HK:XA is the ratio of HK divided by XA, which is a substrate:product pair of kynurenine aminotransferase (see also the description of HKr). In general, metabolite ratios cancel out the influence from potential confounders affecting the components (metabolites) in the nominator and denominator. This explains why the HK:XA is less affected by e.g. renal function and inflammation than e.g. HK alone (2). HK:XA is somewhat less sensitive and specific to PLP concentrations and more influenced by cellular immunity activation than the HKr index. HK:XA has properties that makes it a integrated index for intracellular vitamin B6 status and cellular immunity activation (3).
Assessment of vitamin B6 status.
Matrix: EDTA plasma and serum.
Volume: Minimum volume is 60 µL, but 200 µL is optimal and allows reanalysis.
Preparation and stability: HK decreases while XA is stable during storage of serum/plasma samples. The blood sample must be centrifuged and the plasma/serum fraction put on ice, and frozen.
Reported values: 1.5-2.7.
Intraclass correlation coefficients (ICC): na.
1. Midttun, O., Hustad, S., and Ueland, P.M. (2009). Quantitative profiling of biomarkers related to B-vitamin status, tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Sp 23, 1371-79.
2. Ueland, P.M., Ulvik, A., Rios-Avila, L., Midttun, Ø., and Gregory, J.F. (2015). Direct and Functional Biomarkers of Vitamin B6 Status. Annu Rev Nutr 35, 33-70.
3. Ulvik, A., Theofylaktopoulou, D., Midttun, O., Nygård, O., Eussen, S.J., and Ueland, P.M. (2013). Substrate product ratios of enzymes in the kynurenine pathway measured in plasma as indicators of functional vitamin B-6 status. Am J Clin Nutr 98, 934-940.