Redox signaling may be extracellular

From Physiological Chemistry and Physics and Medical NMR, 16 (1984),175-195. US patent #5,723,502 incorporates this paper by reference.

"Since SOD and catalase (which probably do not penetrate into cells) ameliorate the toxicity of such agents in vivo, a significant fraction of their toxicity may be due to extracellular production of oxygen metabolites, in line with the relative paucity of defense mechanisms in the extracellular space."

"As outlined in Figure 4, in addition to their direct action on cellular constituents (1,7,16,31-33,38-41,96,100,152) oxygen metabolites may also act as specific modulators of the inflammatory process. For example, in vitro active oxygen species can affect the activity of inflammatory immunomodulators such as interferon (153) leucocyte-dependent inflammatory processes (Reviews, 1,7,16,31-32) (154-156), leucocyte clastogenic factors (154-156) , lymphocyte clastogenic factors(157-159), soluble immune response suppressor (160), serum protease inhibitors (161-164), and vascular permeabi1ity-regulating factors.(165,166). Similarly, extracellular active oxygen species may also directly influence platelet (l67-I71) and fibrocyte (63,172) function. They may also be involved in the metabolism and action of such important modulator substances as the prostaglandins (41,68) leucotrienes,(17I), unsaturated fatty acids (41,68-71) and cyclic nucleotides (61-67). They may also influence interactions between lymphokines and macrophages (l56,173) and lymphocyte function (l74) as well as induce histamine release from mast cells (175). The specificity of such effects suggests that active metabolites of oxygen may be acting more as messenger substances than as non-specific chemical reagents." ...(emphasis-added)

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