Worcester Foundation for Biomedical Research

About: Worcester Foundation for Biomedical Research is a based out in . It is known for research contribution in the topics: Estrone & Estrogen. The organization has 2195 authors who have published 2646 publications receiving 115809 citations. The organization is also known as: Worcester Foundation for Experimental Biology.

Metabolism of the antimammary cancer antiestrogenic agent tamoxifen. II. Flavin-containing monooxygenase-mediated N-oxidation.

Abstract: Earlier studies demonstrated that the major metabolites of tamoxifen generated by mammalian liver microsomes are the corresponding N-oxide, N-desmethyl, and 4-hydroxy derivatives. This study examines the enzymatic activity catalyzing the formation of tamoxifen N-oxide by mammalian liver microsomes. Incubations of tamoxifen with liver microsomes from the various species, supplemented with NADPH, yielded the N-oxide, N-desmethyl, and 4-hydroxy derivatives. Inhibition of N-oxide accumulation by mild heat and low concentrations of methimazole in rat liver microsomes indicated that this reaction is catalyzed by the flavin-containing monooxygenase (FMO). Antibodies to NADPH-P-450 reductase inhibited N-demethylation and 4-hydroxylation, but not N-oxidation, supporting the aforementioned conclusion. Purified mouse liver microsomal FMO converted tamoxifen solely into the N-oxide, providing direct evidence for FMO involvement. Human liver microsomes formed the same tamoxifen metabolites, albeit at a much lower rate. Inhibitors of FMO diminished the accumulation of N-oxide by human liver microsomes, indicating involvement of FMO. Tamoxifen-N-oxide was found to be readily reduced to tamoxifen by rat or human liver microsomes, in the presence of NADPH; the extent of reduction was dramatically increased when incubations were supplemented with methimazole. The facile reduction of tamoxifen N-oxide back to tamoxifen suggests that the N-oxide may serve as a storage form for tamoxifen in vivo.

Determination of the transbilayer distribution of fluorescent lipid analogues by nonradiative fluorescence resonance energy transfer.

Abstract: We measured the nonradiative fluorescence resonance energy transfer between 7-nitro-2,1,3-benzoxadiazol-4-yl (NBD) labeled lipids (amine labeled phosphatidylethanolamine or acyl chain labeled phosphatidylcholine) and rhodamine labeled lipids in large unilamellar dioleoylphosphatidylcholine vesicles. Two new rhodamine labeled lipid analogues, one a derivative of monolauroylphosphatidylethanolamine and the other of sphingosylphosphorylcholine, were found to exchange through the aqueous phase between vesicle populations but not to be capable of rapid transbilayer movement between leaflets. Energy transfer from NBD to rhodamine was measured using liposomes with symmetric or asymmetric distributions of these new rhodamine labeled lipid analogues to determine the relative contributions of energy transfer between donor and acceptor fluorophores in the same (cis) and opposite (trans) leaflets. Since the characteristic R0 values for energy transfer ranged from 47 to 73 A in all cases, significant contributions from both cis and trans energy transfer were observed. Therefore, neither of these probes acts strictly as a half-bilayer quencher of NBD lipid fluorescence. The dependence of transfer efficiency on acceptor density was fitted to a theoretical treatment of energy transfer to determine the distances of closest approach for cis and trans transfer. These parameters set limits on the positions of the fluorescent groups relative to the bilayer center, 20-31 A for NBD and 31-55 A for rhodamine, and provide a basis for future use of these analogues in measurements of transbilayer distribution and transport.