The variety of biological systems which are modulated by naturally occurring cyclopropanes has only been realized recently. They range from antibiotic, antiviral, antifungal and insecticidal activities, control of plant growth and fruit ripening, antimycotic, thyromimetic, hormonal, carcinogenic or antitumoral activities, enzyme and gluconeogenesis inhibitions, to neuro­ chemical activity. Cyclopropanes are also major components of membranes, for instance in tuberculosis cells, and key intermediates in many biosynthetic processes. Many synthetic drugs include cyclopropanes in their structure and they are also widely used to probe the mechanisms of biological processes. Naturally occurring cyclopropenes are known to be potent enzyme inhibitors controlling key processes on the fatty acid desaturation cycle and, in one case, to be an antibacterial agent. Synthetic analogues similarly inhibit pheromone production in insects, and the production of bacterial membranes, and interfere with the behaviour of insects by blocking pheromone receptors. When they have been demonstrated, the mechanisms responsible for the specific bioactivity of these systems are also reviewed.

The thiourea isoxyl (thiocarlide; 4,4′-diisoamyloxydiphenylthiourea) is known to be an effective anti-tuberculosis drug, active against a range of multidrug-resistant strains of Mycobacterium tuberculosis and has been used clinically. Little was known of its mode of action. We now demonstrate that isoxyl results in a dose-dependent decrease in the synthesis of oleic and, consequently, tuberculostearic acid in M. tuberculosis with complete inhibition at 3 μg/ml. Synthesis of mycolic acid was also affected. The anti-bacterial effect of isoxyl was partially reversed by supplementing growth medium with oleic acid. The specificity of this inhibition pointed to a Δ9-stearoyl desaturase as the drug target. Development of a cell-free assay for Δ9-desaturase activity allowed direct demonstration of the inhibition of oleic acid synthesis by isoxyl. Interestingly, sterculic acid, a known inhibitor of Δ9-desaturases, emulated the effect of isoxyl on oleic acid synthesis but did not affect mycolic acid synthesis, demonstrating the lack of a relationship between the two effects of the drug. The three putative fatty acid desaturases in the M. tuberculosis genome, desA1, desA2, and desA3, were cloned and expressed in Mycobacterium bovis BCG. Cell-free assays and whole cell labeling demonstrated increased Δ9-desaturase activity and oleic acid synthesis only in the desA3-overexpressing strain and an increase in the minimal inhibitory concentration for isoxyl, indicating that DesA3 is the target of the drug. These results validate membrane-bound Δ9-desaturase, DesA3, as a new therapeutic target, and the thioureas as anti-tuberculosis drugs worthy of further development.