STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 123 



explained by the relative priority of certain synthetic systems for the amino 

 acid pool, the possibihty that these results arise from a selective degradation 

 of the latter enzymes must also be considered.^ 



For a long time, brain proteins were considered to have a very slow 

 turnover, a fact thought to support the concept that turnover reflected 

 population changes rather than events within individual cells. The pheno- 

 mena of chromatolysis behind severed axons and the regeneration of the 

 axon suggests that individual cells possess the capacity for protein turnover, 

 at least imder these abnormal conditions. Using lysine-C^*, an amino acid 

 which more readily penetrates the blood-brain barrier, Lajtha et al. (1957) 

 have demonstrated a rapid uptake of the amino acid into mouse brain, 

 particularly into the microsomal fraction. Furthermore, the half -life of lysine 

 in brain was a maximum of 20 days. Since such cells must maintain a constant 

 size for many years, one would anticipate the existence of a renewal mechan- 

 ism, even as seems to have been found. 



Furthermore, studies of protein degradation m a variety of mammalian cells 

 have revealed that, as in protein synthesis, the release of labeled amino acids 

 requires an energy source and is inhibited by anaerobiosis and dinitrophenol 

 (Simpson, 1953; Steinberg and Vaughan, 1956). The release is also inhibited by 

 the amino acid analog, ^-fluorphenylalanine. These results suggest that the 

 release is not primarily autolytic. A similar result was obtained by Moldave 

 (1957) on the release of labeled amino acids from structures of ascites cells. 



The hypothesis of Hogness et al. concerning the absence of protein turnover 

 relies exclusively on work with E. coli. However, in yeast, conditions of 

 nitrogen deficiency produce enzymatic deadaptation, i.e., the apparent 

 destruction of enzymes (Robertson and Halvorson, 1957), and an initial stage 

 in deadaptation to a-glucosidase mcluded a release of the enzyme from a 

 bound to the soluble state, before the latter also disappeared. In this con- 

 nection, reference may be made to the disappearance of the 840 particles of 

 E. coli during starvation (Dagley and Sykes, 1957). Evidently the " static state 

 of body constituents" in microorganisms also warrants a closer examination. 



2. Nucleic Acids 



In general, the rate of P^^ incorporation into DNA in a given cell parallels 

 the mitotic activity of the cell (with a few exceptions, as in the development 



^ We may note the existence of experiments on enzyme synthesis in ammo acid- 

 deficient rats which are superfically similar to the bacterial mutant experiments of 

 Monod et al. (Cohn, 1954), but which have yielded contrary results. The livers of rats fed 

 nonprotein diets are depleted with respect to a number of enzymes, such as xanthme oxi- 

 dase, succinic oxidase, and cholme oxidase. When such rats are fed diets deficient only m 

 a single amino acid, such as histidine, there are marked restorations of these enzymes 

 (Prigmore et al., 1955). It was suggested by these authors that under these conditions 

 missing amino acids may be salvaged from the body proteins. Such experiments, 

 however, do not take the turnover of cellular populations into account. 



