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BIOTINIDASE ACTIVITY IN BRAIN AND ITS IMPLICATION IN BIOTINIDASE DEFICIENCY

The neurologic features of biotinidase deficiency include myoclonic seizures, ataxia and hearing loss which may occur without overt organic aciduria. Previous studies by Pispa (Ann Med Exp Biol Fenn 43, suppl. 5:1-39, 1965) indicated that there was no biotinidase activity in normal mammalian brains. However, using a sensitive radioassay we have determined that the mean activity in perfused rat brains (n=4) is 9.9 pmol/min/mg protein (range= 6.4-12.0) in the brain stem, 3.5 (2.9-4.3) in the cerebellum and 1.7 (1.0-2.2) in the cerebrum. The mean enzyme activity in human cerebrum is 32 pmol/min/mg (7.2-91; n=6). The mean biotinidase activity in human CSF is 30 pmol/ min/ml (range=0-100; n=44). These results indicate that the brain is capable of recycling biotin. Therefore, in biotinidase deficient individuals the brain's only access to biotin is via transport across the blood-brain barrier. Since the requirement for biotin in the brain appears to be high and studies by Baker et al. (Nutr. Rep. Intl. 27:661-670, 1983) indicate that the concentration of biotin in the CSF is only 1/5 that of the serum, then a severe systemic depletion of biotin would result in concomitant decrease of biotin in the brain. Even if carboxylase turnover is slower in the brain than in other tissues, biotin deficiency may occur sooner in the brain and preferentially affect biotinylation of the carboxylases, primarily pyruvate carboxylase, resulting in a localized accumulation of lactate and/or organic acids which may not be detectable in the urine.