The oxidative fermentation of ethanol in Gluconacetobacter diazotrophicus is a two-step pathway catalyzed by a single enzyme: alcohol-aldehyde Dehydrogenase (ADHa).

Gluconacetobacter diazotrophicus is a N2-fixing bacterium endophyte from sugar cane. The oxidation of ethanol to acetic acid of this organism takes place in the periplasmic space, and this reaction is catalyzed by two membrane-bound enzymes complexes: the alcohol dehydrogenase (ADH) and the aldehyde dehydrogenase (ALDH). We present strong evidence showing that the well-known membrane-bound Alcohol dehydrogenase (ADHa) of Ga. diazotrophicus is indeed a double function enzyme, which is able to use primary alcohols (C2-C6) and its respective aldehydes as alternate substrates. Moreover, the enzyme utilizes ethanol as a substrate in a reaction mechanism where this is subjected to a two-step oxidation process to produce acetic acid without releasing the acetaldehyde intermediary to the media. Moreover, we propose a mechanism that, under physiological conditions, might permit a massive conversion of ethanol to acetic acid, as usually occurs in the acetic acid bacteria, but without the transient accumulation of the highly toxic acetaldehyde.

The active (ADHa) and inactive (ADHi) forms of the PQQ-alcohol dehydrogenase from Gluconacetobacter diazotrophicus differ in their respective oligomeric structures and redox state of their corresponding prosthetic groups.

The membrane-bound alcohol dehydrogenase of Gluconacetobacter diazotrophicus contains one pyrroloquinoline quinone moiety (PQQ), one [2Fe-2S] cluster, and four c-type cytochromes. Here, we describe a novel and inactive enzyme. ADHi, similarly to ADHa, is a heterodimer of 72- and 44-kDa subunits and contains the expected prosthetic groups. However, ADHa showed a threefold molecular mass as compared to ADHi. Noteworthy, the PQQ, the [2Fe-2S] and most of the cytochromes in purified ADHi is in the oxidized form, contrasting with ADHa where the PQQ-semiquinone is detected and the [2Fe-2S] cluster as well as the cytochromes c remained fully reduced after purification. Reduction kinetics of the ferricyanide-oxidized enzymes showed that while ADHa was brought back by ethanol to its full reduction state, in ADHi, only one-quarter of the total heme c was reduced. The dithionite-reduced ADHi was largely oxidized by ubiquinone-2, thus indicating that intramolecular electron transfer is not impaired in ADHi. The acidic pH of the medium might be deleterious for the membrane-bound ADH by causing conformational changes leading to changes in the relative orientation of heme groups and shift of corresponding redox potential to higher values. This would hamper electron transfer resulting in the low activity observed in ADHi.

Rate of oxidant stress regulates balance between rat gastric mucosa proliferation and apoptosis.

We have characterized an experimental model of ethanol-induced chronic gastritis in which a compensatory mucosal cell proliferation is apparently regulated by lipoperoxidative events. Therefore, the present study is an attempt to further assess the participation of oxidant stress during gastric mucosa proliferation, by administering alpha-tocopherol (vitamin E) to rats with gastritis. A morphometric analysis was done, and parameters indicative of oxidant stress, cellular proliferation (including cyclin D1 levels), apoptotic events, and activities of endogenous antioxidant systems were measured in gastric mucosa from our experimental groups. After ethanol withdrawal, restitution of surface epithelium coincided with increased lipid peroxidation and cell proliferation and further active apoptosis. High alpha-tocopherol dosing (100 IU/kg bw) showed a clear antioxidant effect, abolished cell proliferation, and promoted an early and progressive apoptosis, despite vitamin E also enhancing levels of endogenous antioxidants. Indicators of cell proliferation inversely correlated with apoptotic events, and this relationship was blunted by administering vitamin E, probably by affecting translocation of active cyclin D1 into the nucleus. In conclusion, alpha-tocopherol administration inhibited cell proliferation, leading to a predominance of apoptotic events in ethanol-induced gastric damage. Therefore, the timing and magnitude of lipoperoxidative events seemed to synchronize in vivo cell proliferative and apoptotic events, probably by changing the cell redox state.