Rose bengal sensitizes photoinactivation of lipoamide dehydrogenase from pig heart to a constant residual reductase activity resulting from specific destruction of histidine residues. The rate of sensitized photoinactivation is pH dependent and is associated with an ionizable group with pK 6.6 ± 0.2. All steady-state kinetic parameters are markedly reduced by photooxidation. Spectroscopic studies indicate the contribution of oxidized flavin/dithiol to the half-reduced form of the photooxidized enzyme. The proton magnetic resonance spectrum of lipoamide dehydrogenase shows resolved histidine C2 proton peak at δ9.18 ppm and a shoulder at δ9.23 ppm. The shoulder protons are eliminated by the sensitized photooxidation and shifted upfield on deprotonation. At high pH, the characteristic Faraday A term also disappears. These observations suggest that the essential histidine stabilizes the nascent thiolate via the ion pair formation to facilitate the reductase reaction catalyzed by lipoamide dehydrogenase.

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Journal Archives of Biochemistry and Biophysics
Tsai, C.S., Wand, A.J., Godin, J.R.P., & Buchanan, G.W. (1982). Multifunctionality of lipoamide dehydrogenase: Role of histidine residues in reductase reaction. Archives of Biochemistry and Biophysics, 217(2), 721–729. doi:10.1016/0003-9861(82)90553-7