Characterization of aspirin hydrolase of guinea-pig liver cytoplasm

Design and synthesis of compounds that extend yeast replicative lifespan

This past decade has seen the identification of numerous conserved genes that extend lifespan in diverse species, yet the number of compounds that extend lifespan is relatively small. A class of compounds called STACs, which were identified as activators of Sir2/SIRT1 NAD+-dependent deacetylases, extend the lifespans of multiple species in a Sir2-dependent manner and can delay the onset of age-related diseases such as cancer, diabetes and neurodegeneration in model organisms. Plant-derived STACs such as fisetin and resveratrol have several liabilities, including poor stability and relatively low potency as SIRT1 activators. To develop improved STACs, stilbene derivatives with modifications at the 4' position of the B ring were synthesized using a Horner-Emmons-based synthetic route or by hydrolyzing deoxyrhapontin. Here, we describe synthetic STACs with lower toxicity toward human cells, and higher potency with respect to SIRT1 activation and lifespan extension in Saccharomyces cerevisiae. These studies show that it is possible to improve upon naturally occurring STACs based on a number of criteria including lifespan extension.

Cefpodoxime-proxetil hydrolysis and food effects in the intestinal lumen before absorption: in vitro comparison of rabbit and human material

The luminal and mucosal deesterification of the prodrug ester cefpodoxime-proxetil was studied in human duodenal washings in vitro. Enzymatic hydrolysis of the ester, releasing the active third generation cephalosporin, was observed in luminal washing in the same way as it had previously been observed in the rabbit. Eserine and PMSF and HgCl(2) were potent inhibitors of cefpodoxime-proxetil hydrolysis in luminal washing, suggesting the participation of a cholinesterase in the hydrolysis of cefpodoxime-proxetil. These results are in agreement with our previous findings performed in the rabbit. Moreover, cefpodoxime-proxetil directly decreases the acetylcholinesterase activity when tested by a specific enzymatic method. These observations support the hypothesis that the partial oral bioavailability of cefpodoxime-proxetil results from hydrolysis by luminal cholinesterases. In vitro experiments run with rabbit duodenal washing with food components were compared with the pharmacokinetics of cefpodoxime-proxetil in humans. Amino acids, trace elements and vitamins were potent inhibitors for cefpodoxime-proxetil hydrolysis in duodenal washings. Otherwise, lipids (LTC and mixed LCT/MCT) did not interact. In the human, cefpodoxime-proxetil bioavailability is significantly enhanced when tablets are administered with food. The correlation found between animal results and human results in vitro for prospective investigation of a new prodrug ester could be very useful. An in vitro hydrolysis in intestinal animal washings could allow the potentially degraded condition and the food effect of the luminal tract to be assessed before absorption.

Esterase activity in rat hepatocytes

Hydrolysis of acetylsalicylate, benorylate, phenetsal, fluazifop butyl and paraoxon has been studied with freshly isolated rat hepatocytes maintained as a monolayer. Acetylsalicylate and paraoxon were the poorest substrates for hydrolysis whereas benorylate was hydrolysed one hundred times faster. Phenetsal and fluazifop butyl were both hydrolysed at one-tenth of the rate of benorylate. Inhibitor studies with paraoxon, BNPP and physostigmine indicated the involvement of different carboxylesterase isozymes. Studies with acetylsalicylate indicated that uptake of the substrate into the hepatocyte may influence the rate of formation of the hydrolysis product. Studies of hydrolysis in hepatocytes more closely reflect in vivo hepatic hydrolysis than subcellular fractions as cytosolic and microsomal esterases can act in parallel.

Aspirin hydrolyzing esterases from rat liver cytosol

Unlike most esterases, which are predominantly bound to the microsomal fraction, the enzymes hydrolyzing acetylsalicylic acid are present in an equal amount in the cytosol. Two soluble isozymes were purified to homogeneity from rat liver and characterized as serine esterases with a Mr of 35,000. Both had the wide substrate spectrum characteristic of enzymes active in detoxication. Both had a very low Km for acetylsalicylate. Three other cytoplasmic enzymes active with aspirin were observed but these differed in their high Mr (about 220,000) and their lack of reactivity with antibody to one of the homogeneous isozymes.

Human liver and plasma aspirin esterase

The plasma, in addition to the liver, is a major site of hydrolysis of aspirin. Human plasma and liver aspirin esterase activities in samples from a group of patients varied over a two fold range and there was a significant correlation between individual plasma and liver activities. Human liver aspirin esterase was present in the cytosolic and microsomal fractions. Cytosolic and microsomal enzymes had different activities and apparent affinities for aspirin.

Clinical significance of esterases in man

Esterases, hydrolases which split ester bonds, hydrolyse a number of compounds used as drugs in humans. The enzymes involved are classified broadly as cholinesterases (including acetylcholinesterase), carboxylesterases, and arylesterases, but apart from acetylcholinesterase, their biological function is unknown. The acetylcholinesterase present in nerve endings involved in neurotransmission is inhibited by anticholinesterase drugs, e.g. neostigmine, and by organophosphorous compounds (mainly insecticides). Cholinesterases are primarily involved in drug hydrolysis in the plasma, arylesterases in the plasma and red blood cells, and carboxylesterases in the liver, gut and other tissues. The esterases exhibit specificities for certain substrates and inhibitors but a drug is often hydrolysed by more than one esterase at different sites. Aspirin (acetylsalicylic acid), for example, is hydrolysed to salicylate by carboxylesterases in the liver during the first-pass. Only 60% of an oral dose reaches the systemic circulation where it is hydrolysed by plasma cholinesterases and albumin and red blood cell arylesterases. Thus, the concentration of aspirin relative to salicylate in the circulation may be affected by individual variation in esterase levels and the relative roles of the different esterases, and this may influence the overall pharmacological effect. Other drugs have been less extensively investigated than aspirin and these include heroin (diacetylmorphine), suxamethonium (succinylcholine), clofibrate, carbimazole, procaine and other local anaesthetics. Ester prodrugs are widely used to improve absorption of drugs and in depot preparations. The active drug is released by hydrolysis by tissue carboxylesterases. Individual differences in esterase activity may be genetically determined, as is the case with atypical cholinesterases and the polymorphic distribution of serum paraoxonase and red blood cell esterase D. Disease states may also alter esterase activity.

Partial purification and characterization of a microsomal carboxylesterase specific for salicylate esters from guinea-pig liver

Studies on liver carboxylesterases have predominantly involved the use of uncharged ester and amide substrates to monitor activity. A microsomal carboxylesterase (EC 3.1.1.1) from guinea-pig liver microsomes has been identified which specifically hydrolyses aspirin (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773), a substrate which is negatively charged at physiological pH, and this work describes its partial purification and characterization. The enzyme is monomeric, it has a molecular weight of approx. 55 000 and is very sensitive to inhibition by the carboxylesterase inhibitor bis(4-nitrophenyl)phosphate. Although it could not be completely separated from contaminating carboxylesterases, substrate specificity was investigated using the negatively charged esters of salicylic acid. The enzyme is not specific for the acetyl ester of salicylic acid, aspirin, but hydrolyses the longer chain esters more rapidly, with the highest Vmax for the n-octanoyl ester. The enzyme was subject to substrate inhibition which increased with increasing chain length of the fatty acid on the ester, and approached 100% inhibition at concentrations of substrate below critical micellar concentrations.