Biotinidase in the porcine cerebrum

Biotinidase activities found in porcine brains ( n = 3) were as follows: cerebrum, 4.4 ± 0.2 pmol/min per milligram of protein; cerebellum, 7.6 ± 0.3 pmol/min per milligram of protein; medulla, 2.9 ± 0.3 pmol/min per milligram of protein. These values are relatively high compared with the activities in rat or guinea pig brains. Subcellular distribution of biotinidase was found mainly in the soluble cytoplasmic fraction (S3) i.e., in the supernatant of 0.32 m sucrose S2 solution after ultracentrifugation at 105,000 g for 90 min. This is in contrast to the guinea pig livers, in which the subcellular distribution of biotinidase is mainly found in the microsomal fraction. After a seven-step purification (22,200-fold enrichment), porcine brain biotinidase is identified as a single polypeptide by the sodium dodecyl sulfate-polyacrylamide gel electrophoresis system, and its molecular weight is determined as 68,000 Da. The isoelectric point of the enzyme was 4.3. Sialidase treatment strongly suggests the presence of sialyl residues in this enzyme. Amino acid analysis indicates relatively high hydrophilicity and high content of glycine and serine. The enzyme activity is inhibited by organic mercurials, but not by diisopropylfluorophosphate. Abundant soluble biotinidase in brain cytoplasm may play an important role which has not been discovered yet.