Properties of St I and St II, two isotoxins isolated from Stichodactyla helianthus: a comparison

Sticholysins I and II are two highly hemolytic polypeptides purified from t he Caribbean Sea anemone Stichodactyla helianthus. Their high sequence homo logy (93%) indicates that they correspond to isoforms of the same hemolysin . The spectroscopic measurements show a close similarity in the secondary s tructure content, conformation and stability of both toxins. Exposure of th e toxins to high pHs (> 11), a free radical source (AAPH), urea or temperat ure produce permanent changes in the toxin that lead to a significant loss of HA. It is significant to note that this loss of hemolytic activity occur s when other indicators, probably with the only exception of near-UV CD spe ctra, barely detect changes in the protein structure. This emphasizes the s ensitivity of the protein function to changes in the macromolecule conforma tion. The most noticeable difference between both toxins is the considerabl y higher activity of St II, both measured in terms of erythrocyte internal K+ exit or hemolysis; which is related to enthalpic factors. This differenc e is not due to an incomplete association of St I to the membrane. We consi der then that the different pore forming capacity of both toxins in erythro cytes can be explained in terms of the difference in charge of the N-termin al fragment, than can considerably reduce the St I insertion rate in the me mbrane probably due to the negatively charged outer leaflet of the red bloo d cell, without a significant reduction of its capacity to bind to the cell membrane. This electrostatic effect, together with a slightly more relaxed structure in St II, could explain the higher pore forming capacity of St I I in the red blood cell membrane. (C) 2001 Elsevier Science Ltd. All rights reserved.