Characterization of lipid-protein interactions in acetylcholinesterase lipoprotein extracted from bovine erythrocytes.

Acetylcholinesterase was released from bovine erythrocytes in hypo-osmotic sodium phosphate buffer. Initially, about 30% of the enzyme was released in a soluble lipoprotein form, and further incubation resulted in the progressive release of the enzyme in a particulate form. Solubilization of the acetylcholinesterase in the particulate fraction with Lubrol WX (2 mg/ml) resulted in the loss of all lipids except a non-exchangeable fraction identified as cardiolipin. Addition of a mixture of erythrocyte phospholipids to the soluble forms and to the Lubrol WX-solubilized enzyme resulted in the formation of particulate forms of the enzyme with increased partial specific volume and Stokes radius, and a break in the Arrhenius plot of the enzyme activity around 20 degrees C. The break in the Arrhenius plot was abolished by treatment of a soluble enzyme preparation with 1.8 M salt (NaCl) in phosphate buffer, conditions that allowed the extraction of cardiolipin from the enzyme by chloroform/methanol. Failure of the high-salt treatment to decrease the Stokes radius made it unlikely that the bound cardiolipin formed a boundary layer or annulus around the protein. It is suggested that cardiolipin is bound to the core of the dimeric protein structure, thereby controlling the acetylcholinesterase activity.