Flavin adenine dinucleotide metabolism in nucleus

Following its uptake from outside, in the cell riboflavin undergoes an ATP-dependent phosphorylation catalyzed by riboflavin kinase (RFK, E.C. 2.7.1.26) to form FMN, most of which is further converted by FAD synthase (FADS, E.C. 2.7.7.2) into FAD, the cofactor for many flavoenzymes involved in several biological processes in different cellular compartments. We previously demonstrated that beside a cytosolic enzyme (i.e. isoform 2 of human FLAD1 gene product) a mitochondrial isoform of FADS exists (isoform 1) (1, 2), presumably responsible for the biogenesis of mitochondrial flavoenzymes. Interestingly, the possibility of unexpected localizations for FADS has arisen (3) and, meanwhile, at least two new isoforms of FADS have been deposited in databases. Among FAD-dependent enzymes, lysine-specific demethylase-1 (E.C. 1.14.11.B1) is a nuclear protein recently shown to regulate cellular energy balance depending on FAD availability (4). These observation prompted us to investigate on the presence of flavin coenzymes in the nucleus and on a possible metabolism of FAD in this organelle. To this aim HPLC analysis of acid-precipitable flavins in nuclei isolated from rat liver were performed. The observation of a time-dependent decrease of FAD suggested us the existence of a hydrolytic activity, as confirmed by spectrofluorimetric measurements revealing an increase in fluorescence due to conversion of FAD into its more fluorescent precursor FMN/riboflavin (5). Immunoblotting experiments performed on rat liver nuclei using a home-made antibody against FADS revealed the enrichment of an immunoreactive band in respect to the other sub-cellular fractions. Confocal microscopy on several mammalian cells confirmed the existence of FADS co-localizing with nuclear markers. 1. Barile et al., Eur J Biochem, 2000, 15, 4888-900. 2. Torchetti et al., Mitochondrion, 2010, 10, 263-73. 3. Lin et al., J Neurol, 2009, 256, 774-82. 4. Hino et al., Nat Commun, 2012, 3, 758. 5. Brizio et al., Eur J Biochem, 1997, 3, 777-85.