Tryptophan catabolites as metabolic markers of vitamin B-6 status evaluated in cohorts of healthy adults and cardiovascular patients

• Published in: The American journal of clinical nutrition 2020-01, Vol.111 (1), p.178-186
• Main Authors: Ulvik, Arve, Midttun, Øivind, McCann, Adrian, Meyer, Klaus, Tell, Grethe, Nygård, Ottar, Ueland, Per M
• Format: Article
• Language: English
• Subjects: Acids; Aged; Angiography; Anthranilic acid; biomarker; Biomarkers - blood; Cardiovascular Diseases - blood; Cardiovascular Diseases - metabolism; Catabolism; Catabolites; Cohort Studies; Cross-Sectional Studies; Evaluation; Female; Humans; inflammation; Kynurenic acid; Kynurenine - blood; kynurenines; Low concentrations; Male; Markers; Measurement methods; Metabolism; Metabolites; Middle Aged; nutritional status; Pyridoxal Phosphate - blood; Pyridoxine; Regression analysis; Regression coefficients; Regression models; Sex; Substrates; Transaminase; Tryptophan; Tryptophan - metabolism; Vitamin B; Vitamin B 6 - blood; Vitamin B 6 - metabolism; vitamin B-6; Vitamin deficiency; Xanthurenic acid
• ISSN: 0002-9165; 1938-3207
• DOI: 10.1093/ajcn/nqz228

Vitamin B-6 status is routinely measured as pyridoxal 5′-phosphate (PLP) in plasma. Low concentrations of PLP are associated with rheumatic, cardiovascular, and neoplastic diseases. We have previously shown that vitamin B-6 status affects the kynurenine (Kyn) pathway of tryptophan (Trp) catabolism. This study aimed to comprehensively evaluate the use of Kyns as potential markers of functional vitamin B-6 status across 2 large cohorts. We measured circulating concentrations of the first 6 metabolites in the Trp catabolic pathway by LC–MS-MS in the community-based Hordaland Health Study (HUSK; n = 7017) and cardiovascular patient–based Western Norway Coronary Angiography Cohort (WECAC; n = 4161). Cross-sectional and longitudinal associations of plasma PLP with Kyns were estimated using linear and nonlinear regression–based methods. 3′-Hydroxykynurenine (HK), a substrate, and all 4 products formed directly by the PLP-dependent enzymes kynurenine transaminase and kynureninase contributed to the explanation of circulating PLP in multivariable-adjusted regression models. The construct HK:(kynurenic acid + xanthurenic acid + 3′-hydroxyanthranilic acid + anthranilic acid), termed HK ratio (HKr), was related to plasma PLP with standardized regression coefficients (95% CIs) of −0.47 (−0.49, −0.45) and −0.46 (−0.49, −0.43) in HUSK and WECAC, respectively. Across strata of cohort and sex, HKr was 1.3- to 2.7-fold more sensitive, but also 1.7- to 2.9-fold more specific to changes in PLP than a previously proposed marker, HK:xanthurenic acid. Notably, the association was strongest at PLP concentrations < ∼20 nmol/L, a recognized threshold for vitamin B-6 deficiency. Finally, PLP and HKr demonstrated highly sex-specific and corroborating associations with age. The results demonstrate that by combining 5 metabolites in the Kyn pathway into a simple index, HKr, a sensitive and specific indicator of intracellular vitamin B-6 status is obtained. The data also underscore the merit of evaluating alterations in Kyn metabolism when investigating vitamin B-6 and health.