502 Cc. F. BAXTER AND E. ROBERTS 
doxal and adenosine triphosphate’, was inhibited by thiosemicarbazide to a much 
greater extent than GAD: %. Nevertheless, when thiosemicarbazide was adminis- 
tered intraperitoneally to rats, the only significant effect on amino acids in 
brain was upon GABA levels (Figs. 5-10). The apparent specificity of thiosemi- 
carbazide for GAD might be attributed to the loose attachment of PyP to the GAD 
apoenzyme. This would make GAD particularly susceptible to PyP depletion through 
‘t: 12 
Figs. 11-14. Effect of hydroxylamine upon the concentration of amino acids in brain of rat. 
Injected dose NH,OH-HCI (adjusted to pH 6): 75 mg/kg body wt. Animals were decapitated 
90 min after injection. Chromatograms represent extracts of 25 mg equivalent wet weight of 
tissue. Figs. 11, 12: cortex. Figs. 13, 14: diencephalon. Arrows point to GABA. All odd num- 
bered chromatograms represent tissues from normal control animals; all even numbered chro- 
matograms illustrate changes in the tissues of treated animals. 
inhibition of pyridoxal kinase. The depressed levels of cerebral GABA in thiosemi- 
carbazide-treated rats could not be restored to normal levels by the injection of 
pyridoxal or pyridoxine (Figs. 5-10). Even 5h after pyridoxal was administered, 
the levels of GABA in all areas of brain which were tested had not returned to normal 
(Table I). The results give added support to the belief that im vivo the primary 
inhibition by thiosemicarbazide was of pyridoxal kinase. Presumably the resultant 
vitamin B, depletion then decreased GAD activity and GABA levels. The effect of 
a number of hydrazides upon the levels of GABA, glutamic acid and aspartic acid 
in the brains of mice has been reported!®, While thiosemicarbazide inhibited GAD 
in mouse brain, it did not affect the level of GABA. This contrasts with our results 
(Table I, Fig. 6) in rats where both GAD enzyme activity and GABA levels were 
affected by thiosemicarbazide. It was shown also in mice that the administration of 
vitamin B, could not rapidly restore levels of GABA which had been depressed by 
a prior injection of hydrazides other than thiosemicarbazide!®. This finding is in 
References p. 508 
