rate is reached. Elongation is favored at the barbed end of the growing filament. Here, the influence of cytochalasin B strongly depends on the overall conditions for elongation. If ideal physiological conditions are present, the inhibitory influence of cytochalasin B is minuscule. If the conditions are less optimal, elongation can be inhibited by up to 90 percent. Annealing is the last step in polymerization. Cells treated with cytochalasin B and control group cells could not be distinguished. This indicated, that CB has no significant effect at this stage. CB contains a beta-unsaturated ester which can undergo a Michael-type conjugation with nucleophiles. If this is the case, DNA-adduction might be a plausible reaction afterwards. A more suitable reaction seems to be the one with thiol-groups of several biomolecules. The thiol-groups would then no longer be available for disulfide bonds for further actin polymerization and thus a crucial step in actin polymerization is inhibited as the barbed ends of the filaments are blocked. An analogue principle is used by the well-studied capping proteins which are responsible for a natural limiting factor of actin polymerization. The first step in actin polymerization, after polymerization is initiated, is the deprotonation of the thiol group of G-actin. This renders the sulfur atom charged and makes it available for actin polymerization. If cytochalasin B is present in the cell, the deprotonation of thiol is competed. The reactive beta-unsaturated ester group of cytochalasin B reacts with the thiol group of actin via a nucleophilic attack of the charged sulfur onto the beta-carbon atom. This forces the π-bond to get dislocated on the left site of the beta-carbon. Consequently, mesomerism occurs, dislocating the negative charge between the alpha-carbon and the oxygen atom. This step is followed by a protonation step to counteract the negative charge. The hydronium ion needed to do so was produced during the activation of the sulfur atom in an earlier step. Metabolism There are ten possible sites for the in vitro degradation of cytochalasin B. There is not yet any evidence that the same sites are used for degradation in vivo, but evidence has confirmed the in vitro sites. Degradation is initialized by a periodate cleavage of the compound, taking place at carbon 14 and 21. As a result, carbonic acid (A), formaldehyde (B), 5-methylhexane-1,1,6-triol (C) and a large remaining molecule (D) are released. Molecules C and D are then oxidized via Kuhn Roth reaction,