Aspirin esterase(s) - Much known, more to learn

Abstract

Esterases, so called because they catalyze hydrolytic splitting of ester bonds, remain quite intriguing – they partake in the biotransformation of drugs/xenobiotics, but their physiological/biological functions remain unknown. An exception is acetyl cholinesterase that catalyses hydrolysis of the neurotransmitter acetylcholine. Hydrolysis of a xenobiotic/drug can, at times, be catalyzed by different esterases in different tissues or organs. For example, aspirin hydrolysis to salicylate and acetate is catalyzed by carboxylesterases (EC 3.1.1.1) in human gastric mucosa and liver [1], resulting in bioavailability of 65% [2]. Once in the systemic circulation, further hydrolysis is catalyzed by other esterases in the plasma (pseudo cholinesterase, EC 3.1.1.8; and albumin) and red blood cells. Albumin-associated esterase activity is minor, and the main plasma aspirin esterase activity is associated with the plasma cholinesterase [3]. About 80% of aspirin esterase activity in the blood is due to the enzyme in erythrocytes. It is different from that in plasma – there is no significant correlation in levels of the two, and the red cell enzyme is resistant to 1.66 mmol/L physostigmine (in final assay system) that completely inhibits the form in plasma [4]. The suggestion that it is probably ‘non specific’ has been contested by the isolation from human erythrocyte of an esterase with specificity for aspirin by Costello and Green [5]. 2. Factors affecting aspirin esterase activity