Accelerated reaction of nitric oxide with O2 within the hydrophobic interior of biological membranes

Abstract

We demonstrate herein dramatic acceleration of aqueous nitric oxide (NO) reaction with O2 within the hydrophobic region of either phospholipid or biological membranes or detergent micelles and demonstrate that the presence of a distinct hydrophobic phase is required. Per unit volume, at low amounts of hydrophobic phase, the reaction of NO with O2 within the membranes is approximately 300 times more rapid than in the surrounding aqueous medium. In tissue, even though the membrane represents only 3% of the total volume, we calculate that 90% of NO reaction with O2 will occur there. We conclude that biological membranes and other tissue hydrophobic compartments are important sites for disappearance of NO and for formation of NO-derived reactive species and that attenuation of these potentially damaging reactions is an important protective action of lipid-soluble antioxidants such as vitamin E.

Figure 1

(A) Accelerated NO autooxidation in the presence of phospholipid vesicles (liposomes; 40 mg/ml). NO (30 μM) was added in the well-stirred aerated vessel at t = 0 and detected electrochemically. (B) Second-order linear transformation of the data.

Figure 2

Model to predict the effects of a membrane on NO autooxidation.

Figure 3

(A) Second-order plot of NO disappearance in the presence of soy phospholipid vesicles (0.003 g/ml), Triton X-100 (0.003 g/ml), and isolated hepatocyte membranes (0.003 g/ml). (B and C) Determination of v̄_hλ for soy vesicles, Triton X-100, and hepatocyte membranes (B) and three detergents with different CMCs, as denoted by the arrows (C). Z-12, Zwittergent-12 [CMC = 1.21 mg/ml (15)]; OTG, octyl thioglucoside [CMC = 2.77 mg/ml (16)]; OG, octyl glucoside [CMC = 7.3 mg/ml (17)].

Figure 4

Predicted effect of increasing amounts of membrane phase on the total and the membrane reaction of NO with O₂. The upper curve is the percent of the total reaction of NO with O₂ that occurs within the membrane interior as the amount of membrane increases. The lower curve is the fold increase in total reaction of NO with O₂ that results from increasing amounts of membrane.