8. REACTIONS OF INORGANIC SUBSTANCES
395
Little is known about the coupling of hydrogenase to the cytochromes. Hydro genomonas probably has a cytochrome respiratory system and this is coupled functionally to the hydrogenase and the aerobic
oxidation of H 2 , but despite its intrinsic interest this problem has not yet
been investigated. However, cytochromes have been shown to be present
in several bacteria which have a hydrogenase, in particular in the phototrophic organisms Chromatium (109), Chlorobium limicola (110), C.
thiosulfatophilum
(111), Rho { dospirillum rubrum (189), and in the
aerobic nitrogen-fixing bacterium Azotobacter vinelandii; from the latter,
Tissieres (190) extracted and purified two nonautoxidizable cytochromes
c (c 4 and c 5 ) whose E' 0 values, respectively, are +0.300 and +0.320
volt. The functional role of these cytochromes as hydrogen carriers
coupled to hydrogenase has not yet been studied.
Postgate (134) showed that the cytochrome c 3 of D. desulfuricans
is very rapidly reduced by the hydrogenase of this organism and carries
the hydrogen to systems which reduce sulfates and other sulfur compounds. Senez and Pichinoty (187) found that the cytochrome c 3 reduced by hydrogenase is reoxidized chemically, without the aid of
specific reductases, by nitrite and hydroxylamine which are reduced to
ammonia. This shows that the cytochromes can function as electron carriers in anaerobic systems, without any contact with free oxygen. It
also indicates that, in the presence of a natural or artificial carrier with
a suitable potential, hydrogenase can reduce certain substrates, such as
inorganic nitrogen compounds, by nonspecific processes of chemical
type.
6. Interrelations between Hydrogenase and Nitrogen Fixation
Lee and Wilson (191) noted that the cells of A. xAnelandii were
much richer in hydrogenase if they were derived from cells which had
fixed atmospheric nitrogen than if they grew on an ammonium salt as
nitrogen source. They also observed that gaseous hydrogen strongly inhibits the fixation of N 2 by these same bacteria. It was later found that
H 2 inhibits nitrogen fixation by red-clover nodules (172) and the bluegreen alga Nostoc (192), but definite evidence has not yet been obtained for the presence of a hydrogenase in the whole nodule of this
legume, or in pure cultures of the saprophytic bacterium Rhizobium
trifolii (193), or in the cells of Nostoc and of other Cyanophyceae (142).
These data led Wilson and his group (194) to postulate a direct
relationship between nitrogen fixation and hydrogenase and to state
that all hydrogenase-containing organisms should be able to fix nitrogen.
The latter hypothesis was verified by Gest et al. (195) for the photolithotrophic bacterium Rhodospirillum rubrum, which had been known
395
Little is known about the coupling of hydrogenase to the cytochromes. Hydro genomonas probably has a cytochrome respiratory system and this is coupled functionally to the hydrogenase and the aerobic
oxidation of H 2 , but despite its intrinsic interest this problem has not yet
been investigated. However, cytochromes have been shown to be present
in several bacteria which have a hydrogenase, in particular in the phototrophic organisms Chromatium (109), Chlorobium limicola (110), C.
thiosulfatophilum
(111), Rho { dospirillum rubrum (189), and in the
aerobic nitrogen-fixing bacterium Azotobacter vinelandii; from the latter,
Tissieres (190) extracted and purified two nonautoxidizable cytochromes
c (c 4 and c 5 ) whose E' 0 values, respectively, are +0.300 and +0.320
volt. The functional role of these cytochromes as hydrogen carriers
coupled to hydrogenase has not yet been studied.
Postgate (134) showed that the cytochrome c 3 of D. desulfuricans
is very rapidly reduced by the hydrogenase of this organism and carries
the hydrogen to systems which reduce sulfates and other sulfur compounds. Senez and Pichinoty (187) found that the cytochrome c 3 reduced by hydrogenase is reoxidized chemically, without the aid of
specific reductases, by nitrite and hydroxylamine which are reduced to
ammonia. This shows that the cytochromes can function as electron carriers in anaerobic systems, without any contact with free oxygen. It
also indicates that, in the presence of a natural or artificial carrier with
a suitable potential, hydrogenase can reduce certain substrates, such as
inorganic nitrogen compounds, by nonspecific processes of chemical
type.
6. Interrelations between Hydrogenase and Nitrogen Fixation
Lee and Wilson (191) noted that the cells of A. xAnelandii were
much richer in hydrogenase if they were derived from cells which had
fixed atmospheric nitrogen than if they grew on an ammonium salt as
nitrogen source. They also observed that gaseous hydrogen strongly inhibits the fixation of N 2 by these same bacteria. It was later found that
H 2 inhibits nitrogen fixation by red-clover nodules (172) and the bluegreen alga Nostoc (192), but definite evidence has not yet been obtained for the presence of a hydrogenase in the whole nodule of this
legume, or in pure cultures of the saprophytic bacterium Rhizobium
trifolii (193), or in the cells of Nostoc and of other Cyanophyceae (142).
These data led Wilson and his group (194) to postulate a direct
relationship between nitrogen fixation and hydrogenase and to state
that all hydrogenase-containing organisms should be able to fix nitrogen.
The latter hypothesis was verified by Gest et al. (195) for the photolithotrophic bacterium Rhodospirillum rubrum, which had been known