Handbook of Vitamin C Research

Vitamin C Protects from Oxidative DNA Damage… 107Determination of Enzyme ActivitiesFigure 11 shows the effect of vitamin C (1-10µM) on p-nitrophenol hydroxylase(CYP2E1), coumarin hydroxylase (CYP2A6), ethoxyresorufin O-deethylation (CYP1A1) andUDP-glucuronyltransferase (UGT1A4) activity. The p-nitrophenol hydroxylase and coumarinhydroxylase activity decreased with increasing concentrations of vitamin C. However, theethoxyresorufine O-deethylation activity was similar at all the concentrations of vitamin Cused. Vitamin C (10 µM) strongly reduced the coumarin hydroxylase (82%) activity, whilethe p-nitrophenol hydroxylase and the ethoxyresorufine O-deethylation activities wereslightly and weakly reduced (32-19%) respectively. With regard to phase II enzymes, allconcentrations tested of vitamin C (1-10 µM) had a pronounced effect on UDPglucuronyltransferaseactivity (171-178%, respectively).Figure 11.Figure 11. Effect of vitamin C on ( ) p-nitrophenol hydoxylase activity (CYP2E1), ( ) coumarinhydroxylase activity (CYP2A6), ( ) ethoxyresorufin O-deethylation activity (CYP1A1) and ( ) UDPglucuronyltransferaseactivity (UGT1A4). Values are mean of four samples S.D. and are expressedrelative to control. Asterisks indicate significant difference from control ***p 0.001 and ** p 0.01.Effect of Vitamin C Treatment on Cell ViabilityThe effect of vitamin C on HepG2 and HL-60 cell viability was evaluated by twocytotoxicity assays, MTT (Figure 12) and LDH release (Figure 13). Vitamin C was tested atconcentrations ranging from 1 to 100 μM and incubated for 24, 48 and 72 h. As expected,treatment of HepG2 cells with vitamin C showed no toxicity using the MTT assay (Figure 12A). In contrast to the effect noted on HepG2 cells, treatment of HL-60 cells with vitamin Cconcentrations of 50 and 100 μM for 72 h caused a dose-dependent decrease of cell viabilityof about 29 and 46%, respectively (Figure 12 B).