterase (5). All three esterases cleave the 3-acetyl group of heroin producing 6-monoacetylmorphine. However, only hCE-2 hydrolyzes 6-monoacetylmorphine to morphine with high catalytic efficiency (5). Hence, the contribution of hCE-2 to the metabolism of cocaine or heroin is only recently recognized and its relative role in drug ester clearance has not been fully elucidated.Human liver hCE-2 and hCE-1 enzymes belong to the family of carboxylesterases with a subunit size of about 60 kDa that exhibit broad subst...

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Mammalian alcohol dehydrogenase (ADH) is thought to be involved in the reversible oxidation of vitamin A or retinol to retinal for retinoic acid synthesis. Retinoic acid is a potent transcriptional regulator and a morphogen. It was proposed that the competition of consumed ethanol with retinol oxidation by ADH might explain developmental disorders seen with fetal alcohol syndrome. We report herein the relative efficiency (V/Km) of eight human ADH isoenzymes for oxidation of all-trans-retinol and reduction of three retinal isomers (all-trans, 9-cis, and 13-cis-retinal). Class IV sigma sigma and class II pi pi isoenzymes are the most efficient forms, with V/Km values approximately 100 and 30 times greater, respectively, than class I beta 1 beta 1 or gamma 1 gamma 1, sigma sigma exhibits the highest V/Km (1-2 microns-1min-1), followed by pi pi, with V/Km of 0.5-0.6 microns-1min-1 for all-trans-retinol, all-trans-retinal, and 9-cis-retinal. pi pi also has the lowest Km (11-14 microns) for all-trans-retinol and three retinal isomers. alpha alpha shows an intermediate efficiency, with V/Km of 0.09-0.2 microns-1min-1 and a relatively low Km of 16-24 microns for all four substrates. alpha alpha has the highest efficiency of all tested isoenzymes for 13-cis-retinal. Class III chi chi is inactive with all the tested retinoids.(ABSTRACT TRUNCATED AT 250 WORDS)

Methylphenidate is an important stimulant prescribed to treat attention-deficit hyperactivity disorder. It has two chiral centers, but most current commercial formulations consist of the racemic mixture of the threo pair of methylphenidate isomers (d-, l-threo-methylphenidate). The d-isomer is the pharmacologically active component. Numerous studies reported that oral administration of the methylphenidate racemate undergoes first-pass, stereoselective clearance in humans with l-methylphenidate being eliminated faster than d-methylphenidate. Accordingly, the kinetics of hydrolysis of individual enantiomers by purified native and recombinant human liver carboxylesterases CES1A1 and CES2 and a colon isoenzyme CES3 were examined with a liquid chromatography/mass spectrometry assay. The expression of CES1A1, CES2, and CES3 in Sf9 cells and the methods for purification of the three isoenzymes are reported. CES1A1 has a high catalytic efficiency for both d-and l-enantiomers of methylphenidate. No catalytic activity was detected with CES2 and CES3 for either enantiomer. The catalytic efficiency of CES1A1 for l-methylphenidateHence, the catalytic efficiency of CES1A1 for methylphenidate enantiomers agrees with stereoselective clearance of methylphenidate reported in human subjects. Both enantiomers of methylphenidate can be fit into the three-dimensional model of CES1A1 to form productive complexes in the active site. We conclude that CES1A1 is the major enzyme responsible for the first-pass, stereoselective metabolism of methylphenidate.

Capecitabine is an oral prodrug of 5-fluorouracil that is indicated for the treatment of breast and colorectal cancers. A three-step in vivo-targeted activation process requiring carboxylesterases, cytidine deaminase, and thymidine phosphorylase converts capecitabine to 5-fluorouracil. Carboxylesterases hydrolyze capecitabine's carbamate side chain to form 5Ј-deoxy-5-fluorocytidine (5Ј-DFCR). This study examines the steady-state kinetics of recombinant human carboxylesterase isozymes carboxylesterase (CES) 1A1, CES2, and CES3 for hydrolysis of capecitabine with a liquid chromatography/mass spectroscopy assay. Additionally, a spectrophotometric screening assay was utilized to identify drugs that may inhibit carboxylesterase activation of capecitabine. CES1A1 and CES2 hydrolyze capecitabine to a similar extent, with catalytic efficiencies of 14.7 and 12.9 min Ϫ1 mM Ϫ1