Vitamin C attenuates hypochlorite-mediated loss of paraoxonase-1 activity from human plasma

Paraoxonase 1 (PON1) is a cardioprotective enzyme associated with high-density lipoprotein (HDL). We tested the hypothesis that vitamin C protects HDL and PON1 from deleterious effects of hypochlorous acid, a proinflammatory oxidant. In our experiments, HDL (from human plasma) or diluted human plasma was incubated with hypochlorite in either the absence (control) or presence of vitamin C before measuring chemical modification and PON1 activities. Vitamin C minimized chemical modification of HDL, as assessed by lysine modification and accumulation of chloramines. In the absence of vitamin C, chloramines accumulated to 114 ± 4 μmol/L in HDL incubated with a 200-fold molar excess of hypochlorite; but addition of vitamin C (200 μmol/L) limited formation to 36 ± 6 μmol/L (P < .001). In plasma exposed to hypochlorite, IC50 values of 1.2 ± 0.1, 9.5 ± 1.0, and 5.0 ± 0.6 mmol/L were determined for PON1's phosphotriesterase, arylesterase, and (physiologic) lactonase activities, respectively. Vitamin C lessened this inhibitory effect of hypochlorite on PON1 activities. In plasma supplemented with vitamin C (400 μmol/L), PON1 phosphotriesterase activity was 72% ± 17% of normal after incubation with hypochlorite (2 mmol/L), compared with 42% ± 6% for unsupplemented plasma (P < .05). Similar effects were seen for other PON1 activities. In some experiments, vitamin C also appeared to reverse hypochlorite-mediated loss of PON1 phosphotriesterase activity; but this effect was not observed for the other PON1 activities. In conclusion, vitamin C attenuated hypochlorite-mediated loss of PON1 activity in vitro and may, therefore, preserve cardioprotective properties of HDL during inflammation.