88 GENERAL METABOLISM in vitrO 



and the lack of knowledge of acceptors and the reactions in which 

 they take part. At present only hints can be made at the mechanisms 

 involved (see p. 159). Nevertheless, the function of certain of these 

 enzymes can still be explained in terms of a degradative role. Thus 

 in cerebral dispersions diphosphopyridine nucleotide is degraded 

 by a nucleotidase which can operate at rates up to 200 jumoles/g 

 wetwt. hr~i (Mann and Quastel, 1941; Utter et al., 1948; 

 Mcllwain and Rodnight, 1949«, b). The nucleotidase is specific 

 for the oxidized form of the nucleotide and does not degrade the 

 reduced form. A possible role for the enzyme is thus the main- 

 tenance of a high ratio of reduced to oxidized form thereby exert- 

 ing a partial control upon the rate at which oxidative phosphoryla- 

 tion, most of which occurs during the oxidation of reduced 

 diphosphopyridine nucleotide, is able to proceed. Such a role 

 in vivo would require proportions of the two forms different from 

 those found by Clock and McLean (1955). 



Phospholipids 



Synthesis. — When slices of rat cerebral cortex were incubated in 

 oxygenated salines containing glucose, radioactive phosphate was 

 incorporated into the phospholipids of the slices (Schachner et al., 

 1942; Fries et al., 1942). Uptake was comparatively rapid, some 

 0-5% of the labelled inorganic phosphate added to the medium 

 being incorporated into the total phosphohpids within 4 hr. 

 Incorporation was not considered to be due to a total synthesis of 

 lipid but to be the result of a steady exchange between the phos- 

 pholipid and some radioactive precursor. These findings were 

 confirmed with cat cerebral slices by Strickland (1954) and 

 Findlay et al. (1954), who showed that labelling of the lipid 

 phosphorus depended upon the maintenance of conditions which 

 were optimal for oxidative phosphorylation. As in vivo the degree 

 of labelling of the lipids was not uniform. To study incorporation 

 into the different lipids Dawson (1954) hydrolysed lipid fractions 

 under alkaline conditions to yield identifiable fragments containing 

 the phosphorus. It was found that during metabolism in a suitably 

 reinforced brain brei, radioactive phosphorus was incorporated 

 into cephalin diphosphoinositide to an amount some 50-100 times 

 greater than into any other recognizable phospholipid (Dawson, 

 1953). The lipids phosphatidyl serine, phosphatidyl ethanolamine, 

 phosphatidyl choline and sphingomyelin incorporated less than 



