Effects of methyl 9(or 10)-hydroxy-10(or 9)-mercaptostearate and hexadecanethioic S-acid on cupric ion- or 2,2-azo-bis(2-amidinopropane) dihydrochloride (AAPH)-induced oxidation of low density lipoprotein.

The preventive effects of two antioxidants, methyl 9(or 10)-hydroxy-10 (or 9)-mercaptostearate (SH-S) and hexadecanethioic S-acid (thiopalmitic acid, SH-Pal) against the oxidative modification of low density lipoproteins (LDL) induced by cupric ion or a water soluble initiator of peroxyl radicals, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH), were studied by measuring thiobarbituric acid-reactive substances (TBARS). SH-S acted as an effective antioxidant in the oxidative modification of LDL induced by either cupric ion or AAPH. Interestingly, SH-S completely inhibited the formation of fluorescence products and decreased both the fluorescence and alpha-tocopherol content in LDL induced by cupric ion, and reduced 1,1-diphenyl 2-picrylhydrazyl (DPPH) used as a stable free radical model. The antioxidative effect was effectively prevented by the addition of increasing amounts of N-ethylmalemide (NEM) to the system. SH-Pal also inhibited the cupric ion-induced LDL oxidation, but showed little inhibitory effect on the AAPH-induced LDL oxidation. Moreover, SH-Pal was reduced to palmitic acid during the AAPH-induced LDL oxidation. These findings indicate that SH-S protects against oxidative damage of LDL in vitro, and that it acts as a free radical in peroxidation. In addition, this study shows that SH-Pal doesn't act as an efficient antioxidant in AAPH-induced lipid peroxidation.