Abstract
In previous studies, the bioavailability of biotin in humans was estimated from the recovery of biotin in urine; urinary biotin was measured by microbial growth assays or assays of avidin-binding activity. These assays underestimate concentrations of biotin metabolites, which originate from β-oxidation, sulfur oxidation or a combination. We have developed an HPLC/avidin-binding assay that is specific for biotin and its metabolites. With the use of the HPLC/avidin-binding assay, TLC and derivatization with p-dimethylaminocinnamaldehyde, we have identified and quantitated biotin and metabolites in urine from six healthy adults. Of that total, biotin accounted for 32 ± 12%, bisnorbiotin for 52 ± 15%, bisnorbiotin methyl ketone for 7.9 ± 5.8%, biotin-d,l-sulfoxide for 4.0 ± 3.2% and biotin sulfone for 3.6 ± 1.9%. After intravenous administration of 18.4 μmol of biotin, the urinary excretion of biotin metabolites increased 21–130 times above baseline values. Because the biliary excretion of biotin is quantitatively minor (1.9 ± 0.2% of an intravenous [14C]biotin dose in rats), intravenously administered biotin is not exposed to intestinal microorganisms. Thus we conclude that biotin metabolites in human urine originate from biotin catabolism in human tissues rather than biotin catabolism by intestinal microorganisms. With the use of the HPLC/avidin-binding assay, we estimated the bioavailability of biotin in adults from the urinary excretion of biotin and metabolites after ingestion of 2.1, 8.2 and 81.9 μmol of biotin. These data provide evidence that biotin is nearly completely absorbed.
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