Abstract
FAD is a redox cofactor ensuring the activity of many flavoenzymes mainly located in mitochondria but also relevant for nuclear redox activities. The last enzyme in the metabolic pathway producing FAD is FAD synthase (EC 2.7.7.2), a protein known to be localized both in cytosol and in mitochondria. FAD degradation to riboflavin occurs via still poorly characterized enzymes, possibly belonging to the NUDIX hydrolase family. By confocal microscopy and immunoblotting experiments, we demonstrate here the existence of FAD synthase in the nucleus of different experimental rat models. HPLC experiments demonstrated that isolated rat liver nuclei contain ∼300 pmol of FAD·mg(-1) protein, which was mainly protein-bound FAD. A mean FAD synthesis rate of 18.1 pmol·min(-1)·mg(-1) protein was estimated by both HPLC and continuous coupled enzymatic spectrophotometric assays. Rat liver nuclei were also shown to be endowed with a FAD pyrophosphatase that hydrolyzes FAD with an optimum at alkaline pH and is significantly inhibited by adenylate-containing nucleotides. The coordinate activity of these FAD forming and degrading enzymes provides a potential mechanism by which a dynamic pool of flavin cofactor is created in the nucleus. These data, which significantly add to the biochemical comprehension of flavin metabolism and its subcellular compartmentation, may also provide the basis for a more detailed comprehension of the role of flavin homeostasis in biologically and clinically relevant epigenetic events.
Highlights
FAD synthase is known to catalyze the biosynthesis of FAD in cytosol and mitochondria
Notwithstanding the apparent enrichment of the 65-kDa band in the Triton X-100-insoluble pellet, if compared with the -actin band, the majority of FAD synthetase (FADS) bands was shown to be released in the supernatant (5 ng/100 mg of protein), whereas its total amount in the pellet was less than 10%
In opposition to pyridine nucleotide cofactors, whose metabolism has been extensively investigated in the last decades (50 –52), the crucial function of flavin cofactors in cell biology has been quite neglected
Summary
FAD synthase is known to catalyze the biosynthesis of FAD in cytosol and mitochondria. Significance: Nuclear, mitochondrial, and cytosolic FAD synthase pools constitute a flavin network involved in the regulation of cellular metabolism and epigenetic events. The last enzyme in the metabolic pathway producing FAD is FAD synthase (EC 2.7.7.2), a protein known to be localized both in cytosol and in mitochondria. The coordinate activity of these FAD forming and degrading enzymes provides a potential mechanism by which a dynamic pool of flavin cofactor is created in the nucleus. These data, which significantly add to the biochemical comprehension of flavin metabolism and its subcellular compartmentation, may provide the basis for a more detailed comprehension of the role of flavin homeostasis in biologically and clinically relevant epigenetic events. For many years it has been assumed that in eukaryotes FAD biosynthesis occurred only in the cytosol
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