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Enzyme | Cellular brain location | Characteristic | Optimum pH | Substrates with higher potency towards | Cosubstrate | Endogenous inhibitors | References |
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KAT-I/glutamine transaminase K/cysteine conjugate beta-lyase 1 | Glia, astrocytes (nuclei and cytosol), and neurons (cytoplasmic) | (i) KAT I and KAT III share similar genomic structures (ii) KAT I immunoreactivity was found in ventral medulla, nucleus ambiguus, nucleus of the solitary tract, and intramedio lateral cell column of the spinal cord (iii) pKa 7.6 (iv) Enzyme partially purified: Km 875 μmol/L, KYN concentrations ranging from 2 μmol/L to 2 mmol/L (v) Relative percentage of brain activity: Mouse: 25.7 Rat: 15.5 Human: 9.8 (vi) Present high activity in the cerebellum | 9.5–10 | Glutamine | Pyruvate | Glutamine | [84–93] |
Phenylalanine | Tryptophan |
Kynurenine (hKAT-I under physiological pH) | Phenylalanine |
Indole-3-pyruvic acid |
Cysteine |
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KAT-II/α-aminoadipate aminotransferase | Astrocytes | (i) At physiologic KYN concentrations and pH, KAT II catalyzed around 75% of KYNA synthesis in most brain areas (ii) pKa 5.7 (iii) Enzyme partially purified: Km 660 μmol/L, KYN concentrations ranging from 2 μmol/L to 2 mmol/L (iv) Relative percentage of brain activity: Mouse: 12.3 Rat: 58.7 Human: 54.1 | 7.4 | Kynurenine | α-Ketoglutarate | Aminoadipate | [85, 89, 90, 94, 95] |
Glutamate | Pyruvate | Asparagine |
Aminoadipate | Glutamate |
Methionine | Histidine |
Cysteine |
Lysine |
3-Hydroxykynurenine phenylalanine |
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KAT-III/cysteine conjugate beta-lyase 2 | | (i) mKAT III shows activity toward a number of amino acids (ii) mKAT III is more active than hKAT I under basic conditions (iii) Northern blot analysis showed a strong transcript in the liver, kidney, and heart and to a less extent in the brain and testis (iv) Has a higher isoelectric point than KAT I (v) 3-HK decreases mKAT III-catalyzed kynurenine transamination (vi) pKa 8.7 | 9-10 | Glutamine | α-Ketobutyrate | Cysteine | [95–97] |
Histidine | Oxaloacetate | Glutamine |
Methionine | Histidine |
Phenylalanine | Methionine |
Leucine |
Phenylalanine |
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KAT-IV/glutamic-oxaloacetic transaminase 2/mitochondrial aspartate aminotransferase | Mitochondria of astrocytes and neurons | (i) mKAT I, III, and IV showed high resistance to heat treatment (ii) pKa 6.9 (iii) Enzyme partially purified: Km 724 μmol/L, KYN concentrations ranging from 2 μmol/L to 2 mmol/L (iv) Relative percentage of brain activity: Mouse: 63.0 Rat: 25.8 Human: 36.1 | 8.5 | Aspartate | α-Ketoglutarate | Aspartate | [94, 98] |
Glutamate | Glutamate |
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