A Tear Metabolomic Profile Showing Increased Ornithine Decarboxylase Activity and Spermine Synthesis in Thyroid-Associated Orbitopathy

About half of patients with Graves’ disease develop an orbitopathy related to an inflammatory expansion of the periorbital adipose tissue and muscles. We used a targeted metabolomic approach measuring 188 metabolites by mass spectrometry to compare the metabolic composition of tears in patients with active (n = 21) versus inactive (n = 24) thyroid-associated orbitopathy. Among the 44 metabolites accurately measured, 8 showed a significant alteration of their concentrations between the two groups. Two short-chain acylcarnitines, propionylcarnitine and butyrylcarnitine, and spermine showed increased concentrations in the tears of patients with active orbitopathy, whereas ornithine, glycine, serine, citrulline and histidine showed decreased concentrations in this group. In addition, the ratio putrescine/ornithine, representing the activity of ornithine decarboxylase, was significantly increased in patients with active compared to inactive orbitopathy (p = 0.0011, fold change 3.75). The specificity of this candidate biomarker was maintained when compared to a control group with unclassified dry eye disease. Our results suggest that the stimulation of ornithine decarboxylase by TSH receptor autoantibodies in orbital fibroblasts could lead to increased synthesis of spermine, through the increased activity of ornithine decarboxylase, that may contribute to periorbital expansion in Graves’ ophthalmopathy.

Anti-Trypanosomal Potential of The Red Sea Soft Coral Nephthea Mollis Supported by Metabolomic Profiling and Molecular Docking Studies

The total ethanol extract and its derived ethyl acetate fraction of the soft coral Nephthea mollis displayed remarkable in-vitro anti-trypanosomal potential against Trypanosoma brucei with IC50 value of 6.4 and 3.7 (µg/ml, 72 h respectively. Consequently, the total ethanol extract was subjected to LC-HR-ESI-MS metabolomic profiling to discover the constituents that possibly underlie their bioactivities. Therefore, thirty-three secondary metabolites were characterized, among them, sesquiterpenes and diterpenes were found to prevail. In silico modeling was carried out on the dereplicated compounds to provide an insight into their anti-trypanosomal mechanism of action with docking study on ornithine decarboxylase (ORD) which illustrated that five of the dereplicated compounds (‎2-deoxy-12-ethoxy-7-O-methyl lemnacarnol, Nephthenol, ‎4α-O-acetyl-selin-11-en, Eudesma-4,7(11)-diene-8β-ol, and Chabrolidione A) have the highest affinity to the ornithine decarboxylase enzyme. These results highlight the valuable chemical profile of Nephthea mollis as a lead source for anti-trypanosomal natural products.

Structural basis of binding and inhibition of ornithine decarboxylase by 1-amino-oxy-3-aminopropane

Ornithine decarboxylase (ODC) is the rate-limiting enzyme for the synthesis of polyamines (PAs). PAs are oncometabolites that are required for proliferation, and pharmaceutical ODC inhibition is pursued for the treatment of hyperproliferative diseases, including cancer and infectious diseases. The most potent ODC inhibitor is 1-amino-oxy-3-aminopropane (APA). A previous crystal structure of an ODC–APA complex indicated that APA non-covalently binds ODC and its cofactor pyridoxal 5-phosphate (PLP) and functions by competing with the ODC substrate ornithine for binding to the catalytic site. We have revisited the mechanism of APA binding and ODC inhibition through a new crystal structure of APA-bound ODC, which we solved at 2.49 Å resolution. The structure unambiguously shows the presence of a covalent oxime between APA and PLP in the catalytic site, which we confirmed in solution by mass spectrometry. The stable oxime makes extensive interactions with ODC but cannot be catabolized, explaining APA’s high potency in ODC inhibition. In addition, we solved an ODC/PLP complex structure with citrate bound at the substrate binding pocket. These two structures provide new structural scaffolds for developing more efficient pharmaceutical ODC inhibitors.