Thành
Lập Hỗn Hợp Phức Tạp Giữa Hydrogen Peroxide và L-Phenylalanine Decarboxylase
Complex Formation Between Hydrogen
Peroxide and L-Phenylalanine Decarboxylase
Ngo Manh Tran
Department of Nuclear Medicine and Radiobiology, Centre Hospitalier Universitaire,
Sherbrooke, Québec, Canada.
(International Journal of Biochemistry,
3: 61-65, 1972)
Bài này trình bày kết quả nghiên cứu thành lập chất hỗn hợp phức tạp giữa hydrogen peroxide (H2O2) và phenylalanine decarboxylase của vi trùng Streptococcus Faecalis. Đây là lần đầu tiên thiết lập một phương pháp mới dùng phòng ion hóa (vibrating-reed electrometer ionization chamber method) đo liên tục hiện tượng oxýt hóa, không có phân hóa tố (non-enzymatic oxidation), của 3,4-dihydroxyphenylalanine thành thể hơi CO2 và chất dopachrome, trong thử nghiệm in-vitro.
The interaction of hydrogen peroxide with several enzymes, e.g., fungal laccase (Branden et al.) and cytochromic-c peroxidase (Mochan and Nicholls), has been demonstrated recently. H2O2, a toxic intermediate generated from the compound 6-hydroxydopamine Heikkida and Cohen) and monoamine oxidase (Kapeller-Adler), was shown to damage the uptake of several biogenic amines, i.e. dopamine, norepinephrine, and serotonin, into synaptosomes (Iverson). H2O2 was also believed to be responsible for the destruction of nerve terminals (Bloom et al.).
The present paper describes experiments that examine the complex formed between L-phenylalanine decarboxylase of Streptococcus faecalis and hydrogen peroxide. The formation of an enzyme-peroxide complex was shown by changes in ultra-violet absorbance near the 255nm. and 202.5 cm. peaks. Such a complex formation was demonstrated further by an inhibition of 14CO2 production rates from a non-enzymatic oxidation of 14C-labeled dopa when incubated with L-phenylalanine decarboxylase. H2O2 was removed from the complex by either exhaustive dialysis or fluoride.
The result obtained is specific, since no absorbance changes in the spectra of L-phenylalanine decarboxylase alone were seen under the same experimental conditions. Furthermore, a difference between 14CO2 production from [carboxyl-14C] dopa incubated with H2O2 alone, compared with that from H2O2 plus L-phenylalanine decarboxylase, is due to the fact that in the former case H2O2 oxidizes dopa non-enzymatically to CO2 and dopachrome (Tran and Laplante, Clinical Research, 1971); whereas, in the latter case, H2O2 is bound to the enzyme and hence inhibits such a non-enzymatic oxidation of dopa.
We demonstrate further that the enzyme-H2O2 complex is not stable, since H2O2 from the complex can be easily removed either by exhaustive dialysis of the reduced enzyme by fluoride. The mechanism of the formation of L-phenylalanine decarboxylase-H2O2 is not known. H2O2 may not bind to any ion on the enzyme, because no metals have been found in this bacterial enzyme or in other aromatic L-amino-acid decarboxylases (Christensen et al.).
It is of interest to note that the formation of enzyme-peroxide in this experiment was performed in an artificial way, by the addition of H2O2 to L-phenylalanine decarboxylase, and that the bound species was probably not only H2O2 but also its ions, i.e., H2O2- or O2²-. The decomposition of the enzyme-peroxide complex by fluoride may be understood because ions with a high affinity for fluoride can interact strongly with H2O2- (Branden et others).
Bs Trần Mạnh Ngô
