Efficiency of tryptophan-niacin conversion in chickens and ducks

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

NAD+ Kinase Enzymes Are Reversible, and NAD+ Product Inhibition Is Responsible for the Observed Irreversibility of the Human Enzyme

The NAD⁺ kinase (NADK) is the only known enzyme capable of phosphorylating NAD(H) to NADP(H) and therefore it plays a crucial role in maintaining NAD(P)(H) homeostasis. All domains of life contain at least one NADK gene, and the commonly investigated isoforms have been measured, or assumed, to be functionally irreversible. In 1977, the kinetics of native pigeon liver NADK were thoroughly investigated, and it was reported to exhibit reversible activity, such that ATP and NAD⁺ can be formed from ADP and NADP⁺. We hypothesized that the reverse activity of the pigeon enzyme may enable compensation of the high picolinic acid carboxylase (PC) activity present in pigeon livers, which inhibits NAD⁺ biosynthesis from dietary tryptophan. Here, we report the characterization of four recombinantly expressed NADKs and explore their reversible activities. Duck and cat livers have higher PC activity than pigeon livers, and the recombinant duck and cat NADKs exhibit high activity in the reverse direction. The human NADK has an affinity for NAD⁺ that is ∼600 times higher than the pigeon, duck, and cat isoforms, and we conclude that NAD⁺ serves as a potent product inhibitor for the reverse activity of the human NADK, which accounts for the observed irreversible behavior. These results demonstrate that while all four NADKs are reversible, the reverse activity of the human enzyme alone is impeded via product inhibition. This mechanism─the conversion of a reversible to a unidirectional reaction by product inhibition─may be valuable in future metabolic engineering applications.

Excess dietary lysine increases growth of chicks fed niacin-deficient diets, but dietary quinolinic acid has no niacin-sparing activity

Catabolism of Trp and Lys produces alpha-ketoadipic acid as an intermediary metabolite. An alternate pathway of Trp turnover leads to NAD synthesis. We hypothesized that excess Lys might improve the conversion of Trp to niacin by causing a buildup of alpha-ketoadipic acid, thereby endproduct inhibiting the main Trp catabolic pathway and resulting in more niacin synthesis from Trp. Six bioassays were carried out in which 12 to 20 chicks were fed each experimental diet from d 8 to d 20 or 21 posthatching. The basal diet (4 mg/kg of bioavailable niacin) used for all assays was a semipurified corn gluten meal diet fortified with crystalline amino acids to 22.5% CP and 0.96% true digestible Lys. Assay 1 through 3 established the requirements for digestible Trp (0.16%) and bioavailable niacin (19.5 mg/kg) and showed that 0.96% digestible Lys was adequate for chick growth in the presence of adequate Trp and niacin. The fourth assay was done to determine the effect of 1% Lys (1.25% food-grade L-Lys x HCl) on niacin utilization. Excess Lys improved (P < 0.01) weight gain of niacin-deficient chicks. The fifth assay showed that 1% excess food-grade Lys improved weight gain in niacin-deficient (4 mg/kg) chicks but depressed weight gain in niacin-adequate (24 mg/kg) chicks (niacin x Lys interaction, P < 0.01). In assay 6, chicks fed 6 mg/kg of niacin gained faster (P < 0.01) than control chicks, but neither quinolinic acid (100 mg/kg) nor picolinic acid (4,200 mg/kg) elicited a response. These results suggest that excess Lys leads to an accumulation of alpha-ketoadipic acid, which causes endproduct inhibition of the main Trp catabolic pathway to CO(2), therefore increasing flux of 2-amino-3-carboxymuconate semialdehyde to NAD.