Covalent modification of proteins exerts significant effects on their chemical properties and has important functional and regulatory consequences. We now report the identification and verification of an electrically-active form of modified proteins recognized by a group of small molecules commonly used to interact with DNA. This previously unreported property of proteins was initially discovered when the γ-ketoaldehydes were identified as a source of the proteins stained by the DNA intercalators. Using 1,4-butanedial, the simplest γ-ketoaldehyde, we characterized the structural and chemical criteria governing the recognition of the modified proteins by the DNA intercalators and identified Nε-pyrrolelysine as a key adduct. Unexpectedly, the pyrrolation conferred an electronegativity and electronic properties on the proteins that potentially constitute an electrical mimic to the DNA. In addition, we found that the pyrrolated proteins indeed triggered an autoimmune response and that the production of specific antibodies against the pyrrolated proteins was accelerated in human systemic lupus erythematosus. These findings and the apparent high abundance of Nε-pyrrolelysine in vivo suggest that protein pyrrolation could be an endogenous source of DNA mimic proteins, providing a possible link connecting protein turnover and immune disorders.
Lysine N-pyrrolation, converting lysine residues to Nϵ-pyrrole-l-lysine, is a recently discovered post-translational modification. This naturally occurring reaction confers electrochemical properties onto proteins that potentially produce an electrical mimic to DNA and result in specificity toward DNA-binding molecules such as anti-DNA autoantibodies. The discovery of this unique covalent protein modification provides a rationale for establishing the molecular mechanism and broad functional significance of the formation and regulation of Nϵ-pyrrole-l-lysine–containing proteins. In this study, we used microbeads coupled to pyrrolated or nonpyrrolated protein to screen for binding activities of human serum-resident nonimmunoglobin proteins to the pyrrolated proteins. This screen identified apolipoprotein E (apoE) as a protein that innately binds the DNA-mimicking proteins in serum. Using an array of biochemical assays, we observed that the pyrrolated proteins bind to the N-terminal domain of apoE and that oligomeric apoE binds these proteins better than does monomeric apoE. Employing surface plasmon resonance and confocal microscopy, we further observed that apoE deficiency leads to significant accumulation of pyrrolated serum albumin and is associated with an enhanced immune response. These results, along with the observation that apoE facilitates the binding of pyrrolated proteins to cells, suggest that apoE may contribute to the clearance of pyrrolated serum proteins. Our findings uncover apoE as a binding target of pyrrolated proteins, providing a key link connecting covalent protein modification, lipoprotein metabolism, and innate immunity.
In this study, three new garcinoic acid dimers, δ,δbigarcinoic acid (1), δ,δ-bi-O-garcinoic acid (2), and γ,δ-bi-Ogarcinoic acid (3), and a new benzophenone derivative, (8E)-4geranyl-3,5-dihydroxybenzophenone (4), as well as seven known compounds (5−11) were isolated from the seeds of Garcinia kola. The structures of the new compounds were elucidated using MALDI-TOF-MS and spectroscopic data, including 1D and 2D NMR and electronic circular dichroism spectra. All of the isolated compounds were evaluated for their antimicrobial activity against two oral pathogens, Porphyromonas gingivalis and Streptococcus sobrinus. Among them, 4 and δ-garcinoic acid (6) exhibited antimicrobial activity against both of these microorganisms (MICs of 31.3−62.5 μM for P. gingivalis and 15.6−31.3 μM for S. sobrinus). These results indicate that some chemical constituents in G. kola seeds have potential application in the prevention of oral diseases.
Efforts to isolate compounds from an Indonesian member of the ginger family, Kaempferia angustifolia, yielded three known molecules, identified as (+)-crotepoxide (1), (+)-pipoxide chlorohydrin (2), and flavokawain A (FKA, 3). All three compounds strongly inhibited triglyceride accumulation in 3T3-L1 murine pre-adipocytes at 10 µg/mL, and compounds 1 and 2 were both cytotoxic at this concentration. To determine the biological activities of natural 3, flavokawains A (3), B (4) and C (5) were synthesized. While 4 was cytotoxic, both 3 and 5 potently inhibited differentiation of murine pre-adipocytes and reduced triglyceride accumulation (EC50 = 64.4 and 26.1 µM, respectively) with relatively weak cytotoxicity. Thus, the electron-donating group on the aromatic B ring may contribute to the highly selective anti-obesity activity.
The lichen Usnea baileyi is a fruticose lichen belonging to the Usnea genus. It is well known as a rich source of natural xanthone dimers and possesses various bioactivities. Nevertheless, the chemical investigation on this type of lichen is still rare as most of researches reported its components without structural elucidation. Herein, in the continuous study on this type of lichen, we further isolate xanthone dimers from the dichloromethane extract and explore three new xanthone dimers, eumitrins F−H (1−3). Their structures were elucidated unambiguously by spectroscopic analyses, including high resolution electrospray ionisation mass spectrometry (HRESIMS), 1D and 2D nuclear magnetic resonance spectroscopy (1D and 2D NMR), and DP4 probability. All compounds were evaluated for their enzyme inhibition against αglucosidase, tyrosinase, and antibacterial activity. They revealed moderate antimicrobial and weak tyrosinase inhibition. For α-glucosidase inhibition, compound 3 displayed the most significant inhibitory against α-glucosidase possessing an IC50 value of 64.2 µM.
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