The intracellular concentration of protein may be as high as 400 mg per ml; thus it seems inevitable that within the cell, numerous protein-protein contacts are constantly occurring. A basic biochemical principle states that the equilibrium of an association reaction can be shifted by ligand binding. This indicates that if within the cell many protein-protein interactions are indeed taking place, some fundamental characteristics of proteins would necessarily differ from those observed in traditional biochemical systems. Accordingly, we measured the effect of eight different proteins on the formation of homodimeric triosephosphate isomerase from Trypanosoma brucei (TbTIM) from guanidinium chloride unfolded monomers. The eight proteins at concentrations of micrograms per ml induced an important increase on active dimer formation. Studies on the mechanism of this phenomenon showed that the proteins stabilize the dimeric structure of TbTIM, and that this is the driving force that promotes the formation of active dimers. Similar data were obtained with TIM from three other species. The heat changes that occur when TbTIM is mixed with lysozyme were determined by isothermal titration calorimetry; the results provided direct evidence of the weak interaction between apparently unrelated proteins. The data, therefore, are strongly suggestive that the numerous protein-protein interactions that occur in the intracellular space are an additional control factor in the formation and stability of proteins.

A nanofiltration (NF) membrane pilot plant was tested to treat water from a spring located in the Tula Valley. Conventional physicochemical parameters and the pathogenic content were analyzed at the inlet and outlet (permeate) of the process. Ninety-five per cent removal of heterotrophic bacteria was obtained by the membrane and complete removal of all other pathogens was achieved. The membrane process performed better than the municipal chlorination facility. The NF process also removed most of the organic matter and removal of emerging pollutants varied from 5 to 100%, depending on the compound. A softening process prevented the membrane system from clogging, and continuous operation was carried out for more than 1 month with minimal maintenance.