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Trimethylamine oxide (TMO) promotes anaerobic growth of Fscherichia coli. It has been suggested that an electron transport chain for the reduction of TMO consists of at least b-, c-type cytochromes, and TMO re ductase in bacterial cells.1) Mombrane fragments from cells grown in the presence of TMO show a reduced minus oxidized difference spectrum of cytochromes with a maximum of α peak at 555 nm at room temperature (25°C).2) 2-Heptyl-4-hydroxy -quinoline-N-oxide (HOQNO), which potently inhibits the reduction of TMO,3) prevents partially the re -oxidation of reduced cytochromes.2) The objective of this report is to describe spectral properties of a difference spectrum of the cytochromes reduced and partially oxidized in the presence of HOQNO. The methods for growing the bacteria and for preparing cell membrane fragments were the same as those described previously.2) The procedures for recording reduced minus oxidized difference spectra of cytochromes were also the same as those reported previously.2) Cytochromes reduced by formate in the presence of HOQNO exhibited a spectrum with absorption maxima ofα peak at 555 nm, β peak at 527 nm (Fig. 1A), and γ peak at 427 nm (data not shown). The reduced cytochromes, when reoxidized by TMO, showed a spectrum with maxima of 556, 526 (Fig. 1B), and 429 nm (data not given) for the respective peaks of a, β, and γ. The α peak had a distinct shoulder at 561-562 nm, as reported in our previous paper.2) A difference spectrum between the reduced and oxidized forms showed maxima at about 554, 526 (Fig. 1C), and 424 nm (data not shown). The α peak had no distinct shoulder on either the short or long wavelength flank. Possibly, the absorption maximum of 554nm corresponds to that of 549.5 and//or 551 nm, which have been recorded at 77 K for a spectrum of fully reduced cytochromes.2) The maxima of these relatively low wavelengths are known to be those of c-type cytochromes.4) HOQNO is supposed to inhibit electron fl ow between band c-type cytochfomes.5) The cytochromes of the 554 nm-peak were reoxidized by TMO without the inhibitian (Fig. 1C), suggesting that they are the c-type. While the c-type increase in level with the increase in TMO concentrations in culture media, the b-type do not.8) The c-type cytochromes are thus essential for the reduction of TMO in bacterial cells. Fig. 1. Reduced minus oxidized difference spectra of cytochromes in membrane fragments in the presence of HOQNO. Membrane fragment preparations (3ml) containing 6.0mg protein were incubated for 15 min in the presence of HOQNO (0.13mM), The following procedures were performed: (A) formate (50 μl of 200mM) was added and incubated for 60 min to reduce cytochromes, followed by the addition of distilled water (50 μl) and incubated for another 60 min. (B) After all cytochromes were reduced, TMO (50 μl of 400mM) was added and incubated for 60 min to reoxidize the reduced cytochromes. (C) Difference spectrum of (A) minus (B). All incubations were carried out at 25°C.

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Cytochromes for the Reduction of Trimethylamine Oxide in &lt;i&gt;Escherichia coli&lt;/i&gt;: Reduced Minus Oxidized Difference Spectra in the Presence of 2-Heptyl-4-hydroxyquinoline-&lt;i&gt;N&lt;/i&gt;-oxide