THE OCCURRENCE OF A NEW FLAVIN DINUCLEOTIDE (FAD-X)

Identification of human and rat FAD-AMP lyase (cyclic FMN forming) as ATP-dependent dihydroxyacetone kinases

Rat liver FAD-AMP lyase or FMN cyclase is the only known enzymatic source of the unusual flavin nucleotide riboflavin 4',5'-cyclic phosphate. To determine its molecular identity, a peptide-mass fingerprint of the purified rat enzyme was obtained. It pointed to highly related, mammalian hypothetical proteins putatively classified as dihydroxyacetone (Dha) kinases due to weaker homologies to biochemically proven Dha kinases of plants, yeasts, and bacteria. The human protein LOC26007 cDNA was used to design PCR primers. The product amplified from human brain cDNA was cloned, sequenced (GenBank Accession No. ), and found to differ from protein LOC26007 cDNA by three SNPs. Its heterologous expression yielded a protein active both as FMN cyclase and ATP-dependent Dha kinase, each activity being inhibited by the substrate(s) of the other. Cyclase and kinase activities copurified from rat liver extracts. Evidence supports that a single protein sustains both activities, probably in a single active center. Putative Dha kinases from other mammals are likely to be FMN cyclases too. Future work will profit from the availability of the structure of Citrobacter freundii Dha kinase, which contains substrate-interacting residues conserved in human Dha kinase/FMN cyclase.

Fluorimetric HPLC detection of endogenous riboflavin 4',5'-cyclic phosphate in rat liver at nanomolar concentrations

We have investigated the biological occurrence of riboflavin 4',5'-cyclic phosphate (cyclic FMN or cFMN), the flavin product known to be formed in vitro from FAD by the rat liver enzyme FAD-AMP lyase (cyclizing) or FMN cyclase (EC 4.6.1.15). The expected difficulties were the low concentration of the compound, the tendency of the more abundant FAD to decompose chemically to cFMN, and the acid lability of cFMN itself. A protocol was devised to prepare deproteinized rat liver extracts, avoiding conditions prone to the chemical formation of cFMN and making exposure to conditions of cFMN degradation as short as possible. In these extracts, cFMN was assayed by HPLC with fluorimetric detection. The identity of liver cFMN was confirmed by its HPLC separation from other known flavins, its coelution with authentic cFMN, and its susceptibility to acid degradation, yielding a mixture of 5'-FMN and 4'-FMN. The amount of total cFMN recovered in the liver extracts was 22+/-11 pmol/g fresh tissue. Careful control experiments were performed to rule out the possibility that this could be a chemical product of FAD degradation during extract preparation. These controls indicated that, on average, 97% of the measured extract concentration of cFMN, i.e., about 21+/-10 pmol/g, was already present in the liver at the beginning of the process and was extracted from the tissue. This figure is likely to be an underestimation of the hepatic content, as indicated by control experiments.

Purification, characterization, and substrate and inhibitor structure-activity studies of rat liver FAD-AMP lyase (cyclizing): preference for FAD and specificity for splitting ribonucleoside diphosphate-X into ribonucleotide and a five-atom cyclic phosphodiester of X, either a monocyclic compound or a cis-bicyclic phosphodiester-pyranose fusion

An enzyme with FAD-AMP lyase (cyclizing) activity, splitting FAD to AMP and riboflavin 4',5'-phosphate (cFMN), was recently identified [Fraiz, F., et al. (1998) Biochem. J. 330, 881-888]. Now, it has been purified to apparent homogeneity from a rat liver supernatant, by a procedure that includes affinity for ADP-agarose (adsorption required the activating cation Mn2+ and desorption required its removal), to a final activity of 2.2 units/mg after a 240-fold purification with a 15% yield. By SDS-PAGE, only one protein band was observed (Mr = 59 000). The correspondence between protein and enzyme activity was demonstrated by renaturation after SDS-PAGE, by gradient ultracentrifugation followed by analytical SDS-PAGE, and by native PAGE with visualization of enzyme activity by fluorescence. A native Mr of 100 000 (ultracentrifugation) or 140 000 (gel filtration) indicated that FAD-AMP lyase could be a dimer. The enzyme required millimolar concentrations of Mn2+ or Co2+, exhibited different optimum pH values with these cations (pH 8.5 or 7.3, respectively), and was strongly inhibited by ADP or ATP, but not by dADP, dATP, or the reaction products AMP and cFMN. A specificity study was conducted with 35 compounds related to FAD, mostly nucleoside diphosphate-X (NDP-X) derivatives. Besides FAD, the enzyme split 11 of these compounds with the pattern NDP-X --> NMP + P=X. Structure-activity correlations of substrates, nonsubstrates, and inhibitors, and the comparison of the enzymic reactivities of NDP-X compounds with their susceptibilities to metal-dependent chemical degradation, pinpointed the following specificity pattern. FAD-AMP lyase splits ribonucleoside diphosphate-X compounds in which X is an acyclic or cyclic monosaccharide or derivative bearing an X-OH group that is able to attack internally the proximal phosphorus with the geometry necessary to form a P=X product, either a five-atom monocyclic phosphodiester or a cis-bicyclic phosphodiester-pyranose fusion. For instance, NDP-glucose and GDP-alpha-L-fucose were substrates, but dTDP-glucose, NDP-mannose, and GDP-beta-L-fucose were not. Judging from kcat/Km ratios, we found the best substrate to be FAD, followed closely by ADP-glucose (kcat/Km only 2-fold lower, but not a physiological compound in mammals), whereas other substrates exhibited 50-500-fold lower kcat/Km values. However, there was no evidence for specific flavin recognition. Instead, what seems to be recognized is the NDP moiety of NDP-X, with a strong preference for ADP-X. Splitting would then depend on the presence of an adequate X-OH group. The possibility that, besides FAD, there could be in mammals other ADP-X substrates of FAD-AMP lyase is discussed, with emphasis placed on some ADP-ribose derivatives.