458
ERIC E. CONN
may account for the cyanide-resistant respiration encountered in some
plant tissues (15). Both of these cytochromes are autoxidizable, and the
former is reduced by DPNH and TPNH. They therefore provide a
means for linking the oxidation of respiratory substrates to 0 2 in a
manner independent of the cyanide-sensitive cytochrome oxidase.
III. Oxidative Phosphorylation
A. EARLY STUDIES
The cytochrome respiratory pathway has been described in detail
because of its intimate association with the process of oxidative phosphorylation. Kalckar (133) was among the first to show that the conversion of inorganic phosphate to sugar phosphates in tissue extracts
could be made dependent upon respiration. In subsequent studies he
demonstrated that the addition of substrate to such extracts increased
both the phosphorylation and the oxygen uptake.
Somewhat later Belitzer and Tsibakowa (134) reported that, in
studies carried out in muscle minces, approximately 2 moles of phosphocreatine were formed for every atom of 0 2 consumed by the
oxidation of different substrates. This observation of the ratio of phosphorus esterified to oxygen consumed (the P/O ratio) was an extremely
important finding, for it demonstrated that more than one phosphorylative step could occur in the transfer of a single pair of electrons from
substrate to oxygen. Although one of the phosphorylation steps observed in their studies might well have been associated with substrate
oxidation, the findings of Belitzer and Tsibakowa implied that at least
one phosphorylation step was associated with electron transfer through
the cytochrome system.
The significance of these findings was recognized by others working
in the field at that time and they were confirmed. Interest centered in
the phosphorylation associated with the oxidation of pyruvate because
this compound, an end product of the glycolytic sequence, could be
oxidized to completion by means of the Krebs cycle. In a detailed
paper, Ochoa (135) examined pyruvate oxidation in heart preparations
and observed P/O ratios in the range of 2.0. Phosphate losses were estimated as high as 60% due to the very active dephosphorylation of ATP
that occurred. Therefore, Ochoa corrected the value of 2.0 to 3.0. Thus,
15 phosphorylations representing the conservation of perhaps as much
as 150 kcal, were believed to be associated with the oxidation of pyruvate. Ochoa also clearly stated the problem of localization of the
sites of phosphorylation in the electron transport scheme and made the
first attempt to demonstrate phosphorylation associated with the oxida-
ERIC E. CONN
may account for the cyanide-resistant respiration encountered in some
plant tissues (15). Both of these cytochromes are autoxidizable, and the
former is reduced by DPNH and TPNH. They therefore provide a
means for linking the oxidation of respiratory substrates to 0 2 in a
manner independent of the cyanide-sensitive cytochrome oxidase.
III. Oxidative Phosphorylation
A. EARLY STUDIES
The cytochrome respiratory pathway has been described in detail
because of its intimate association with the process of oxidative phosphorylation. Kalckar (133) was among the first to show that the conversion of inorganic phosphate to sugar phosphates in tissue extracts
could be made dependent upon respiration. In subsequent studies he
demonstrated that the addition of substrate to such extracts increased
both the phosphorylation and the oxygen uptake.
Somewhat later Belitzer and Tsibakowa (134) reported that, in
studies carried out in muscle minces, approximately 2 moles of phosphocreatine were formed for every atom of 0 2 consumed by the
oxidation of different substrates. This observation of the ratio of phosphorus esterified to oxygen consumed (the P/O ratio) was an extremely
important finding, for it demonstrated that more than one phosphorylative step could occur in the transfer of a single pair of electrons from
substrate to oxygen. Although one of the phosphorylation steps observed in their studies might well have been associated with substrate
oxidation, the findings of Belitzer and Tsibakowa implied that at least
one phosphorylation step was associated with electron transfer through
the cytochrome system.
The significance of these findings was recognized by others working
in the field at that time and they were confirmed. Interest centered in
the phosphorylation associated with the oxidation of pyruvate because
this compound, an end product of the glycolytic sequence, could be
oxidized to completion by means of the Krebs cycle. In a detailed
paper, Ochoa (135) examined pyruvate oxidation in heart preparations
and observed P/O ratios in the range of 2.0. Phosphate losses were estimated as high as 60% due to the very active dephosphorylation of ATP
that occurred. Therefore, Ochoa corrected the value of 2.0 to 3.0. Thus,
15 phosphorylations representing the conservation of perhaps as much
as 150 kcal, were believed to be associated with the oxidation of pyruvate. Ochoa also clearly stated the problem of localization of the
sites of phosphorylation in the electron transport scheme and made the
first attempt to demonstrate phosphorylation associated with the oxida-