The efficiency of antioxidants delivered by liposomal transfer

Abstract

Phenolic antioxidants of the hydroxychroman class, α-tocopherol ( α-TOC) and 2,2,5,6,7-pentamethyl-6-hydroxychroman (PMHC), and the hindered phenols 2,3-dihydro-5-hydroxy-2,2,4-trimethylnaphtho[1,2- b]furan (NFUR), 2,6-di- tert-butyl-4-methoxyphenol (DBHA), and 2,6-di- tert-butyl-4-methyl phenol (BHT), were delivered into oxidizable ( acceptor) liposomes of dilinoleoylphosphatidylcholine (DLPC) or 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC) from saturated donor liposomes of dimyristoylphosphatidylcholine (DMPC) by liposomal transfer. The antioxidant activities, k inh, by the inhibited oxygen uptake method were compared with the k inh s determined when the antioxidants were introduced into the liposomes by coevaporation from organic solvents. The peroxidations were initiated using either thermal initiators, water-soluble azo- bis-amidinopropane hydrochloride (ABAP), lipid-soluble azo- bis-2,4-dimethylvaleronitrile (ADVN) and di- tert-butylhyponitrite (DBHN), or the photoinitiator benzophenone. The antioxidants PMHC, NFUR, DBHA, and BHT transferred rapidly between liposomes, but several hours of incubation were needed to transfer α-TOC. The average k inh s in liposomes, in the relative order NFUR≃DBHA>PMHC>BHT≃ α-TOC, were markedly lower than known values in organic solvent. k inh values in liposomes appear to be controlled by effects of hydrogen bonding with water and by restricted diffusion of antioxidants, especially in the case of α-TOC. Product studies of the hydroperoxides formed during inhibited oxygen consumption were carried out. The cis,trans/trans,trans (c,t/t,t) product ratios of the 9- and 13-hydroperoxides formed from PLPC during inhibited peroxidation by PMHC were similar for both the coevaporated and liposomal transfer procedures. The c,t/t,t ratio for the same concentration of α-TOC, 1.52, compares to a value of 1.69 for PMHC at the start of the inhibition period. The higher c,t/t,t ratio observed for NFUR in DLPC, which varied between values of 7.0 at the start of the inhibition to about 1.8 after the break in the induction period, is a reflection of the increased hydrogen atom donating ability of the antioxidant plus the increased concentration of oxidizable lipid provided by DLPC.