Punaglandins, chlorinated prostaglandins, function as potent Michael receptors to inhibit ubiquitin isopeptidase activity.

Cyclopentenone prostaglandins exhibit unique antineoplastic activity and are potent growth inhibitors in a variety of cultured cells. Recently the dienone prostaglandin, Delta(12)-PGJ(2), was shown to preferentially inhibit ubiquitin isopeptidase activity of the proteasome pathway. It is theorized that isopeptidase inhibition and general cytotoxicity of prostaglandins depend on olefin-ketone conjugation, electrophilic accessibility, and the nucleophilic reactivity of the endocyclic beta-carbon. Delta(12)-PGJ(2), which contains a cross-conjugated alpha,beta-unsaturated ketone, was a potent inhibitor of isopeptidase activity, whereas PGA(1) and PGA(2) with simple alpha,beta-unsaturated pentenones were significantly less potent and PGB(1) with a sterically hindered alpha,beta-unsaturated ketone was inactive. To further investigate the proposed mechanism, punaglandins, which are highly functional cyclopentadienone and cyclopentenone prostaglandins chlorinated at the endocyclic alpha-carbon position, were isolated from the soft coral Telesto riisei. They were then assayed for inhibition of ubiquitin isopeptidase activity and antineoplastic effects. The punaglandins were shown to inhibit isopeptidase activity and exhibit antiproliferative effects more potently than A and J series prostaglandins. Also, the cross-conjugated dienone punaglandin was more potent than the simple enone punaglandin. The ubiquitin-proteasome pathway is a vital component of cellular metabolism and may be a suitable target for antineoplastic agents. These newly characterized proteasome inhibitors may represent a new chemical class of cancer therapeutics.