Novel Amino Acid Derived Natural Products from the Ascidian Atriolum robustum: Identification and Pharmacological Characterization of a Unique Adenosine Derivative

Investigation of the methanolic extract of the Australian ascidian Atriolum robustum led to the isolation and characterization of five new amino acid derived structures (1-5). The structures were elucidated employing spectroscopic techniques (NMR, MS, UV, and IR). The absolute stereochemistry of 1 and 2 was established by chemical degradation, derivatization, and chiral GC-MS analysis. Structures 4 and 5 are complex nucleosides containing rare methylthioadenosine and methylsulfinyladenosine moieties, respectively. In radioligand binding studies the 5′-deoxy-5′ -methylthioadenosine-2′,3′-diester 4 exhibited affinity for A and A adenosine receptors with K values below 10 μM. Its affinity was somewhat lower for A (K = 17 μM) and much lower for A adenosine receptors. Analytical experiments using capillary electrophoresis showed that compound 4 was stable under the conditions of radioligand binding studies. Incubation with carboxylesterase resulted in slow hydrolysis of the adenosine derivative to 5′-deoxy-5′-methylthioadenosine (MTA), which was about 10-fold more potent at adenosine receptors than compound 4. Thus, the 2′,3′ -diester derivative 4 may act as a lipophilic prodrug of MTA in addition to its own adenosine receptor activity. GTP shift experiments indicated that the adenosine derivative was a partial agonist at A adenosine receptors of rat brain cortical membranes. Compound 4 inhibited cAMP accumulation in Chinese hamster ovary (CHO) cell membranes recombinantly expressing the human A adenosine receptor, thus indicating that the adenosine derivative also acted as a partial agonist at AARs. Homology models of the A and the A adenosine receptors in their putative active and inactive conformations were built and used for docking of the sterically demanding compound 4. It was found that this ligand fit well into the binding pockets of both receptor subtypes because of its highly flexible structure, although in somewhat different binding modes.