Studies of the metabolism of a - tocopherol stereoisomers in rats using [ 5 - methyl - 14 C ] SRR - and RRR - a - tocopherol

We investigated the distribution and metabolism of SRR a -tocopherol ( SRR a -Toc), synthetic a -Toc compared with RRR a -Toc, in rats after a single oral administration of 2 mg (20 m Ci) SRR and RRR a -[5-methyl14 C]Toc. In the liver, there was no difference in the recovery of radioactivity until 12 h after administration, and it reached a maximum of 4.4% of the dose after 12 h, but in other tissues, radioactivity derived from RRR a -Toc was clearly higher than that derived from SRR a -Toc after 12 h. For 96 h after administration, urinary excretions of SRR a -Toc were 7.8% of the dose and significantly greater than that of RRR a -Toc, which was 1.3% of the dose. On the other hand, total fecal excretions of SRR and RRR a -Toc were 87.6% and 83.0%, respectively. Therefore, radioactivity in the urine was assumed to have transferred out of the liver. Furthermore, the urine samples were hydrolyzed with 3 N methanolic HCl and analyzed by high performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry. The results showed that about 73% of the total radioactivity injected into HPLC was found to be 2,5,7,8-tetramethyl-2-(2 9 -carboxyethyl)-6hydroxy chroman ( a -CEHC), as well as RRR a -Toc. Thus, there is no difference between SRR a -Toc and RRR a -Toc in metabolic pathways, and it is suggested that SRR a -Toc discriminated in the liver is rapidly metabolized by the liver and excreted as the conjugate of a -CEHC in the urine. — Kaneko, K., C. Kiyose, T. Ueda, H. Ichikawa, and O. Igarishi. Studies of the metabolism of a -tocopherol stereoisomers in rats using [5-methy114 C] SRR and RRR a -tocopherol. J. Lipid Res. 2000. 41: 357–367. Supplementary key words RRR a -tocopherol • SRR a -tocopherol • RRR a -[5-methyl14 C]tocopherol • SRR a -[5-methyl14 C]tocopherol • a -CEHC • a -tocopherol metabolism in vivo • distribution of radioactivity in rat • detection of LC-MS • urine metabolites a -Tocopherol ( a -Toc) has three chiral centers in its phytyl tail (2, 4 9 and 8 9 ), which could result in a total of eight different stereoisomers. Naturally occurring a -Toc consists of a single stereoisomer, 2 R , 4 9 R , 8 9 R a -Toc ( RRR a -Toc), whereas the synthetic form, allrac a -Toc, contains equimolar stereoisomers. a -Toc is used as a dietary supplement and a food additive in both its forms. However, in the rat resorption–gestation assay, relative biopotencies of individual a -Toc acetate ( a -Toc Ace) stereoisomers of 100% ( RRR ), 90% ( RRS ), 73% ( RSS ), 57% ( RSR ), 60% ( SSS ), 37% ( SRS ), 31% ( SRR ), and 21% ( SSR ) were established (1), and the biopotency of synthetic a -Toc was therefore found to be different from the natural form. The study of the distribution of a -Toc stereoisomers in vivo suggested that the 2 R -configuration especially affects the biodiscrimination between the eight a -Toc stereoisomers (2–6). This discrimination is related to the a -Toc transfer protein ( a -TTP) that is present in the liver (7–9). a -Toc stereoisomers are equally well absorbed from the intestine (10) and transported to the liver, but only 2 R -isomers are preferentially secreted in association with nascent very low density lipoprotein. This is dependent on the charactarization of a -TTP. After that, 2 S -isomers remaining in the liver rapidly disappeared (11, 12). Tocopherol has two possible metabolic pathways. One leads to the degradation of the side chain after opening of the chroman ring and the other to the degradation of the side chain while retaining the chroman ring. In 1956, Simon et al. (13, 14) investigated urinary metabolites of rabbits and humans administered 10 to 15 mg (1.5 to 2.0 m C) of a -Toc. After hydrolysis, these metabolites were identified as 2-(3-hydroxy-3-methyl-5-carboxypentyl)-3,5,6-trimethylbenzoquinone ( a -Toc acid) and its lactone, so this was assumed to be the metabolic pathway for a -Toc (13, 14). In this pathway, first the chroman ring is opened, leading to a -tocopherylquinone ( a -TQ). a -TQ is then reduced to the a -hydroquinone, conjugated with glucuronic acid, and degraded by b -oxidation, ultimately being excreted to the urine. After that, two additional minor meAbbreviations: a -Toc, a -tocopherol; a -Toc-Ace, a -Toc-acetate; a -TTP, a -Toc transfer protein; a-Toc acid, 2-(3-hydroxy-3-methy1-5-carboxypentyl)-3,5,6-trimethy1benzoquinone; a-TQ, a-tocopherylquinone; a-CEHC, 2,5,7,8-tetramethy1-2-(29-carboxyethy1)-6-hydroxy chroman; S-LLU-a, 2,7,8-trimethy1-2-(29-carboxyethy1)-6-hydroxy chroman; LSC, liquid scintillation counter; HPLC, high performance liquid chromatography; MS, mass spectrometry; UV, ultraviolet detector; RD, radiometric detector; a-CEHC-Me, methyl ester of a-CEHC. 1 To whom correspondence should be addressed. at P E N N S T A T E U N IV E R S IT Y , on F ebuary 1, 2013 w w w .j.org D ow nladed fom