Biotinidase catalyzes the hydrolysis of the
vitamin biotin from proteolytically degraded
biotin-dependent carboxylases. This key reaction makes the
biotin available for reutilization in the biotinylation of newly synthesized apocarboxylases. This latter reaction is catalyzed by holocarboxylase
synthetase (HCS) via synthesis of 5'-biotinyl-AMP (B-
AMP) from
biotin and
ATP, followed by transfer of the
biotin to a specific
lysine residue of the apocarboxylase substrate. In addition to carboxylase activation, B-
AMP is also a key regulatory molecule in the transcription of genes encoding apocarboxylases and HCS itself. In humans, genetic deficiency of HCS or
biotinidase results in the life-threatening disorder
biotin-responsive
multiple carboxylase deficiency, characterized by a reduction in the activities of all
biotin-dependent carboxylases. Although the clinical manifestations of both disorders are similar, they differ in some unique neurological characteristics whose origin is not fully understood. In this study, we show that
biotinidase deficiency not only reduces net carboxylase biotinylation, but it also impairs the expression of carboxylases and HCS by interfering with the B-
AMP-dependent mechanism of transcription control. We propose that
biotinidase-deficient patients may develop a secondary HCS deficiency disrupting the altruistic tissue-specific
biotin allocation mechanism that protects brain metabolism during
biotin starvation.