A naturally occurring molecular form of human plasma cholinesterase is an albumin conjugate

Abstract

Human plasma cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) consists of four main molecular forms designated as C 1, C 2, C 3 and C 4 according to their electrophoretic mobility on gels. The major component, C 4, is the tetrameric form; C 1 and C 3 are the monomeric and dimeric forms, respectively. The C 2 form, which has an apparent free electrophoretic mobility higher than that of the three size isomers, and, moreover, a higher isoelectric point, was found to be a covalent conjugate between the cholinesterase monomer and serum albumin. This result is supported by the following arguments: the non-catalytic subunit of C 2 was found to be a carbohydrate-free protein of apparent molecular mass 65 kDa that could not be labelled by diisopropylfluorophosphonate in the labelling conditions of esterases. It possesses a high affinity for a long-chain aliphatic ligand (a substituted octadecylamine) and for Cibacron blue F3 GA, and could be adsorbed on an immunoadsorbent for albumin. The two subunits of C 2 are disulfide bridge linked; the active center of the cholinesterase subunit is partly masked by the albumin molecule. The conjugation reaction very likely occurs in the hepatic cell and not in plasma.