630 F. Dickens 



estimates of total cellular TPN, judging from Table 1, could be in error by as 

 much as 100 /ig/g of tissue. In tissues other than Uver, the extent of bound 

 pyridine nucleotides has not yet been determined, and this information would 

 be very desirable. 



It is just possible for example that the extremely low levels of total TPN 

 in tumour tissue, which were consistently observed by Clock and McLean 

 (1957), might in part be due to a high proportion of the bound form of TPN 

 in tumour mitochondria, but this possibility has not yet been tested experi- 

 mentally. Morton (1958) has already commented on the low values of DPN 

 per nucleus in tumour tissue, compared with normal tissues on the same basis, 

 and has devised an ingenious hypothesis that a low cytoplasmic DPN may 

 in fact be a stimulus to cell division in tumours. However, when expressed 

 on the customary basis of tissue weight, the levels of total DPN in tumours 

 found in our laboratory was about in the middle of the range of that for 

 normal tissues (20-27 /^moles DPN/lOOg), while that of TPN was only 

 0-8-4-2//moles/100 g; nevertheless there is evidence of the existence of an 

 active HMP shunt, which requires TPN, in tumours (Dickens, 1958). The 

 difficulty of reconciling these facts makes the search for a hidden supply of 

 TPN in tumours still more necessary. 



Quite apart from this recent work of Purvis, it has of course long been 

 recognized that there are differences in the accessibiUty of pyridine nucleotides 

 in cells ; some being bound to enzymes or cellular structure, and some in the 

 free state. Thus Lehninger (1958) has pointed out that the phosphorylating 

 membrane fragments, obtained by digitonin treatment of liver mitochondria, 

 retain only one-fortieth of the DPN (relative to cytochrome a as standard) 

 that the intact mitochondria possessed, yet the submitochondrial fragments 

 have five times the activity of the whole mitochondria in oxidizing ^-hydroxy- 

 butyrate. Tliis indicates the very much higher activity of a structurally bound 

 form of DPN, which is believed to be that concerned in oxidative phosphory- 

 lation. It also gives a further reason for the belief that only a small part of the 

 pool of mitochondrial pyridine nucleotide is metabolically active in any given 

 enzymic reaction. It is interesting to note that the bound malic dehydrogenase 

 in these digitonin-treated particles did not react with mitochondrially bound 

 DPN, although the bound j5-hydroxybutyrate dehydrogenase readily did so 

 (Lehninger, 1958). 



As regards the oxidation of extramitochondrial pyridine nucleotides by the 

 mitochondrial oxidative chain, this subject has been excellently reviewed 

 recently by Ernster (1958), who distinguishes rather sharply between phos- 

 phorylating and non-phosphorylating DPNH-cytochrome c reductase 

 systems. The phosphorylating type is considered to be intra-mitochondrial 

 and to be sensitive to Amytal (believed to block the flavoprotein-DPN 

 stage; Chance, 1956) and to Antimycin A (cf. Fig. 1). The extra-mito- 

 chondrial system requires free cytochrome c and oxidizes DPNH by a 



