444
ERIC E. CONN
tributed in living cells. The only exception appeared to be certain
anaerobic bacteria (6).
The most obvious property of the cytochromes was their ability to
absorb light in characteristic regions of the spectrum. The pigments
were defined in terms of position of the absorption maxima. In heart
muscle cytochromes a, b, and c absorbed maximally at 605, 562, and
550 τημ respectively. The absorption of the cytochromes not only permitted identification of the pigments but also provided clues to their
chemical nature and to their biological role.
In 1933 Keilin (6) claimed an important role for the cytochromes
in cellular respiration based on observations of the following nature.
The cytochromes were almost ubiquitous in nature. They were capable
of alternate oxidation and reduction, a property which could be observed directly in living cells (bacteria, yeast, insects). The oxidation
and reduction of the cytochromes was affected by well known inhibitors
of cellular respiration such as KCN, NaN 3 , and CO. One of the pigments, cytochrome c, was readily extracted from bakers' yeast and
partially purified. When reduced chemically and added to a homogenate
of heart muscle, the reduced cytochrome c was oxidized by 0 2 . The
oxidized cytochrome c on the other hand was rapidly reduced when a
normal cellular metabolite such as succinate was added to the muscle
preparation. These last two observations clearly indicated the ability
of the cytochromes to function as respiratory carriers and meant that
the cytochromes were to play another important role. This was the
resolution of a controversy of this period concerning biological oxidations. The cytochromes provided a functioning link between the hydrogen activation theory of Wieland and the oxygen activation concept
championed by Warburg.
Warburg's contribution to the mechanism of cellular respiration resulted from his fundamental studies on the role of heavy metals as
catalysts in oxidation reactions (7). His experiments on the inhibition
of cellular respiration by various substances led him to suggest the
existence of an iron enzyme linking molecular oxygen to cell metabolism.
Warburg named this enzyme Atmungsferment and provided evidence
that it was a heme-protein. His studies on the photodissociation of the
CO-inhibited enzyme led to the publication of the relative absorption
spectrum of this enzyme in 1929 (8). An enzyme system, indophenol
oxidase, which catalyzed the oxidation of a mixture of α-naphthol and
paraphenylenediamine was shown by Keilin to be widely distributed
in nature and was inhibited by KCN, NaN 3 , and CO (the latter was
photoreversible). Since this oxidase system also catalyzed the oxidation
of reduced cytochrome c, Keilin renamed the enzyme cytochrome oxi-
ERIC E. CONN
tributed in living cells. The only exception appeared to be certain
anaerobic bacteria (6).
The most obvious property of the cytochromes was their ability to
absorb light in characteristic regions of the spectrum. The pigments
were defined in terms of position of the absorption maxima. In heart
muscle cytochromes a, b, and c absorbed maximally at 605, 562, and
550 τημ respectively. The absorption of the cytochromes not only permitted identification of the pigments but also provided clues to their
chemical nature and to their biological role.
In 1933 Keilin (6) claimed an important role for the cytochromes
in cellular respiration based on observations of the following nature.
The cytochromes were almost ubiquitous in nature. They were capable
of alternate oxidation and reduction, a property which could be observed directly in living cells (bacteria, yeast, insects). The oxidation
and reduction of the cytochromes was affected by well known inhibitors
of cellular respiration such as KCN, NaN 3 , and CO. One of the pigments, cytochrome c, was readily extracted from bakers' yeast and
partially purified. When reduced chemically and added to a homogenate
of heart muscle, the reduced cytochrome c was oxidized by 0 2 . The
oxidized cytochrome c on the other hand was rapidly reduced when a
normal cellular metabolite such as succinate was added to the muscle
preparation. These last two observations clearly indicated the ability
of the cytochromes to function as respiratory carriers and meant that
the cytochromes were to play another important role. This was the
resolution of a controversy of this period concerning biological oxidations. The cytochromes provided a functioning link between the hydrogen activation theory of Wieland and the oxygen activation concept
championed by Warburg.
Warburg's contribution to the mechanism of cellular respiration resulted from his fundamental studies on the role of heavy metals as
catalysts in oxidation reactions (7). His experiments on the inhibition
of cellular respiration by various substances led him to suggest the
existence of an iron enzyme linking molecular oxygen to cell metabolism.
Warburg named this enzyme Atmungsferment and provided evidence
that it was a heme-protein. His studies on the photodissociation of the
CO-inhibited enzyme led to the publication of the relative absorption
spectrum of this enzyme in 1929 (8). An enzyme system, indophenol
oxidase, which catalyzed the oxidation of a mixture of α-naphthol and
paraphenylenediamine was shown by Keilin to be widely distributed
in nature and was inhibited by KCN, NaN 3 , and CO (the latter was
photoreversible). Since this oxidase system also catalyzed the oxidation
of reduced cytochrome c, Keilin renamed the enzyme cytochrome oxi-