Section 4: DISCUSSION 



In Section 2, data on tissue respiration in in- 

 vertebrates are presented; in Section 3 some of 

 these data plus others from the same investiga- 

 tions are analyzed. In the present section we 



discuss the broader implications of certain stud- 

 ies presented here and suggest some conclusions 

 regarding invertebrate tissue respiration that 

 may be drawn from them. 



ENZYMES OF CITRIC ACID CYCLE 



Respiratory activity of a tissue preparation, 

 or fraction thereof, when measured in the pres- 

 ence of added substrate, can be attributed solely 

 to an enzyme acting on the added substrate only 

 if controls are included to indicate any incre- 

 ment of respiratory activity due to action of 

 other enzymes on other substrates that may re- 

 sult from oxidation of the original substrate. 

 For example, in order to determine the authentic 

 Q!-ketoglutaric dehydrogenase activity of insect 

 sarcosomes, one should measure, as did Lewis 

 and Slater (1954), the respiratory rate of the 

 preparation first in the presence of Of-ketoglutar- 

 ate and subsequently in the presence of a-keto- 

 glutarate plus malonate, the latter being a sub- 

 stance that inhibits succinic dehydrogenase. One 

 may then attribute a difference in respiratory 

 rate to authentic a-ketoglutaric dehydrogenase 

 activity (although one must still recognize the 

 possibility that, with malonate present, accumu- 

 lation of succinate may affect the rate at which 

 a -ketoglutarate is oxidized). 



Without controls such as described above, 

 one may not necessarily conclude that, because 

 a particular citric cycle substrate is metabol- 

 ized, only the enzyme acting specifically upon 

 that substrate is being assayed. Other compo- 

 nents of the chain of enzymes may be concerned 

 with the reaction, and the assay may be a meas- 

 ure of their activity as well. 



Consequently in Section 2 we make no mention 

 of individual enzymes or enzyme systems. We in- 

 dicate merely that a given assay measures endog- 

 enous respiration or, alternatively, respiration 

 in the presence of added substrate or substrates. 



Although the data of Section 2 do not always 

 justify a conclusion regarding the activity of any 

 particular enzyme of the citric acid cycle, never- 

 theless they clearly indicate that at least some 

 enzymes of that cycle are present in the tissues 

 of invertebrates. 



For other reviews dealing with this subject, 

 see Krebs (1954), Gumbman, Brown, and Tappel 

 (1958), and Hammen and Osborne (1959). 



CYTOCHROME SYSTEM 



There is convincing evidence for the presence 

 of cj^ochrome oxidase as the terminal oxidase 

 in tissues of certain mollusks and arthropods 

 (see data of Section 2; see also Shappirio and 

 Williams, 1957a, 1957b: Tappel, 1960; Pablo and 

 Tappel, 1961; Sacktor, 1961). The possibility 

 that cytochrome oxidase may be part of the 

 terminal electron transfer system of other in- 

 vertebrates, including some sponges and coe- 

 lenterates, is suggested by the work of Robbie 



(1949). Cyanide sensitivity is usually taken to 

 indicate that cytochrome oxidase and certain 

 other enzymes may be involved in the terminal 

 electron transport system. Except under unusual 

 conditions (see pp. 82-83) cyanide insensitivity 

 is generally considered evidence for the non- 

 involvement of these enzymes. With cyanide as 

 a respiratory poison, Robbie recorded marked 

 inhibition of endogenous respiration in all in- 

 vertebrate tissues so treated, with the exception 



81 



