IDO2 and major depressive disorder: There was also a significant positive relationship between the KYN/TRP ratio, indicative of cellular immune activation and activation of the TRP-degrading enzyme indoleamine 2,3-dioxygenase (Ulvik et al., 2016), and PA (B6 catabolite) at baseline in patients with depression (ρ ​= ​0.35, p ​= ​0.001), as well as between KYN/TRP and the PAr (ρ ​= ​0.301, p ​= ​0.003), and a negative relationship between PLP and KYN/TRP (ρ ​= ​−0.26, p ​= ​0.01) and between nicotinamide and KYN/TRP (ρ ​= ​−0.30, p ​= ​0.003).