ESR and electron nuclear double resonance characterization of the cholesterol oxidase from Brevibacterium sterolicum in its semiquinone state.

Reduction of the flavin of cholesterol oxidase from Brevibacterium sterolicum, at pH values above 7, by sodium dithionite or light irradiation in the presence of EDTA (either in the presence or absence of deazariboflavin) was found to occur through a stable intermediate state. This intermediate had an optical spectrum characteristic of a flavin anionic semiquinone. The rate and extent of reduction were pH-dependent. No semiquinone intermediate was detected during reduction by these agents at pH values of 6.5 or below or at any pH when dehydroisoandrosterone, a protein substrate analogue, was used as reductant. No intermediate radical was detected during the reoxidation process. Treatment of cholesterol oxidase semiquinone with dehydroisoandrosterone did not convert the semiquinone intermediate to the fully reduced state. The absorption coefficient of oxidised cholesterol oxidase at 470 nm is 10.3 M-1 cm-1. The ESR signal of Brevibacterium sterolicum cholesterol oxidase semiquinone is centred at g = 2.004. The linewidth of the signal was 1.48 mT when the protein was studied in H2O or D2O. These data are in agreement with those reported for anionic semiquinones. The linewidths were the same when measured either at X-band or at S-band frequencies, indicating that line broadening is due to hyperfine interactions. The linewidth decreased to 1.43 mT when the substrate, dehydroisoandrosterone, was added. Electron nuclear double resonance (ENDOR) spectroscopy of cholesterol oxidase semiquinone provided further information about the interactions of the flavin radical with protons. A group of signals, with couplings of 9-12 MHz, is attributed to protons on 8-CH3 (Aiso = 10.9 MHz) and on C6 (Aiso = 9 MHz) of the flavin ring. No change in these hyperfine coupling constants was detected when the protein was studied in D2O. However, the hyperfine coupling constant attributed to protons on 8-CH3 decreased by 0.98 MHz when the ENDOR spectrum of the cholesterol oxidase semiquinone was studied in the presence of dehydroisoandrosterone (Aiso = 9.92 MHz). A second group of signals was observed with hyperfine couplings less than 2.5 MHz. Some of these weak couplings disappeared when the protein was transferred to D2O, or when the substrate, dehydroisoandrosterone, was present. These signals are attributed to displaced water protons, or to exchangeable protons from amino-acid residues on the protein near the flavin binding site, involved in substrate stabilisation.