Action of proteases on human plasma cholinesterase isoenzymes

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

Human plasma cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) consists of four main molecular forms designated as C1, C2, C3 and C4 according to their electrophoretic mobility on gels. The major component, C4, is the tetrameric form; C1 and C3 are the monomeric and dimeric forms, respectively. The C2 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 C2 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 C2 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.

Quantitative monitoring by high-performance liquid chromatography of the dissociation of human serum cholinesterase by limited proteolysis

Proteolytic action on human serum cholinesterase, a tetrameric enzyme, results in a partial disintegration which can be recorded only qualitatively by time-consuming electrophoretic techniques. In this study, a rapid high-performance liquid chromatographic method was used for the separation and determination of the active dissociation products. Separation of the cholinesterase subunits was accomplished by high-performance gel permeation chromatography on a combination of DIOL columns (Zorbax GF 450/GF 250) in 0.2 M phosphate buffer (pH 7.0). Detection and quantification of enzyme activity in the fractionated eluate were carried out using a Flexigem analyser (substrate, butyrylthiocholine). On limited tryptic digestion of partially purified human ChE, up to three peaks of enzyme activity could be identified. Their elution volumes corresponded to apparent molecular masses of 480,000, 270,000 and 120,000, indicating, in addition to the tetrameric holoenzyme, a dimeric and a monomeric form. Quantification of the relative amounts of individual enzyme activity peaks revealed that in the course of degradation, the dimer appeared first, followed by the monomer. This suggests that the first step in the sequence of dissociation is cleavage of the tetramer into a pair of dimers, then further into the monomeric subunit. During the incubation with trypsin, a significant change in the pattern of the different peaks had already occurred when the total enzyme activity was only slightly reduced.

Human-serum cholinesterase subunits and number of active sites of the major component

The major C4 component of human serum cholinesterase was highly purified by a two-step procedure involving chromatography on DEAE-cellulose and preparative disc electrophoresis. The final product was about 8 000-fold purified with a yield of 64%. The subunit structure was determined by 8M urea polyacrylamide disc electrophoresis and by the sedimentation equilibrium centrifugation method in 5M guanidine hydrochloride. It was found that the C4 enzyme has a tetrameric structure. The subunits are equal in size and charge and a molecular weight comparable to that of the C1 enzyme from native serum. The major C4 enzyme and the minor C1 enzyme were subjected to an 'active enzyme centrifugation'. It was found that the C4 enzyme was a tetramer and the C1 enzyme was a monomer in the presence of substrate. The number of diisopropylphosphofluoridate-binding sites was measured from the molar ratio of bound diisopropylphosphate to protein. A value close to two binding sites was found for the C4 enzyme.

Characterization of alternative molecular forms of xanthine oxidase in the mouse

1. Two major forms of xanthine oxidase are demonstrated for the mouse. On polyacrylamide-gel electrophoresis the duodenal form migrates faster towards the anode than that of the liver. Both forms also differ in their (NH(4))(2)SO(4) precipitation patterns and sucrose-density-gradient molecular-weight determinations. 2. The liver form is fully converted into the duodenal form by incubation at 37 degrees C with 2.5mg of crude trypsin/ml for 1(1/2)h, without loss of activity. The trypsin-treated liver form behaves like the normal duodenal form as characterized by electrophoresis, (NH(4))(2)SO(4) precipitation patterns, and sucrose-density-gradient molecular-weight determinations. 3. Partial conversion is also brought about by purified trypsin and chymotrypsin, but not with beta-carboxypeptidase or lipase. The conversion is inhibited by soya-bean trypsin inhibitor. 4. In embryo mice the duodenal form is similar to the liver form on electrophoresis. 5. These studies indicate, as might be expected, that the duodenal form is a modified version of the liver enzyme, probably caused by proteolytic alteration.