446
ERIC E. CONN
of cytochrome c catalyzed by particulate preparations is more complex
and is discussed in detail below (Section II, C).
Another enzymatic activity, the reduction of dyes such as methylene
blue by reduced pyridine nucleotides, has been ascribed to a group of
enzymes known as the diaphorases. In contrast to the cytochrome reductases, the diaphorases do not react readily with cytochrome c and
consequently their physiological function, if any, is unknown. A current view holds that the diaphorases are a modified form of cytochrome
reductase which no longer can react with cytochrome c (25).
In addition to accepting hydrogens from the reduced pyridine
nucleotide, flavoproteins can react directly with metabolites. Glucose,
xanthine, aldehydes, and amino acids are examples of organic substrates
which can be oxidized directly by flavoproteins. The properties of these
enzymes and of other flavoproteins are described in recent reviews (25,
26).
3. Pyridine Nucleotides
The pyridine nucleotide TPN
+ was discovered in 1931 (27) by Warburg and Christian. The observation that nicotinamide was a component
of the coenzyme stimulated experiments on the mechanism of action of
the nucleotide. As a result of these studies, DPN
+ , discovered 30 years
earlier (28), was shown to be closely related to TPN
+ . The structure of
TPN+ remained unsettled until 1953, when the position of the third
phosphate group could definitely be assigned to the 2'-hydroxyl of the
ribose attached to adenine. Knowledge of the chemistry and function
of the pyridine nucleotides and their dehydrogenases has progressed
steadily (29, 30). Again the most important single property of these coenzymes is their ability to undergo reversible oxido-reductions. In the
presence of the appropriate dehydrogenase, hydrogen atoms are transferred between substrate and coenzyme. The process involves a direct
transfer of one of the hydrogen atoms from the substrate to the pyridine
nucleotide in a stereochemically specific manner (31). The other hydrogen is released in solution as H
+ ion (Eq. 5).
Substrate-H 2 + DPN+ ^ Substrate + DPNH + H
+
(5)
The list of dehydrogenases requiring DPN
+ or TPN
+ is large (29, 30).
Since DPNH and TPNH can be reoxidized in the presence of oxidized
substrates and the appropriate dehydrogenase, the pyridine nucleotides
can serve as hydrogen carriers between two substrate molecules. The
most familiar example of this action is the coupling of the oxidation of
glyceraldehyde-3-phosphate to the reduction of pyruvate or acetaldehyde, which occurs during glycolysis or alcoholic fermentation. In the
oxidation of cellular metabolites by molecular 0 2 , however, the function
ERIC E. CONN
of cytochrome c catalyzed by particulate preparations is more complex
and is discussed in detail below (Section II, C).
Another enzymatic activity, the reduction of dyes such as methylene
blue by reduced pyridine nucleotides, has been ascribed to a group of
enzymes known as the diaphorases. In contrast to the cytochrome reductases, the diaphorases do not react readily with cytochrome c and
consequently their physiological function, if any, is unknown. A current view holds that the diaphorases are a modified form of cytochrome
reductase which no longer can react with cytochrome c (25).
In addition to accepting hydrogens from the reduced pyridine
nucleotide, flavoproteins can react directly with metabolites. Glucose,
xanthine, aldehydes, and amino acids are examples of organic substrates
which can be oxidized directly by flavoproteins. The properties of these
enzymes and of other flavoproteins are described in recent reviews (25,
26).
3. Pyridine Nucleotides
The pyridine nucleotide TPN
+ was discovered in 1931 (27) by Warburg and Christian. The observation that nicotinamide was a component
of the coenzyme stimulated experiments on the mechanism of action of
the nucleotide. As a result of these studies, DPN
+ , discovered 30 years
earlier (28), was shown to be closely related to TPN
+ . The structure of
TPN+ remained unsettled until 1953, when the position of the third
phosphate group could definitely be assigned to the 2'-hydroxyl of the
ribose attached to adenine. Knowledge of the chemistry and function
of the pyridine nucleotides and their dehydrogenases has progressed
steadily (29, 30). Again the most important single property of these coenzymes is their ability to undergo reversible oxido-reductions. In the
presence of the appropriate dehydrogenase, hydrogen atoms are transferred between substrate and coenzyme. The process involves a direct
transfer of one of the hydrogen atoms from the substrate to the pyridine
nucleotide in a stereochemically specific manner (31). The other hydrogen is released in solution as H
+ ion (Eq. 5).
Substrate-H 2 + DPN+ ^ Substrate + DPNH + H
+
(5)
The list of dehydrogenases requiring DPN
+ or TPN
+ is large (29, 30).
Since DPNH and TPNH can be reoxidized in the presence of oxidized
substrates and the appropriate dehydrogenase, the pyridine nucleotides
can serve as hydrogen carriers between two substrate molecules. The
most familiar example of this action is the coupling of the oxidation of
glyceraldehyde-3-phosphate to the reduction of pyruvate or acetaldehyde, which occurs during glycolysis or alcoholic fermentation. In the
oxidation of cellular metabolites by molecular 0 2 , however, the function