82 GENERAL METABOLISM in vitrO 



used to esterify glucose to glucose-6-phosphate in the presence of 

 hexokinase or to convert creatine to phosphocreatine (see also 

 Colowick et al, 1941; Goranson and Elrulkar, 1949). In the 

 presence of sodium fluoride, added at concentrations sufficient to 

 inhibit adenosine triphosphatase, some 2-3 atoms of inorganic 

 phosphate were consumed for each atom of oxygen yielding 

 phosphorus/oxygen ratios of 2-3-0. 



These experiments have been confirmed with a variety of 

 dispersions (Case and Mcllwain, 1951; Clowes and Keltch, 1952; 

 Lee and Eiler, 1953) and more particularly with mitochondrial 

 preparations oxidizing a wide variety of substrates (Table 13). 

 Examination of the phosphorus/oxygen ratios obtained at different 

 temperatures suggests that the efficiency of oxidative phosphory- 

 lation decreases as the temperature increases. Such a phenomenon 

 has been previously suggested by Christie et al. (1953), with brain 

 preparations, and by Dianzani (1954) with liver preparations. 

 Also the phosphorylative activity of cerebral mitochondria, as 

 judged by phosphorus/oxygen ratios in excess of 1-5, is maintained 

 for a much longer period if measurements are carried out at 18° 

 rather than at 37° (Brody and Bain, 1952). Thus at 37 °C maximum 

 activity was maintained for up to 20 min while at 18° it could be 

 maintained for up to 70 min. On the other hand Aldridge (1957) 

 obtained a steady and linear rate of oxygen consumption combined 

 with phosphorus/oxygen ratios of 2-2-3 at 37° over a period of 

 1-1 J hr. Examination of Fig. 1 of Brody and Bain (1952) reveals 

 that at 37 °C the uptake of phosphate by the mitochondria in the 

 first 10-20 min of the experiment was initially more rapid than was 

 the oxygen uptake, thereafter the uptake of phosphate remained 

 constant while the oxygen uptake steadily increased. At 18 °C the 

 rates of phosphate and oxygen uptake were slower and almost 

 constant over a much longer period of time, thus contributing to 

 an almost stable phosphorus/oxygen ratio. 



Differences of this type are at least partly due to the nature of the 

 preparation employed. Thus under suitable conditions, mito- 

 chondrial preparations can be obtained in which the adenosine 

 triphosphatase activity is not apparent and in which the oxygen 

 uptake can be stimulated by the addition of hexokinase and glucose 

 (Aldridge, 1957). The particles in these preparations are considered 

 to have suffered little physical damage. Concern over differences 

 between preparations is of less importance if the preparations are 



