ANTIOXIDANT AND NEUTROPHIL-INHIBITING PROPERTIES OF NEW 2-O-METHYL-6-(ALKYLTHIO)ASCORBIC ACID-DERIVATIVES

A series of new 6-(alkylthio)ascorbic acids was synthesized, and their inhibitory effects on lipid peroxidation and the oxidative burst of h uman neutrophils were tested. Of 12 structurally different lipophilic ascorbic acid derivatives S-n-hexadecyl-2-O-methyl-6-deoxy-6-thio-L-as corbic acid (7b; B-003) inhibited the Fe2+/ADP-induced lipid peroxidat ion of rat liver microsomes with an IC50 value of 2 mu M. In human neu trophils, 7b most potently inhibited the fMLP-induced oxidative burst in a cell density-dependent manner with an IC50 value Of 0.6 mu M at 5 x 10(5) cells/mL. Shorter alkyl chain lengths decreased the inhibitor y potency for both lipid peroxidation and oxidative burst, but in gene ral no correlation was found between the two parameters. Likewise, 6-S -n-hexadecyl-3-O-methyl-6-thio-L-ascorbic acid (7c; B-015), the regioi somer of 7b, was a potent antioxidant but did not affect the oxidative burst. Since superoxide anions generated by xanthine/ xanthine oxidas e were not quenched by 7b, it became evident that its target was somew here between receptor stimulation and NADPH-oxidase activation. By mea suring the cellular concentrations of 7b and 7c, an accumulation of th e first was found explaining its potency and the dependence on cell de nsity. Expecting a pK(a) value of 3.3 for 7b and 7.7 for 7c a protonop hore action of 7b was likely and could be verified by the drop in intr acellular pH (pH(i)) which did not occur with 7b. Ionophores such as n igericin, CCCP, or propionic acid also lowered the pH(i) but did not i nhibit the oxidative burst, indicating that the pH(i) drop was not the cause for this inhibition. 7b also strongly inhibited the fMLP-induce d secretion of azurophilic (IC50 = 7 mu M) and specific (IC50 = 2.5 mu M) granules.