This paper reports the first direct characterization of flavin (noncovalently bound FMN) in energy coupling site I of the mitochondrial respiratory chain. Thermodynamic parameters of its redox reactions were determined potentiometrically monitoring the g = 2.005 signal of its free radical form in isolated bovine heart NADH:ubiquinone oxidoreductase (complex I). The midpoint redox potentials of consecutive one-electron reduction steps are Em1/0 = -414 mV and Em2/1 = -336 mV at pH 7.5. This corresponds to a stability constant of the intermediate flavosemiquinone state of 4.5 x 10(-2). The pK values of the free radical (Fl.-<=>>FlH.) and reduced flavin (FlH-<=>>FlH2) were estimated as 7.7 and 7.1, respectively. The potentiometrically obtained g = 2.005 flavin free radical EPR signal revealed an unusually broad (2.4 mT) and pH-independent peak-to-peak line width. The spin relaxation of flavosemiquinone in complex I is much faster than that of flavodoxin due to strong dipole-dipole interaction with iron-sulfur cluster N3. Guanidine, an activator of NADH-ferricyanide reductase activity of complex I, was found to have a strong stabilizing effect on the flavin free radical generated both by equilibration with the NADH/NAD+ redox couple and by potentiometric redox titration. The addition of guanidine also leads to a slight modification of the EPR spectrum of iron-sulfur cluster N3. Anaerobic titration of flavosemiquinone free radical with the strictly n = 2 NADH/NAD+ and APADH/APAD+ redox couples revealed that nucleotide binding narrows the EPR signal line width of the flavin free radical to 1.7 mT and changes a shape of the titration curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Perforin is involved in cell-mediated cytotoxicity and mutations of its gene (PRF1) cause familial hemophagocytic lymphohistiocytosis (FLH2). PRF1 sequencing in 190 patients with multiple sclerosis and 268 controls detected two FLH2-associated variations (A91V, N252S) in both groups and six novel mutations (C999T, G1065A, G1428A, A1620G, G719A, C1069T) in patients. All together, carriers of these variations were more frequent in patients than in controls (phenotype frequency: 17 vs 9%, P=0.0166; odds ratio (OR)=2.06, 95% confidence interval (CI): 1.13-3.77). Although A91V was the most frequent variation and displayed a trend of association with multiple sclerosis (MS) in the first population of patients and controls (frequency of the 91V allele: 0.076 vs 0.043, P=0.044), we used it as a marker to confirm PRF1 involvement in MS and assessed its frequency in a second population of 966 patients and 1520 controls. Frequency of the 91V allele was significantly higher in patients than in controls also in the second population (0.075 vs 0.058%, P=0.019). In the combined cohorts of 1156 patients and 1788 controls, presence of the 91V allele in single or double dose conferred an OR=1.38 (95% CI=1.10-1.74). These data suggest that A91V and possibly other perforin variations indicate susceptibility to MS.

Light-induced fluorescence changes (LIFCs) were detected in sporangiophores of the blue-light-sensitive fungus Phycomyces blakesleeanus (Burgeff). The LIFCs can be utilized as a spectrophotometric assay for blue-light photoreceptors and for the in vivo characterization of their photochemical primary reactions. Blue-light irradiation of sporangiophores elicited a transient decrease and subsequent regeneration of flavin-like fluorescence emission at 525 nm. The signals recovered in darkness in about 120 min. In contrast to blue light, near-UV (370 nm) caused an increase in the fluorescence emission at 525 nm. Because the LIFCs were altered in a light-insensitive madC mutant with a defective photoreceptor, the fluorescence changes must be associated with early photochemical events of the transduction chain. Action spectra for the fluorescence changes at 525 nm showed major peaks near 470 and 600 nm. Double-pulse experiments involving two consecutive pulses of either blue and near-UV, blue and red, or near-UV and red showed that the responses depended on the sequence in which the different wavelengths were applied. The results indicate a blue-light receptor with intermediates in the near-UV, blue and red spectral regions. We explain the results in the framework of a general model, in which the three redox states of the flavin photoreceptor, the oxidized flavin (Fl), the flavo-semiquinone (FlH*), and the flavo-hydroquinone (FlH2) are each acting as chromophores with their own characteristic photochemical primary reactions. These consist of the photoreduction of the oxidized flavin generating semiquinone, the photoreduction of the semiquinone generating hydroquinone, and the photooxidation of the flavo-hydroquinone regenerating the pool of oxidized flavins. The proposed mechanism represents a photocycle in which two antagonistic photoreceptor forms, Fl and FlH2, determine the pool size of the biological effector molecule, the flavo-semiquinone. The redox changes that are associated with the photocycle are maintained by redox partners, pterins, that function in the near-UV as secondary chromophores.

Thermodynamic parameters of succinate dehydrogenase flavin were determined potentiometrically from the analysis of free radical signal levels as a function of the oxidation-reduction potential. Midpoint redox potentials of consecutive 1-electron transfer steps are -127 and -31 mV at pH 7.0. This corresponds to a stability constant of intermediate stability, 2.5 x 10(-2), which suggests flavin itself may be a converter from n = 2 to n = 1 electron transfer steps. The pK values of the free radical (FlH . in equilibrium Fl . -) and the fully reduced form (FlH2 in equilibrium FlH-) were estimated as 8.0 +/- 0.2 and 7.7 +/- 0.2, respectively. Succinate dehydrogenase flavosemiquinone elicits an EPR spectrum at g = 2.00 with a peak to peak width of 1.2 mT even in the protonated form, suggesting the delocalization in the unpaired electron density. A close proximity of succinate dehydrogenase flavin and iron-sulfur cluster S-1 was demonstrated based on the enhancement of flavin spin relaxation by Center S-1.

Photosensitized splitting of cis-syn- and trans-syn-1,3-dimethyluracil dimers by 2',3',4',5'-tetraacetylriboflavin in acetonitrile containing a trace of perchloric acid was studied by laser flash photolysis. Protonation of the flavin prior to excitation resulted in excited singlet and triplet states that abstracted an electron from the dimers and yielded the protonated flavin radical (FlH2.+), which was detected by absorption spectroscopy. Electron abstraction by the excited singlet state predominated over abstraction by the triplet state. Approximately one-third to one-half of the excited states quenched by the trans-syn dimer yielded FlH2.+, the balance presumably undergoing back electron transfer within the geminate radical ion pair generated by the initial electron transfer. A covalently linked dimer-flavin exhibited very inefficient flavin radical ion formation, consistent with the known low efficiency of dimer splitting in this system. These results constitute the first identification of a flavin radical ion intermediate in photosensitized pyrimidine dimer splitting.

BACKGROUND

Familial hemophagocytic lymphohistiocytosis 2 (FHL2) is the most common familial type of hemophagocytic lymphohistiocytosis with immune dysregulation. FHL2 patients have mutations in the perforin gene which cause overactivation and proliferation of cytotoxic T lymphocytes and natural killer cells. Perforin is the key component of the cytolytic granule response function of cytotoxic T lymphocytes and natural killer cells. Perforin dysfunction causes a cytotoxic immune deficiency with a clinical outcome of uncontrolled and continuous immune stimulation response. This excessive stimulation leads to continuous systemic inflammation and, ultimately, multiorgan failure. Radical therapy is hematopoietic stem cell transplantation which is limited by the availability of a donor. Exacerbations of inflammatory attacks require a palliative immunosuppressive regimen. There is a need for an alternative or adjuvant therapy to maintain these patients when immunosuppression is ineffective or a donor is not available. Beneficial actions of mesenchymal stem cells (MSCs) have been shown in autoimmune diseases in clinical trials and are attributed to their immune-modulatory properties. This study aimed to assess the immune-modulatory effect of MSCs in an in-vitro model of FHL2.

METHODS

We generated a targeted mutation in the perforin gene of NK92 cells to create an in-vitro FLH2 model using Crispr/Cas technology. A coculture setup was employed to assess the immunomodulatory efficacy of MSCs.

RESULTS

Engineered NK92 clones did not show PRF1 mRNA expression and failed to secrete perforin upon phorbol myristate acetate-ionomycin stimulation, providing evidence for a valid FHL2 model. Coculture media of the engineered cells were investigated for the abundance of several cytokines. Coculture with MSCs revealed a reduction in major proinflammatory cytokines and an induction in anti-inflammatory and immunomodulatory cytokines compared to the parental NK92 cells.

CONCLUSIONS

This study shows the ameliorating effect of MSCs as an adjuvant immune modulator toward the therapy of FHL2 patients. MSCs are supportive therapy candidates for FHL2 patients under circumstances where prolonged immunosuppression is required to gain time before allogeneic hematopoietic stem cell transplantation.

5-Deazariboflavin was observed to undergo one electron-equivalent reduction on reaction with .CO2-, DS2-. or a mixture of .CO2- and eaq-. This is attributed to formation of .FlH, followed by dimerization: 2 5-DRFlH.----(5-DRFlH)2 a process not previously confirmed. In contrast, 3-deazariboflavin, 1-deazariboflavin 7-chlororiboflavin, 2-thioriboflavin, lumiflavin, flavin adenine dinucleotide and riboflavin all underwent two electron-equivalent reductions to FlH2 or FlH-species via the disproportionation: 2.FlH----FlH2+Fl Spectra of the reduced species are reported for 3-deazariboflavin, 1-deazarboflavin and 2-thioriboflavin. The anion radicals of dithiothreitol (DS2-.) and lipoamide (LS2-.) did not effect reduction beyond the two electron equivalence point. However, .CO2- was seen to effect further reactions with the dihydroforms of the higher redox potential flavins 7-chlororiboflavin and 2-thioriboflavin at pH 3 to 10, but for the other flavins only at or below pH 2. Except for the sulphur-containing 2-thioriboflavin, the protonated DS2.H and LS2.H radicals were unable to reduce any of the flavins at pH less than or equal to 3. This cause a 'falloff' in the yield of the reduced flavin which is in accord with the reaction rate constants and radical pK values. Despite their differences in absorbance, the reactions of DS2.H and LS2.H towards flavins were similar in all aspects examined. Several new rate constants are reported.

Background: Familial hemophagocytic lymphohistiocytosis type 2 (FHL2) is a rare genetic disorder presenting with fever, hepatosplenomegaly, and pancytopenia secondary to perforin-1 (PRF1) mutation. FLH2 has been described in Chinese but usually presents after 1 year old. We describe a female Chinese neonate with FHL2 secondary to compound heterozygous PRF1 mutation with symptom onset before 1 mo old. We review Chinese FHL2 patients in the literature for comparison.

Case summary: A 15-d-old female neonate was referred to our hospital for persistent fever and thrombocytopenia with diffuse petechiae. She was born to a G5P3 mother at 39 wk and 4 d via cesarean section secondary to breech presentation. No resuscitation was required at birth. She was described to be very sleepy with poor appetite since birth. She developed a fever up to 39.5°C at 7 d of life. Leukocytosis, anemia, and thrombocytopenia were detected at a local medical facility.

Conclusion: A literature review identified 75 Chinese FHL2 patients, with only five presenting in the first year of life. Missense and frameshift mutations are the most common PRF1 mutations in Chinese, with 24.8% having c.1349C>T followed by 11.6% having c.65delC. The c.658G>C mutation has only been reported once in the literature and our case suggests it can be pathogenic, at least in the presence of another pathogenic mutation such as c.1066C>T.

Keywords: Case report; Compound heterozygous; Familial hemophagocytic lymphohistiocytosis; Hemophagocytic lymphohistiocytosis; Neonate; Perforin-1.