TDO2 and major depressive disorder: For example, ammonia, produced by amino acid deamination, and hydrogen sulfide, produced from methionine and cysteine degradation, are toxic [61–63]; indoles, produced by tryptophanase, and γ-aminobutyric acid, produced by glutamate decarboxylase, are neurotransmitters [64, 65]; branched-chain fatty acids, produced by branched-chain amino acid degradation, have been suggested to be anti-inflammatory [66, 67]; and proline metabolism has been linked to depression [68] and enteric infections [69].