FREE AMINO ACIDS IN NERVOUS TISSUE 479 
Other amino acids (Table III). The compounds grouped under this designation occur 
in amounts from the barely detectable to approx. 0.5 wmole/g in vertebrate nervous 
tissue (much larger amounts are present in invertebrate nerve) and include the essen- 
tial amino acids and those that may be indispensable under certain conditions!®. 
In addition to the substances listed in Table III, the following have been reported 
to occur in nervous tissue, though little information is available concerning the 
actual amounts present: hydroxyproline!?, citrulline’® (not detected in the cat, 
however!®), hydroxylysine phosphate!!, cysteinesulfinic acid®®, and hypotaurine?®, 
The amide of cysteic acid occurs in squid giant nerve axoplasm*”: 48; its concentration 
is 4.86 wmoles/g fresh axoplasm of Loligo pealit, 0.42 uwmole/g in Dosidicus gigas". 
Also present in small amounts in squid giant nerve axoplasm are methionine sulf- 
oxide and citrulline*®: 0.32 and 0.43 wmole/g respectively in Loligo, 0.21 and 0.16 
umole/g in Dosidicus**, 
The distribution of lysine in various parts of the monkey brain’! and the distri- 
bution in human brain of dihydroxyphenylalanine!™’, leucine and isoleucine”? have 
been determined. In several species that have been studied, the concentration of 
tryptophane is much higher in the hypothalamus, pons and mid-brain than in other 
parts of brain!’’. In mice, the concentration of lysine in the brain is higher at birth 
(0.55 wmole/g) than in the young animal (0.35 wmole/g) or adult (0.26 wmole/g), 
(ref. 105). Leucine decreases from 0.15 wmole/g in the newborn to 0.07 in the adult}, 
There is more arginine, tyrosine, and y-aminobutyric acid in fetal mouse brain than 
in the brains of the mothers. In man, leucine and isoleucine tend to increase from 
the fetal to the adult brain!’. 
Peptides. The following peptides are definitely known to occur in brain: glutathione 
(Table I), homocarnosine, carnosine and the so-called Substance P. Hydrolysis of 
protein-free extracts of cat brain results in unexplained increases of histidine, lysine, 
and of a substance in the chromatographic position of f-aminoisobutyric acid!®; in 
hydrolyzed extracts of rat brain there are increases in the amounts of leucine, valine, 
histidine, isoleucine, and proline!#®. BOULANGER AND BISERTE*! have separated dog, 
pig and beef-brain extracts into a basic plus neutral fraction and three acidic frac- 
tions. After hydrolysis, the basic plus neutral fraction gave /-alanine, proline, 
aspartic acid and glutamic acid; one acidic fraction gave large amounts of /-alanine 
and y-aminobutyric acid after hydrolysis; another, small amounts of several amino 
acids; and the third, large amounts of aspartic acid, glutamic acid and cysteic acid. 
None of the parent substances, which could be peptides, appear to have been studied 
further. It should be noted that in the work on the cat!®, there was an appearance 
after hydrolysis of glutamic acid, glycine, cysteic acid and aspartic acid that could 
be accounted for as arising from glutathione, glutamine and N-acetylaspartic acid. 
The increase in histidine after hydrolysis suggests the presence of carnosine; 
carnosine was reported absent in dog brain!*! and only possibly present in ox brain", 
but there have been several recent reports of its occurrence? ®® 134, ABRAHAM et al." 
state that carnosine was found in all brains examined, at values varying from almost 
none in human brain to 0.03 wmole/g in beef brain. It has recently been demon- 
strated that y-aminobutyryl-L-histidine (homocarnosine) also occurs in brain!™?, at 
values ranging up to 0.1-0.3 wmole/g in human, monkey, beef and rabbit brain’; 
0.18 wmole/g was found in rabbit spinal cord, none was detectable in rabbit sciatic 
References p. 482/485 
