Cytolytic

Entamoeba histolytica is an anaerobic rhizopod that occurs in tropical and subtropical areas. It can cause intestinal and extraintestinal manifestations. It is transmitted orally by ingestion of cysts that develop into trophozoites in the large intestine. Amebic trophozoites release several cytolytic factors, e.g. amoe-bapore, which enable the parasite to invade tissue. In intestinal amoebiasis, E. histolyticatrophozoites invade the intestinal mucosa, causing a form of ulcerative colitis with bloody and mucous diarrhoea. Extraintestinal manifestation of amebiasis results in abscess formation, usually in the liver but sometimes in the brain. 

The characteristics and possible mechanisms of action of cytolytic peptides isolated from sea anemones during the past 20 years are described. These agents fall into three categories (1)... 

Cytolytic peptides derived from 16 species of anemones currently comprise this group (Table I). It is likely that as additional species are examin in the future... 

Homogenates of MetruUum senile, possibly the world s most common large sea anemone, yield extracts that are powerfully hemolytic for washed mammalian erythrocytes (22). The active substance, metridiolysin, is a protein of molecular weight approximately 80,000. In contrast to the sphingomyelin-inhibitable toxins, metridiolysin is an acidic protein having a pi of about 5. It is thermolabile and is inactivat by proteolytic enzymes. The optimal pH for hemolysis is between 5 and 6, and at pH 8 the lysin is inactive. It can be dissociated into two subunits of unequal size. Besides being cytolytic in vitro, metridiolysin is lethal when injected intravenously into mice. As shown in Table IV erythrocytes from the horse or dog are about a hundred times as sensitive to lysis as those from the mouse, and erythrocytes from other animals tested are intermediate in sensitivity. 

In contradistinction to site-specific molecules there are other toxins which, in a sense, create their own specificity. Thus ionophore toxins, by opening up the cell membrane to sodium and calcium ions, create their own specificity. Prime examples of this are found in cytolytic toxins in general, and jellyfish toxins and palytoxin in particular. 

Many of the toxins obtained from coelenterates and echinoderms, because of their hemolytic or cytotoxic actions, are assumed to have a general disruptive action on cell membranes. However, since many of these toxins are capable of forming pores or channels in the plasma membrane of cells, their cytolytic actions may be a result of this highly selective action. On the other hand, the saponins from starfish and sea cucumbers have a direct lytic action as a result of their detergent action on the integrity of cells. 

Marine toxins are not always acutely toxic. This may be particularly so for toxins which are used to deter competing occupants for living space since they often have comparatively slow actions on growth. With such toxins, the procedures for the evaluation of acutely lethal toxins cannot apply. However, interesting discoveries may be made by using the simplest of screen of alcoholic extracts for cytolytic actions as exemplified in Table I of Shier 109),... 

Modeling Pardaxin Channel. The remarkable switching of conformation in the presence of detergents or phospholipid vesicles (5) suggests that pardaxin is a very flexible molecule. This property helps to explain the apparent ability of pardaxin to insert into phospholipid bilayers. In addition, it is consistent with the suggestion that the deoxycholate-like aminoglycosteroids (5,7) present in the natural secretion from which pardaxin is purified (5) serve to stabilize its dissociated conformation. The question of the mechanism by which pardaxin assembles within membranes is important for understanding pore formation and its cytolytic activity (5). 

The natural killer cells (NK) are the host s primary innate immune responders against viral infections. Studies have shown morphine to suppress the cytolytic activity of NK cells (Shavit et al. 2004). In vivo studies carried out in the Indian rhesus macaques looked at chronic morphine administration and SIV the equivalent of HIV in apes. This group concluded that morphine contributed to the pathogenesis of Simian Immunodeficiency Virus (SIV) infection and that this contribution occurred in conjunction with the replication of viral proteins including Tat (Noel and Kumar 2006 Noel et al. 2006). 

Weltzem, H.U. (1979). Cytolytic and membrane-perturbing properties of lysophosphatidylcholine. Biochim. Biophys. Acta 559, 259-287. 

Wagner L, Yang OO, Garcia-Zepeda EA, et al. Beta-chemokines are released from HIV-l-specihc cytolytic T-cell granules complexed to proteoglycans. Nature 1998 391(6670) 908-911. 

Limited studies are available that indicate the distribution of cytolytic agents inversely matches the locations of so-called "seed beds" of P. brevis. 

The inhibitory effects of in vitro sterol addition (21) showed large percentage kills (83% and 91% respectively). The in vitro addition of ergosterol (10.1 mM) to P brevis cell cultures with Filipin (1.5 mM) showed complete inhibition of the cytolytic effect. Ergosterol (10.1 mM) added to P. brevis cell cultures with cell extract showed a 10% reduction in cell mortality. The addition of ergosterol alone (control) showed no cytolytic effect at the experimental concentration (10.1 mM). 

The latter behavior is significant because the P. brevis would be rendered ineffective and ichthyotoxins would presumably not be released from sessile cells. All fractions (Fig. 1) were investigated by Pabon and Martin (25). After initial screening of fractions (Figure 1) for activity, it appeared that sessile-formation activity was associated with first ten fractions (minus cytolytic fraction 7), and further investigation indicated that fraction 4 was responsible for induction of cytolysis (at 500 ppb). Clearly, additional studies are in order, but the evidence of a mechanism that causes an additional disfunction of P. brevis cells is encouraging. 

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