pH dependence, substrate specificity and inhibition of human kynurenine aminotransferase I.
Human kynurenine aminotransferase I/glutamine transaminase K (hKAT-I) is an important multifunctional enzyme. This study systematically studies the substrates of hKAT-I and reassesses the effects of pH, Tris, amino acids and alpha-keto acids on the activity of the enzyme. The experiments were comprised of functional expression of the hKAT-I in an insect cell/baculovirus expression system, purification of its recombinant protein, and functional characterization of the purified enzyme. This study demonstrates that hKAT-I can catalyze kynurenine to kynurenic acid under physiological pH conditions, indicates indo-3-pyruvate and cysteine as efficient inhibitors for hKAT-I, and also provides biochemical information about the substrate specificity and cosubstrate inhibition of the enzyme. hKAT-I is inhibited by Tris under physiological pH conditions, which explains why it has been concluded that the enzyme could not efficiently catalyze kynurenine transamination. Our findings provide a biochemical basis towards understanding the overall physiological role of hKAT-I in vivo and insight into controlling the levels of endogenous kynurenic acid through modulation of the enzyme in the human brain.
Han
Qian
Q
Department of Pathobiology, University of Illinois at Urbana-Champaign, Urbana, IL 61802, USA.
Li
Junsuo
J
Li
Jianyong
J
D000586:Amination; D001483:Base Sequence; D001921:Brain / Q000201:enzymology; D002021:Buffers; D002384:Catalysis; D017931:DNA Primers; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007737:Kynurenine / Q000378:metabolism; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D013057:Spectrum Analysis; D013379:Substrate Specificity; D000637:Transaminases / Q000037:antagonists & inhibitors / Q000378:metabolism*
D002021:Buffers; D017931:DNA Primers; D011994:Recombinant Proteins; D007737:Kynurenine; D000637:Transaminases; C021481:kynurenine-oxoglutarate transaminase