UV INACTIVATION OF ENZYMES IN SUPRAMOLECULAR COMPLEXES OF BIOLOGICAL MEMBRANES. THE PHENOMENON OF PHOTOCHEMICAL ALLOTOPY

Abstract. The photosensitivity of erythrocyte acetylcholinesterase (AChE) is different in its free and membrane-bound states. The modification of the structure of membraneous lipids by phospholipases A₂, C and D or by cholesterol depletion is accompanied by a change in AChE photosensitivity. UV light was demonstrated to induce cooperative structural transitions in the erythrocyte membrane. This follows from the data obtained by circular dichroism and solubilization in detergents. In contrast to free AChE, UV light acts on the membraneous enzyme as a mixed inhibitor (simultaneous change in V _max and K _m). The anomalous behaviour of membrane-bound enzyme, termed the phenomenon of photochemical allotopy, is associated with a modification of the structure within the microenviron-ment of the residual AChE. The phenomenon depends on membrane integrity, and disappears after treatment of erythrocyte ghosts with ultrasound, trypsin, phospholipases and neuraminidase and remains unchanged in cholesterol-depleted membranes. The nature and localization of events responsible for this phenomenon are discussed.