244
GEORGE G. LATIES
oxygen dependence, albeit the response to CO differed completely in
the two types of tissue.
The reversibility by light of CO inhibition is a characteristic of cytochrome oxidase, and distinguishes it from other heavy metal oxidases
which may combine with CO. The inhibition of respiration in fresh
slices by CO, together with the reversibility by light of that inhibition,
and the relative indifference of the respiration to low p Ö2 , sufficiently
indicate cytochrome oxidase to be functional in fresh potato slices. The
activity of both polyphenol oxidase (Ingraham, 1955) and ascorbic
acid oxidase (Thimann et al, 1954) is dependent upon oxygen tension
even at oxygen concentrations of 21% and higher.
It can be estimated that from 80 to 100% of the respiration of freshly
cut potato disks is mediated by cytochrome oxidase. After one day at
25° C. cytochrome oxidase activity is no longer manifest in intact slices,
and a CO-resistant oxidase is found to function in its stead (Thimann et
al, 1954; Schade and Levy, 1949). Cytochrome oxidase is still present,
however, and can be demonstrated in mitochondria isolated from incubated tissue (Hackett, 1956b). It is most interesting to compare the
events which occur in potato tissue as a function of time, with the ontogenetic changes in respiration characteristics evinced by barley roots
(James, 1953b; Fritz and Beevers, 1955; Honda, 1955).
The nature of the oxidase which develops in incubated potato disks
is uncertain, although both Thimann et al (1954), and Levy and Schade
(1948) agree that it is insensitive to CO. According to Thimann and his
co-workers, the affinity of the oxidase for 0 2 is comparable to that of
cytochrome oxidase; but Levy and Schade have stated that it is sensitive
to a low p 0 o- This conclusion, however, was reached on the basis of
studies with tissue homogenates, and was based on the assumption that
their preparation was homogeneous as to oxidase activity, although their
own data (see Schade and Levy, 1949; Fig. 2) seem to indicate that it
possessed a heterogeneous character. The possibility is strong that the
residual activity which they observed at a p 02 of 2.5%, and which increased with aging of the tissue, was due to this new unknown oxidase.
In fact, the increase in total activity of the homogenate through 100
hours can be accounted for entirely by the increase in activity at 2.5% 0 2 ,
and the observations of the two groups of workers are thus in accord.
The greater sensitivity to a low po 2 of the older slices, which Schade
and Levy (1949) observed and which led them to their conclusions, may
be explained on another basis. The respiration of the slices was found
to rise twofold or more in 24 hours at 25° C. As Thimann and co-workers
have shown, a faster rate of respiration leads more readily to a state
where the rate of diffusion of 0 2 into the tissue becomes limiting. This
GEORGE G. LATIES
oxygen dependence, albeit the response to CO differed completely in
the two types of tissue.
The reversibility by light of CO inhibition is a characteristic of cytochrome oxidase, and distinguishes it from other heavy metal oxidases
which may combine with CO. The inhibition of respiration in fresh
slices by CO, together with the reversibility by light of that inhibition,
and the relative indifference of the respiration to low p Ö2 , sufficiently
indicate cytochrome oxidase to be functional in fresh potato slices. The
activity of both polyphenol oxidase (Ingraham, 1955) and ascorbic
acid oxidase (Thimann et al, 1954) is dependent upon oxygen tension
even at oxygen concentrations of 21% and higher.
It can be estimated that from 80 to 100% of the respiration of freshly
cut potato disks is mediated by cytochrome oxidase. After one day at
25° C. cytochrome oxidase activity is no longer manifest in intact slices,
and a CO-resistant oxidase is found to function in its stead (Thimann et
al, 1954; Schade and Levy, 1949). Cytochrome oxidase is still present,
however, and can be demonstrated in mitochondria isolated from incubated tissue (Hackett, 1956b). It is most interesting to compare the
events which occur in potato tissue as a function of time, with the ontogenetic changes in respiration characteristics evinced by barley roots
(James, 1953b; Fritz and Beevers, 1955; Honda, 1955).
The nature of the oxidase which develops in incubated potato disks
is uncertain, although both Thimann et al (1954), and Levy and Schade
(1948) agree that it is insensitive to CO. According to Thimann and his
co-workers, the affinity of the oxidase for 0 2 is comparable to that of
cytochrome oxidase; but Levy and Schade have stated that it is sensitive
to a low p 0 o- This conclusion, however, was reached on the basis of
studies with tissue homogenates, and was based on the assumption that
their preparation was homogeneous as to oxidase activity, although their
own data (see Schade and Levy, 1949; Fig. 2) seem to indicate that it
possessed a heterogeneous character. The possibility is strong that the
residual activity which they observed at a p 02 of 2.5%, and which increased with aging of the tissue, was due to this new unknown oxidase.
In fact, the increase in total activity of the homogenate through 100
hours can be accounted for entirely by the increase in activity at 2.5% 0 2 ,
and the observations of the two groups of workers are thus in accord.
The greater sensitivity to a low po 2 of the older slices, which Schade
and Levy (1949) observed and which led them to their conclusions, may
be explained on another basis. The respiration of the slices was found
to rise twofold or more in 24 hours at 25° C. As Thimann and co-workers
have shown, a faster rate of respiration leads more readily to a state
where the rate of diffusion of 0 2 into the tissue becomes limiting. This