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The oligomerization of lysozyme in aqueous solution was investigated by Monte Carlo simulations as a function of protein concentration, pH, and electrolyte screening. Lysozyme was modeled as a hard sphere with embedded pH-dependent discrete charges and with an attractive 1/r⁶-potential representing nonspecific short-range attraction. The magnitude of the 1/r⁶-potential was adjusted to reproduce experimental second virial coefficients. Radial distribution functions, structure factors, cluster size distributions, and orientation correlations were determined at various conditions. It was observed that increasing protein concentration, or decreasing the electrostatic repulsion between protein molecules by either reducing the protein charge or increasing the ionic strength, promoted cluster formation. Structure factors and equilibrium constants obtained were compared to those obtained experimentally and were found to capture the experimentally obtained effects of pH and ionic strength. The influence of the location of the hydrophobic site was also examined.

The complexation between one polyelectrolyte and one protein has been examined by employing a simple model system solved by Monte Carlo simulations. The polyelectrolyte was composed of a sequence of negatively charged hard spheres, and the protein was represented by a hard sphere with embedded pH-dependent discrete charges, the positions of which were taken from lysozyme. A short-range attractive interaction between the polyelectrolyte and the protein accounting for hydrophobic interactions completed the model. The complexation was found to depend decisively on the charge status of the protein model as well as on the presence of the short-range attractive interaction. In particular, the complexation weakens at decreasing ionic strength except for the highest positive protein net charge considered, and in the absence of the short-range attraction, a positively charged protein was required to obtain a complex. The distribution of the polyelectrolyte beads was inhomogeneous at the protein surface, and the polyelectrolyte contracted upon complexation. Finally, the protein model with discrete charges gave a stronger complex than the corresponding protein model with a homogeneous surface charge density.

Five angusticeps-type toxins, F7, F8 and C10S2C2 from Dendroaspis angusticeps and C and FS2 from D. polylepis polylepis, were tested for action on the chick biventer cervicis nerve-muscle, the frog rectus abdominis muscle and the mouse phrenic nerve-diaphragm preparations. In the chick muscle, none of these toxins exhibited any stimulatory effect up to 100 micrograms/ml. In the frog muscle, the response to acetylcholine, but not to carbachol, was enhanced dose dependently by F7 and C. No appreciable effect was observed with the other three toxins. In the mouse diaphragm, also only F7 and C augmented responses to indirect stimulation and produced spontaneous fasciculations. On tetanic stimulation, a marked Wedensky inhibition was observed. Their stimulatory effect was abolished by d-tubocurarine. In the presence of d-tubocurarine as well as in the denervated mouse diaphragm, neither toxin increased responses to direct stimulation. In low-calcium (0.6 mM) or high magnesium (4.2 mM) medium, the stimulatory effect of both toxins was markedly attenuated. The resting membrane potential of the mouse diaphragm was not changed. The amplitude and frequency of MEPPs and the quantal content and the half-decay time of EPPs was increased. Both toxins also produced a stimulatory effect on the isolated guinea-pig ileum, which was abolished by atropine. In the rat atrial preparation, both toxins caused negative inotropic and chronotropic effects, which were reversed by atropine. If pretreated with atropine, these effects were completely prevented. Both F7 and C markedly inhibited the cholinesterase activity of the homogenized mouse diaphragm and frog rectus abdominis muscle but not that of the chick biventer cervicis muscle.(ABSTRACT TRUNCATED AT 250 WORDS)