Lipoamidase and biotinidase deficiency: evidence that lipoamidase and biotinidase are the same enzyme in human serum.

Late-onset multiple carboxylase deficiency depends on biotinidase deficiency and is inherited as an autosomal recessive trait. Lipoamidase deficiency in humans has not been previously reported, using the natural substrate lipoyllysine for lipoamidase. In this report we describe a simultaneous decrease in both lipoamidase and biotinidase activity in serum from a 21 month-old boy with a profound biotinidase deficiency. Lipoamidase activity in human serum was determined with both lipoyllysine (epsilon-N-(D,L-lipoyl)-L-lysine) and N-D,L-lipoyl-p-aminobenzoate as substrates. Biotinidase activity was determined with both biocytin (epsilon-N-(D-biotinyl-L-lysine) and N-D-biotinyl-p-aminobenzoate as substrates. Our findings indicate that lipoamidase activity and biotinidase activity in human serim are due to the same enzyme, but a "residual activity" was usually found when N-D,L-lipoyl-p-aminobenzoate was used as a substrate. Compared with the activity in control serum, this "residual activity" was little affected by inhibition with biocytin, indicating that a small fraction of a modified biotinidase probably exists.