The neutral red uptake assay provides a quantitative estimation of the number of viable cells in a culture. It is one of the most used cytotoxicity tests with many biomedical and environmental applications. It is based on the ability of viable cells to incorporate and bind the supravital dye neutral red in the lysosomes. Most primary cells and cell lines from diverse origin may be successfully used. Cells are seeded in 96-well tissue culture plates and are treated for the appropriate period. The plates are then incubated for 2 h with a medium containing neutral red. The cells are subsequently washed, the dye is extracted in each well and the absorbance is read using a spectrophotometer. The procedure is cheaper and more sensitive than other cytotoxicity tests (tetrazolium salts, enzyme leakage or protein content). Once the cells have been treated, the assay can be completed in <3 h.

Redox properties of cytochrome b559 (Cyt b559) and cytochrome c550 (Cyt c550) have been studied by using highly stable photosystem II (PSII) core complex preparations from a mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus with a histidine tag on the CP43 protein of PSII. Two different redox potential forms for Cyt b559 are found in these preparations, with a midpoint redox potential ( E'(m)) of +390 mV in about half of the centers and +275 mV in the other half. The high-potential form, whose E'(m)is pH independent, can be converted into the lower potential form by Tris washing, mild heating or alkaline pH incubation. The E'(m) of the low-potential form is significantly higher than that found in other photosynthetic organisms and is not affected by pH. The possibility that the heme of Cyt b559 in T. elongatus is in a more hydrophobic environment is discussed. Cyt c550 has a higher E'(m)when bound to the PSII core (-80 mV at pH 6.0) than after its extraction from the complex (-240 mV at pH 6.0). The E'(m) of Cyt c550 bound to PSII is pH independent, while in the purified state an increase of about 58 mV/pH unit is observed when the pH decreases below pH 9.0. Thus, Cyt c550 seems to have a single protonateable group which influences the redox properties of the heme. From these electrochemical measurements and from EPR controls it is proposed that important changes in the solvent accessibility to the heme and in the acid-base properties of that protonateable group could occur upon the release of Cyt c550 from PSII.

Cytochrome c 550 is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c 550 we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by ؉125 mV and ؊30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c 550 is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c 550 was also constructed. It showed a lowered affinity for Cl ؊ and Ca 2؉ as reported earlier for the cytochrome c 550 -less Synechocystis 6803 mutant, but it showed a shorter lived S 2 Q B Ϫ state, rather than a stabilized S 2 state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c 550 .Cytochrome c 550 (cyt c 550 ), 1 present in cyanobacteria and red algae, was first discovered by Holton and Myers (1) as a soluble monoheme c-type cytochrome. The cyt c 550 has a molecular mass of about 15 kDa, His/His coordination and a very low redox potential around Ϫ260 mV (for review see Ref.2). showed that cyt c 550 is stoichiometrically bound to the Photosystem II (PS II), activates oxygen evolving activity and allows the binding of the 12 kDa protein, another extrinsic component of the cyanobacterial PS II involved in oxygen evolution. The three-dimensional structure of the PS II from two thermophilic cyanobacteria strains confirmed that cyt c 550 binds to the luminal PS II surface in the vicinity of the D1 and CP43 proteins (6 -8). The phenotype of the ⌬psbV (cyt c 550 -less) mutant of Synechocystis PCC 6803 was already characterized with respect to PS II activity (9 -11). The ⌬psbV and the double ⌬psbV/⌬psbU (encoding the 12 kDa protein) mutants were unable to grow in the absence of Ca 2ϩ and Cl Ϫ ions, their PS II activity decreased to 40% of the wild type, and they showed a very rapid inactivation of the enzyme in the dark (after 2 h, the activity decreased about 90% whereas in the wild type only 25% of the activity was lost). A slight retardation in O 2 release from the S 3 state was also observed in these mutants. Another effect observed in these Synechocystis PCC 6803 mutants was a large intensity decrease of the B band of thermoluminescence (TL) and an upshift in the temperature maxima of the ...