548 G. GUROFF AND S. UDENFRIEND 
180 a * Le ee af ao =a 
160;- a = 
Pa ae 
y ~~ PLASMA 
~ 
140|— K tS 4 
/ ~ 
/ Pee | 
120 k “a 
MUSCLE 

MUSCLE p= —— 
= 
=- 
PLASMA 
| 
L-TYROSINE | 
———— D-TYROSINE 7 
| 

0 =) icon OO 120 
MINUTES 
Fig. 3. Uptake of tyrosine by gastrochemius muscle of rat in vivo. 
then, takes up L-tyrosine more rapidly than D-tyrosine but the marked stereospecifi- 
city and the inhibition of tyrosine uptake by other amino acids shown in brain is not 
evident in muscle. It can be noted that some stereoselectivity has also been observed 
in studies on tyrosine uptake by diaphragm 7m vitro”. 
Another phase of these studies has been the uptake of tyrosine by brain 7 vitro?. 
These experiments have been conducted in an attempt to localize the uptake system 
and obtain more information about its biochemical mechanism. Work on amino 
acid uptake by brain slices has appeared!*—!§ but the mechanism of this uptake is 
obscure. 
The uptake of L-tyrosine by brain slices exposed to 1 mM L-tyrosine dissolved in 
Krebs—Ringer—bicarbonate buffer (Fig. 4) was rapid and concentrative. Observation 
TABLE III 
EFFECT OF OTHER COMPOUNDS ON THE UPTAKE OF L-TYROSINE 
BY BRAIN AND MUSCLE in vivo 

Tissue-to-plasma ratio (30 min) 

Compound* 

Brain Muscle 
Control 0.63 0.88 
L-Tryptophane 0.20 0.70 
L-Isoleucine 0.18 0.71 
p-Fluoro-pL-phenylalanine 0.27 0.77 
pL-Norleucine 0.29 0.75 
p-Hydroxyphenylacetic acid 0.51 0.72 
L-Glutamic acid 0.52 0.72 
L-Glutamine 0.43 0.68 

* Compounds were injected (1000 mg/kg) simultaneously with 
L-tyrosine (500 mg/kg). 
References p. 553 
