Distribution of biotin in human plasma: most of the biotin is not bound to protein.

Estimates of the plasma concentration of biotin differ considerably. Variation in detectability of biotin bound covalently to protein is one potential source of disagreement. In this study we determined the amount of biotin covalently bound to plasma protein. First, greater than 99% of free and reversibly bound biotin was removed by dialysis; then greater than 90% of covalently bound biotin was released by acid hydrolysis. For plasma samples from 11 normal adults, the ratio of covalently bound biotin to free biotin was 0.15 +/- 0.09 (mean +/- SD). Taking into account the additional biotin that is reversibly bound to protein, this study provides evidence that approximately 12% of total biotin in plasma is covalently bound, 7% is reversibly bound, and 81% is free. We conclude that covalently bound biotin cannot account for the reported sixfold increase in biotin detected after acid hydrolysis. We speculate that the reported increase was an artifact caused by substances produced during acid hydrolysis of plasma.