Plasma esterase activity in patients with aspirin-sensitive asthma or urticaria

Aspirin hydrolysis in plasma is a variable function of butyrylcholinesterase and platelet-activating factor acetylhydrolase 1b2 (PAFAH1b2)

Aspirin is rapidly hydrolyzed within erythrocytes by a heterodimer of PAFAH1b2/PAFAH1b3 but also in plasma by an unidentified activity. Hydrolysis in both compartments was variable, with a 12-fold variation in plasma among 2226 Cleveland Clinic GeneBank patients. Platelet inhibition by aspirin was suppressed in plasma that rapidly hydrolyzed aspirin. Plasma aspirin hydrolysis was significantly higher in patients with coronary artery disease compared with control subjects (16.5 ± 4.4 versus 15.1 ± 3.7 nmol/ml/min; p = 3.4 × 10(-8)). A genome-wide association study of 2054 GeneBank subjects identified a single locus immediately adjacent to the BCHE (butyrylcholinesterase) gene associated with plasma aspirin hydrolytic activity (lead SNP, rs6445035; p = 9.1 × 10(-17)). However, its penetrance was low, and plasma from an individual with an inactivating mutation in BCHE still effectively hydrolyzed aspirin. A second aspirin hydrolase was identified in plasma, the purification of which showed it to be homomeric PAFAH1b2. This is distinct from the erythrocyte PAFAH1b2/PAFAH1b3 heterodimer. Inhibitors showed that both butyrylcholinesterase (BChE) and PAFAH1b2 contribute to aspirin hydrolysis in plasma, with variation primarily reflecting non-genetic variation of BChE activity. Therefore, aspirin is hydrolyzed in plasma by two enzymes, BChE and a new extracellular form of platelet-activating factor acetylhydrolase, PAFAH1b2. Hydrolytic effectiveness varies widely primarily from non-genetic variation of BChE activity that affects aspirin bioavailability in blood and the ability of aspirin to inhibit platelet aggregation.

Intracellular erythrocyte platelet-activating factor acetylhydrolase I inactivates aspirin in blood

Aspirin (acetylsalicylic acid) prophylaxis suppresses major adverse cardiovascular events, but its rapid turnover limits inhibition of platelet cyclooxygenase activity and thrombosis. Despite its importance, the identity of the enzyme(s) that hydrolyzes the acetyl residue of circulating aspirin, which must be an existing enzyme, remains unknown. We find that circulating aspirin was extensively hydrolyzed within erythrocytes, and chromatography indicated these cells contained a single hydrolytic activity. Purification by over 1400-fold and sequencing identified the PAFAH1B2 and PAFAH1B3 subunits of type I platelet-activating factor (PAF) acetylhydrolase, a phospholipase A(2) with selectivity for acetyl residues of PAF, as a candidate for aspirin acetylhydrolase. Western blotting showed that catalytic PAFAH1B2 and PAFAH1B3 subunits of the type I enzyme co-migrated with purified erythrocyte aspirin hydrolytic activity. Recombinant PAFAH1B2, but not its family member plasma PAF acetylhydrolase, hydrolyzed aspirin, and PAF competitively inhibited aspirin hydrolysis by purified or recombinant erythrocyte enzymes. Aspirin was hydrolyzed by HEK cells transfected with PAFAH1B2 or PAFAH1B3, and the competitive type I PAF acetylhydrolase inhibitor NaF reduced erythrocyte hydrolysis of aspirin. Exposing aspirin to erythrocytes blocked its ability to inhibit thromboxane A(2) synthesis and platelet aggregation. Not all individuals or populations are equally protected by aspirin prophylaxis, the phenomenon of aspirin resistance, and erythrocyte hydrolysis of aspirin varied 3-fold among individuals, which correlated with PAFAH1B2 and not PAFAH1B3. We conclude that intracellular type I PAF acetylhydrolase is the major aspirin hydrolase of human blood.

Substrate specificity of purified rabbit liver esterase ES-1

1. Substrate hydrolysis by two purified rabbit liver esterase-1 allozymes (ES-1A and ES-1B) was compared under conditions differing in substrate, pH and temperature. ES-1A and ES-1B activities had a similar pH and temperature dependency and similar thermal stability profile. 2. There were marked differences in specific activity of ES-1A and ES-1B. ES-1A hydrolysed procaine more rapidly than ES-1B, but was less active towards aspirin. The acetate and propionate esters of p-nitrophenyl were hydrolysed slower by ES-1A than by ES-1B. 3. The effect of substrate concentration on ES-1A activity did not comply with the Michaelis-Menten kinetics, which may be due to so-called substrate activation. 4. At identical substrate concentration, pH and temperature, selected artificial esters were better substrates for ES-1A than selected physiological substrates. Beta-Naphthyloctanoate was found to be a suitable substrate for ES-1A. 1,3-Dioctanoylglycerol was hydrolysed at a rate of only 2% of that of beta-naphthyloctanoate. 5. With methyl, p-nitrophenyl, beta-naphthyl and 4-methylumbelliferyl esters as substrates, ES-1A activity is influenced by length and structure of the acyl moiety. Likewise, ES-1A activity is influenced by the nature of the alkyl moiety of acetate esters. With acetate and methyl esters, branched chains when compared with unbranched chains reduced the esterase activity of ES-1A. Elongation of the acyl moiety up to four or five C atoms gradually raised the velocity of methyl, p-nitrophenyl, and 4-methylumbelliferyl ester hydrolysis by ES-1A. A similar pattern was found for the length of the alkyl moiety of acetate esters. 6. The high degree of similarity between the observed substrate specificity of rabbit ES-1A and that reported earlier for rat ES-10, suggests that these two esterases have a common evolutionary origin.