Lipoamidase activity in human serum is due to biotinidase

Lipoamidase, as determined by lipoyl-p-aminobenzoic acid (L-pABA) hydrolyzing activity, and biotinidase in human serum have similar pH profiles, molecular weights, thermostabilities, and are similarly inhibited by p-hydroxymercuribenzoate and not inhibited by phenylmethylsulfonylfluoride. A monospecific polyclonal antibody prepared against biotinidase immunoprecipitated > 95% of serum L-pABA hydrolyzing activity and an identical proportion of biotinidase activity. In addition, children with profound biotinidase deficiency (< 10% normal serum activity) have greatly reduced levels of L-pABA hydrolyzing activity in serum (< 15% of mean normal activity) and obligate heterozygotes have activities intermediate between that of normal and profoundly deficient individuals. These results indicate that most, if not all, of the L-pABA hydrolyzing activity in human serum is due to biotinidase. Moreover, since the K_m of L-pABA hydrolysis by serum is high, it is unlikely that lipoic acid is recycled in the serum by biotinidase.