METHYLENE-BLUE PLUS LIGHT-INDUCED LIPID-PEROXIDATION IN RAT-LIVER MICROSOMES - INHIBITION BY NICOTINAMIDE (VITAMIN-B-3) AND OTHER ANTIOXIDANTS

Methylene blue plus visible light, in the presence of oxygen, induced lipid peroxidation in rat liver microsomes, as assessed by the formati on of thiobarbituric acid reactive substances (TBARS), lipid hydropero xides and the loss of membrane-bound enzymes. Peroxidation was enhance d by deuteration of the buffer and inhibited by scavengers of singlet oxygen (O-1(2)) and superoxide (O-2(-)). The damage induced seemed to be mainly due to Type II involving O-1(2) and to a lesser extent Type I reactions with O-2(-) and hydroxyl radical ((OH)-O-.) as intermediat es. Nicotinamide or vitamin B-3, an endogenous metabolite occurring at high concentrations in tissues, had a relatively high rate constant o f 1.8 x 10(8) M(-1) s(-1) with (1)0(2) and had a significant inhibitor y effect on lipid peroxidation induced by photosensitization. This eff ect was both time- and concentration-dependent, high inhibition being associated with millimolar concentrations. Chemically related endogeno us compounds like tryptophan and isonicotinic acid also had significan t inhibitory properties. Similar protective effects were observed with natural antioxidants such as beta-carotene, canthaxanthin, lipoic aci d, glutathione, alpha-tocopherol and to a lesser extent ascorbic acid. Nicotinamide was a more effective antioxidant than ascorbic acid. It also showed a similar inhibitory effect against NADPH-ADP-Fe3+-induced lipid peroxidation. Our results suggest that nicotinamide had signifi cant ability to protect against photosensitization-induced cytotoxicit y and cell damage and that it may do so by its ability to react with O -1(2) and other reactive oxygen species.