INVITED DISCUSSION 
UPTAKE OF TYROSINE BY BRAIN IN VIVO AND EN VITRO 
GORDON GUROFF anp SIDNEY UDENFRIEND 
Laboratory of Clinical Biochemistry, National Heart Institute, 
National Institutes of Health, Bethesda, Md. (U.S.A.) 
Our interest in the last few years has centered around the transport of tyrosine 
into the brain! ?. The use of tyrosine was prompted by the availability of a rapid, 
specific, sensitive chemical method of assay® and by the realization that the insolubil- 
ity of this amino acid would allow intraperitoneal depots to maintain high constant 
levels in blood plasma for an extended period. The selection of brain for these in- 
vestigations involved, among other things, the long-term interest of our research 
group in the neuropharmacology of hydroxylated aromatic compounds. 
20 sz | eae 
80}- 
70};- 

PLASMA 





60+ 
wu 
° 
T 
g/g (or ml) 
a 
T 
Ww 
° 
20 



() | Ji L 
() 30 60 90 120 
MINUTES 
Fig. 1. Tyrosine distribution in brain and plasma after intraperitoneal injection of L-tyrosine. 
The uptake of amino acids by brain cannot be reviewed here but it can be noted 
that certain specific amino acids have been extensively studied** and that the up- 
take of several different amino acids has been investigated under specified con- 
ditions® 1°. The prevailing conclusion drawn from these past studies is that excess 
amino acids from blood enter the brain to a very limited extent or not at all. On the 
other hand, administration of labeled amino acids leads to rapid exchange between 
blood and brain®: ! indicating that the brain substance is freely permeable to amino 
acids. These contradictory lines of evidence have not yet been resolved. 
References p. 553 
