Synechococcus sp. RF-1, a unicellular N(2)-fixing cyanobacterium, can grow photosynthetically and diazotrophically in continuous light. How the organism protects its nitrogenase from damage by oxygen is unclear. In cyanobacerial cells, electron transport carriers associated with photosynthesis and respiration are all on the thylakoid membranes and share some common components, including plastoquinone pool and cytochrome b (6) f complex, and the pathways are interacting with each other. In this work, a pulse amplitude modulation (PAM) fluorometer (PAM-101) and an O(2) electrode are used simultaneously to study the chlorophyll a fluorescence and to monitor O(2) exchanges in Synechococcus sp. RF-1 cells. At the CO(2) compensation point, the photochemical quenching activity remained high unless the O(2) was exhausted by the glucose oxidase system (GOS). It indicates that in addition to CO(2), O(2) can also act as electron acceptor to receive electrons derived from Q(A). Studies with various inhibitors of the electron transport chain demonstrated that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and salicylhydroxamic acid (SHAM) inhibited the photoreduction of O(2), while glycolaldehyde, disalicylidenepropanediamine (DSPD), methyl viologen (MV) and KCN did not. These results imply that a KCN-resistant and SHAM-sensitive oxidase transfers electrons generated from Photosystem II to O(2) between cytochrome b (6) f complex and ferredoxin. When SHAM blocked this alternative electron transport pathway, the dinitrogen-fixing activity decreased significantly. The results indicate that a novel oxidase may function as an intracellular O(2)-scavenger in Synechococcus sp. RF-1 cells.

Head and neck squamous cell carcinoma (HNSCC) is a common cancer of the oral cavity. Cisplatin (CDDP) is the ideal chemo-radiotherapy used for several tumor types, but resistance to the drug has become a major obstacle in treating patients with HNSCC. 5-methoxytryptophan (5-MTP), a 5-methoxyindole metabolite of tryptophan metabolism, reduces inflammation-mediated proliferation and metastasis. This study aimed to assess the anti-oral cancer activity of 5-MTP when used alone or in combination with CDDP. Results showed that CDDP dose dependently reduced the growth of SSC25 cells but not 5-MTP. The combination of CDDP and 5-MTP exerted additional inhibitory effect on the growth of SSC25 cells by attenuating the phosphorylation of STAT3. In the 4-nitroquinoline-1-oxide-induced oral cancer mouse model, 5-MTP sensitized the reduction effect of CDDP on tumorigenesis, which restricted the tongue tissue in hyperkeratotic lesion rather than squamous cell carcinoma. The combination of CDDP and 5-MTP may be a potent therapeutic strategy for HNSCC patients with radiotherapy.

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