Formation of Heme-iron Complexes with Nitric Oxide (NO) and Peroxynitrite ( ONOO-) after Ultraviolet Radiation as a Protective Mechanism in Rat Skin

Ultraviolet C (UVC)-irradiated microvessels isolated from rat skin release free nitrogen radicals, i.e. nitric oxide (NO), peroxynitrite (ONOO-) and nitrosocompounds formed from L-arginine. During UVC radiation of microvessels, heme (Fe3+) is released from hemoglobin and reacts with NO to form nitrosyl-heme (Fe2+-NO). The hydroxyl radical (OH•) produced is attached to heme-iron (Fe3+) to form hematin. ONOO- then binds to Fe(OH) and the complex Fe[(OH)ONOO-]2- is formed. Thus, in cases of increased oxidative stress, the free heme can act as an endogenous scavenger of OH• and ONOO-. Furthermore, Fe-NO and Fe[(OH)ONOO-]2- can act as NO donor and as antioxidant in redox cyclic iron-centered heme reactions, respectively. The scavenging-antioxidant properties of heme complexes, which allow it to protect the cells from the cytotoxic effects of the oxygen and/or nitrogen free radicals, were verified by estimating the changes in membrane fluidity of microvessels after UVC radiation. The present study indicates that UVC radiation of the skin acts as a potent stimulator for the formation of Fe-NO and Fe[(OH)ONOO-]2- in microvasculature with cytoprotective effects.

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