METABOLISM IN THE NORMAL FUNCTIONAL STATE 29 



phosphoprotein fraction than into the fraction associated with the 

 particles. Recently, Vladimirov et al. (1957) described a fraction, 

 isolated from a lipid free brain residue, which was not a phospho- 

 protein but nevertheless exchanged its phosphorus at a comparable 

 rate. 



The finding that ribose nucleic acid exchanges its phosphorus at 

 an appreciable rate (Fig. 5) is of considerable interest. It is now 

 generally accepted that ribonucleic acid is in some way involved in 

 protein synthesis, though it is not necessarily the case that protein 

 synthesis and ribonucleic acid synthesis should proceed simul- 

 taneously. Recently it has been shown, contrary to earlier belief, 

 that the proteins of brain are metabolically active, exchanging their 

 aminoacids for others entering from the blood stream (Gaitonde 

 and Richter, 1956, 1957; Clouet and Richter, 1959; Furst et al, 

 1958; Lajtha, 1958). It is likely that in the brain as in other cells 

 the turnover of ribonucleic acid and protein synthesis will be 

 found to be interdependent (cf. Einarson, 1957). Exchange of 

 phosphorus with desoxy ribonucleic acid is extremely slow, the 

 significance of which is not entirely clear. It may be partly 

 associated with the absence of cell proliferation in the adult brain. 

 It should perhaps also be emphasized that the measurements so far 

 made concern phosphorus exchange only. There is no reason to 

 believe that other parts of the molecule exchange equally slowly. 

 Present evidence (see Einarson, 1957, for summary) tends to sug- 

 gest the contrary though biochemical evidence is lacking. 



Incorporation of phosphorus into the residual organic phos- 

 phorus fraction is similarly a process the significance of which is 

 obscure. The nature of this fraction has been commented upon 

 above. Generally, measurement of change in the radioactivity of 

 phosphorus in compounds of this type does no more than indicate 

 the existence of a rapid or slow rate of exchange. It is for further 

 work to establish the significance of such exchange. 



References 



Abood, L. G., Gerard, R. W., Banks, J. and Tschirgi, R. D. (1952) 



Amer. J. Physiol 168, 728. 

 Agren, G. and Engstrom, L. (1956) Acta. chem. scand. 10, 489. 

 Albaum, H. G., Tepperman, J. and Bodansky, O. (1946) J^. biol. chem. 



164, 45. 



