Lipoic acid (thioctic acid) analogs, tryptophan analogs, and urea do not interfere with the assay of biotin and biotin metabolites by high-performance liquid chromatography/avidin-binding assay

Lipoic acid, urea, and tryptophan show structural similarities to the vitamin biotin. If these compounds can successfully compete with biotin or a biotinylated protein for binding to avidin, their presence in serum and urine would cause artifacts in avidin-binding assays for biotin. We assessed the ability of lipoic acid and certain analogs, urea, and L-tryptophan and tryptophan derivatives to interfere with the measurement of 16 biotin analogs by a high-performance liquid chromotography (HPLC)/avidin-binding assay. In this assay, compounds are separated by reversed-phase HPLC, followed by assay of each fraction based on binding to avidin horseradish peroxidase. At physiologic concentrations, neither lipoic acid analogs (d-lipoate, l-lipoate, d,l-lipoamide, bisnorlipoate, β-hydroxybisnorlipoate, and tetranorlipoate) nor tryptophan derivatives exhibited detectable avidin-binding. Minor avidin-binding was seen for urea and L-tryptophan; binding ratios relative to biotin were 1 x 10^-9 and 3 x 10^-6, respectively. Urea did not co-elute on HPLC with any of the 16 biotin analogs, but L-tryptophan did co-elute with bisnorbiotin methyl ketone. However, because the relative avidin affinity of L-tryptophan is five orders of magnitude smaller than that of bisnorbiotin methyl ketone, we conclude that none of the tested compounds are likely to interfere with the measurement of biotin or its metabolites by an HPLC/avidin-binding assay.