Neonatal screening for biotinidase deficiency.

Children with juvenile-onset multiple carboxylase deficiency lack biotinidase activity (biotinamide amidohydrolase, EC 3.5.1.12) in the liver and other tissues. Hence, little free biotin is metabolically available, resulting in seizures, acidosis, and serious neurological damage. As the absence of hepatic biotinidase activity is reflected in serum, assessment of biotinidase status can easily be made from a blood sample. A convenient qualitative procedure for screening infants has been employed in order to estimate serum levels of biotinidase in as little as 10 microliters of sample. This colorimetric procedure detects the formation of free p-aminobenzoate cleaved from the substrate, N-biotinyl-p-aminobenzoate at pH 6.0. The assay is easily performed and has a low incidence of false positive results. A kinetic assay for serum biotinidase has also been developed using biotinyl-p-nitroanilide (BpNA) as substrate. When 50 microliters of biotinidase positive serum was incubated with 0.2 mM BpNA in phosphate buffer at pH 6.0, an increase in absorbance was observed at 405 nm. The rate of change in absorbance was followed kinetically on the Roche Cobas BIO analyzer at 37 degrees C. Monitoring the increase in absorbance of para-nitroanilide every 60 seconds over 30 minutes demonstrated linearity from 10 to 30 minutes. In comparing results from this kinetic assay on 48 randomly selected sera with those obtained using a colorimetric procedure, a correlation coefficient of 0.85 was obtained. Several false positive results were observed in clearly lipemic sera.