Effect of Dietary Phytol on the Expression of α-Amino-β-carboxymuconate-ε-semialdehyde Decarboxylase, a Key Enzyme of Tryptophan-niacin Metabolism, in Rats

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α-Amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) plays a key role in the regulation of NAD biosynthesis or the production of quinolinate from tryptophan (Trp). We investigated in this study the effect of phytol, a phytochemical known as a peroxisome proliferator-activated receptor α (PPARα) ligand, on NAD synthesis and ACMSD expression in rats. Male Sprague-Dawley rats were fed a diet containing 0.5%, 1%, or 2% phytol for 7 d. Phytol decreased the ACMSD activity and its mRNA expression in a dose-dependent manner in the liver. Phytol similarly and significantly suppressed ACMSD mRNA expression in primary rat hepatocytes. However, the mRNA expression of ACO (a known PPARα target gene) was higher in the low-phytol groups than in the high-phytol group in vivo and in vitro. Phytol increased the blood NAD level by suppressing ACMSD mRNA expression in the liver of the rats. It is possible that this mechanism occurred by the activation of PPARα and also of other transcriptional factors.