Much fundamental information about the structure of the active centre in cholinesterases has been derived from studies on changes of activity with pH. These enzymes show a maximum activity at about pH 7-8. On the acid side, activity falls off in approximately the same manner for all substrates studied. However, in the alkaline range great differences were observed between different esters (Bergmann, Segal, Shimoni & Wurzel, 1956). The bell-shaped form of the pH-activity curves has been interpreted to indicate two different components of the esteratic site, one of which (the nucleophilic group G1) is inactivated competitively

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Chromatography of human myeloperoxidase (MPO) on a heparin-agarose column demonstrated a tight association of the protein with the resin. The electrophoretic mobility of mixtures of MPO and heparin in polyacrylamide gels under nondenaturing conditions was consistent with a strong interaction of the cationic enzyme with the polyanionic polysaccharide. Purified MPO prebound to bovine aorta endothelial cells (BAEC) and supplemented with hydrogen peroxide dose- and time-dependently abrogated the interaction of coagulation factor IX (FIX) with factor IX-binding protein (FIXBP) on the surface of BAEC reflecting oxidative modification of the binding protein. This inactivation of FIXBP required the presence of chloride implicating hypochlorite in the reaction. Hypochlorite and activated neutrophils exerted a similar effect. The oxidative modification of FIXBP was only partially dependent on the addition of hydrogen peroxide and was abolished by exogenous heparin which displaced MPO from the cell surface, emphasizing the functional differences between cell-bound and free enzyme.

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