Control of Oxygen Utilization 113 



long interval for their consumption. a-Kg is one substrate 

 with which it is possible to demonstrate the cyclic control of 

 respiration, and a typical record is presented in Fig. 16. If the 

 mitochondria are pretreated with ADP and phosphate, 

 addition of a-Kg will cause reduction of cytochrome b which 

 proceeds to a maximum value, remains in a steady state, and 

 declines as the added substrate is expended. The cycle can 

 be repeated by a further addition of a-Kg. With this particular 

 -substrate, the control is almost as effective as that obtained 

 with ADP. 



Nature of intramitochondrial pyridine nucleotide 



Various experiments indicate that the inhibitory substance 

 responsible for respiratory control is present in the mito- 

 chondria in amounts closely corresponding to their pyridine 

 nucleotide content. Among a number of data in favour of this 

 conclusion, two are especially clear: (a) the titration of intra- 

 mitochondrial reduced pyridine nucleotide by added ADP in 

 a 1 : 1 stoichiometric reaction (Chance, 1958), and {b) the 

 rapid phosphorylation of added ADP to ATP (adenosine 

 triphosphate) in amounts stoichiometrically related to the 

 intramitochondrial pyridine nucleotide (Schachinger, Eisen- 

 hardt and Chance, 1958). Some recent experiments give 

 evidence that intramitochondrial pyridine nucleotide is a 

 bound form of pyridine nucleotide that is extremely labile in 

 the presence of substances which modify the mitochondrial 

 structure, as is indicated by the fluorescent light emitted by 

 mitochondria when they are illuminated at wavelengths 

 between 340 and 366 m(jL (Chance and Conrad, 1958; Chance 

 and H. Baltscheffsky, 1958). Fig. 17 shows that the maximum 

 for the emission spectrum of mitochondrial fluorescence 

 occurs at 443 m(jL, in almost exact agreement with the maxi- 

 mum for light emission for pyridine nucleotide bound to yeast 

 alcohol dehydrogenase (Duysens and Kronenberg, 1957). 

 Many data indicate that this fluorescent material is intramito- 

 chondrial reduced pyridine nucleotide. Perhaps the most 

 convincing are those based upon the close correspondence of 



