Studies on the catalytic behaviour of a cholinesterase-like abzyme in an AOT microemulsion system

The hydrolytic activity of a monoclonal catalytic antibody (9A8) (abzyme) with acetylcholinesterase-like activity was investigated in water-in-oil (w/o) microemulsions (reverse micelles) based on sodium bis-2-(ethylhexyl)sulfosuccinate (AOT) in isooctane, using p- and o-nitrophenylacetate (p-and o-NPA) as substrates. The dependence of the abzyme hydrolytic activity on the molar ratio of water to surfactant (w(o)) showed a bell-shaped curve, presenting a maximum at w(o)=11.1. An increase of the AOT concentration at constant w(o), resulted in a decrease of the catalytic activity suggesting a possible inhibition effect of the surfactant. The incorporation of the abzyme into the reverse micelle system caused a blue shift of the fluorescence emission maximum by a magnitude of 7-10 nm depending on the w(o) value. This result indicates that the antibody molecule, or a large part of it, is located in the aqueous microphase of the system. Kinetic studies showed that the hydrolysis of p-and o-NPA in microemulsion system as well as in aqueous solution follows Michaelis-Menten kinetics. The catalytic efficiency (k(cat)/K(m)) in w/o microemulsion was significant lower than in aqueous solution.