Lysenin is a
protein of 33?kDa in the coelomic fluid (CF) of the earthworm Eisenia foetida. It differs from other biologically active
proteins, such as fetidins, eiseniapore, and coelomic cytolytic factor (CCF-1), that have been found in Eisenia foetida, in terms of both its biochemical and its
biological characteristics. The large coelomocytes and free chloragocytes in the typhlosole of Eisenia foetida appear to be the cells that produce
lysenin since the
mRNA for
lysenin and immunoreactive
lysenin have been found in these cells.
Lysenin binds specifically to
sphingomyelin (SM) but not to other
phospholipids in cell membranes. After binding to the cell membranes of target cells,
lysenin forms oligomers in an SM-dependent manner, with subsequent formation of pores with a hydrodynamic diameter of approximately 3?nm. The biochemical interactions between
lysenin and SM in cell membranes are responsible for the pharmacological activities of
lysenin and of CF that contains
lysenin in vertebrates, such as
hemolysis, cytotoxicity, and contraction of smooth muscle in vitro and vasodepressor activity and lethality in vivo. When incubated with SM-
liposomes, CF and
lysenin lost some or all of their activity, an observation that suggests that SM might be involved in the induction of the various activities of
lysenin and CF. However, in general,
lysenin is neither cytotoxic nor lethal to invertebrates. An attempt has been made to explain the differences in the responses to
lysenin and CF between vertebrates and invertebrates in terms of the presence or absence of SM in the various animals. Among Protostomia, SM is absent in Lophotrochozoa, with the exception of some molluscan species, but it is present in Ecdysozoa, with the exception of Nematomorpha and flies. Among Deuterostomia, Echinodermata and Hemichordata lack SM but SM is found in Chordata. Thus, the difference in terms of the response to
lysenin between invertebrates and vertebrates cannot be fully explained by reference to the presence or absence of SM in the organism.
Lysenin and its antiserum have made it possible to localize SM in the cell membranes. They should be a useful tool for studies of membrane physiology and the role of SM.