BIOTIN AND BIOTIN ANALOGS IN HUMAN URINE - BIOTIN ACCOUNTS FOR ONLY HALF OF THE TOTAL

In studies using avidin-binding assays to measure the urinary excretio n of biotin, biotin is sometimes assumed to be equal to the detectable avidin-binding substances present. High performance liquid chromatogr aphy was used to separate avidin binding substances in human urine, an d the chromatographic fractions were assayed for avidin-binding substa nces (biotin and biotin analogs) by a sensitive, specific assay based on binding of biotin to [I-125]avidin. In a study of ten normal adults , substantial amounts of avidin-binding substances other than biotin w ere detected, two of which were bisnorbiotin and biotin sulfoxide. The se biotin analogs were initially identified by their chromatographic p roperties, and identities were confirmed by chemical conversion. The p resence of avidin-binding substances in addition to biotin may have co nfounded previous measurements of the urinary excretion of biotin usin g avidin-binding assays. Because bioassay methods for biotin often use organisms for which one or more of these biotin analogs are growth fa ctors, measurements of biotin in urine using some bioassay methods are likely to overestimate the concentrations of biotin.