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Tetrahydro-1,8-naphthyridinol analogues of alpha-tocopherol as antioxidants in lipid membranes and low-density lipoproteins.

Nam TG, Rector CL, Kim HY, Sonnen AF, Meyer R, Nau WM, Atkinson J, Rintoul J, Pratt DA, Porter NA

Center in Molecular Toxicology and Vanderbilt Institute of Chemical Biology, Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, USA.

Recently we demonstrated that the C(7)-unsubstituted tetrahydro-1,8-naphthyridin-3-ol has more than an order of magnitude better peroxyl radical trapping activity than alpha-tocopherol (alpha-TOH) in inhibited autoxidations in benzene. In order to prepare analogues more structurally related to alpha-TOH for further studies in vitro and in vivo, we developed synthetic approaches to C(7)-monoalkyl and C(7)-dialkyl analogues using a sequence involving (1) AgNO3-mediated hydroxymethyl radical addition to 1,8-naphthyridine, (2) regioselective alkyllithium addition by cyclic chelation in a nonpolar solvent, (3) iodination of the naphthyridine at C(3), and (4) CuI-medidated benzyloxylation of the aryl iodide followed by catalytic hydrogenolysis. An alpha-TOH isostere was prepared by a Wittig coupling of a C16 side chain identical to that of alpha-TOH to the naphthyridinols. The C(7)-mono- and dialkyl analogues exhibited more than an order of magnitude higher antioxidant activity (k(inh) = (5.3-6.1) x 10(7) M(-1) s(-1)) than alpha-TOH (k(inh) = 0.35 x 10(7) M(-) s(-1)) in benzene, as determined by a newly developed peroxyl radical clock. In addition to the strong antioxidant activity in benzene, the closest alpha-TOH analogue (naphthyridinol-based tocopherol, N-TOH) showed excellent inhibition of the oxidation of cholesteryl esters in human low-density lipoprotein and spared endogenous alpha-TOH in these experiments. Lateral diffusion of N-TOH in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine liposomes was comparable to that of alpha-TOH, suggesting that it will have good antioxidant characteristics in both membranes and lipoproteins. Furthermore, a binding assay using a fluorescent tocopherol analogue showed that N-TOH binds to recombinant human tocopherol transfer protein better than alpha-TOH itself, suggesting that distribution of unnatural antioxidants such as N-TOH in vivo is possible.

Published 16 August 2007 in J Am Chem Soc, 129(33): 10211-9.
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