358 2. ANALOGS OF ENZYME REACTION COMPONENTS 



histidine is noncompetitive indicates that the histidine site is different 

 from the catalytically active site. ATP and 5'-P-ribosyl-PP protect the 

 enzyme against inactivation by trypsin, and histidine eliminates this pro- 

 tection, so it is possible that histidine modifies the enzyme configuration 

 by combining with the feedback site. 



Aminooxyacetate 



An interesting analog studied recently in connection with the physiolog- 

 ically important y-aminobutyrate (GABA) is aminooxyacetate (+H3N — 

 — CH2COO"). This substances inhibits the GABA: a-ketoglutarate trans- 

 aminase of E. coli very potently (40% at 0.0033 mM and 100% at 0.33 

 mM when both substrates are 27 mM), so it was tested on a similar enzyme 

 from brain and found to inhibit as strongly (Wallach, 1960). Aminooxyace- 

 tate in doses of 6-50 mg/kg elevates the brain GABA in several species as 

 much as 4- to 5-fold, peak levels being reached about 6 hr after administra- 

 tion and high levels remaining up to 24 hr (Wallach, 1961 a; Schumann et al., 

 1962). Reinvestigation of the transaminase inhibition led to a K^ of 0.0075 

 mM (Z,„ is 9.66 mM for a-ketoglutarate and 27.6 mM for GABA); the 

 inhibition is competitive with respect to both substrates. The alanine: 

 «-ketoglutarate transaminase from rat liver and heart is also potently 

 inhibited, around 60-80% by 0.0001 mM aminooxyacetate at pH 6.8 

 (Hopper and Segal, 1964). Indeed, this transaminase seems to be more 

 sensitive than either the GABA:a-ketoglutarate or aspartate:or-ketoglutar- 

 ate transaminase. 



Inasmuch as GABA has been implicated in certain convulsive disorders 

 (e. g., GABA formation in epileptic brains is apparently depressed), amino- 

 oxyacetate was administered to animals made convulsive with thiosemi- 

 carbazide and methionine sulfoximine (DaVanzo et al., 1961). Anticonvul- 

 sant activity was observed but there is some doubt if this is correlated with 

 the brain GABA levels since the time relations are not correct. The effects 

 of aminooxyacetate on central nervous system function are complex. 

 There is first a progressive depression and muscular relaxation with loss of 

 certain reflexes, but at high doses tonic-clonic convulsions occur (DaVanzo 

 et al., 1964 a). Pyridoxal-P antagonizes these convulsions and it was postu- 

 lated that oxime formation occurs between aminooxyacetate and pyri- 

 doxal-P. Pyridoxal-P does not reverse the transaminase inhibition in the 

 brain (Wallach, 1961 b). W^allach also suggested that a depletion of suc- 

 cinic semialdehyde, which arises from GABA by transamination, might 

 also play a role in the convulsant action. Since a pyridoxal deficiency is pro- 

 duced in rats by the administration of aminooxyacetate, DaVanzo et al. 

 (1964 b) postulated another possible mechanism of action, namely, the 

 inhibition of pyridoxal kinase, inasmuch as McCormick and Snell (1961) 

 had shown this enzyme to be rather potently inhibited by the condensation 



