Scheme 1.
D609 inhibits PC (Ng et al., 2004) and SM (Luberto and Hannun, 1998) synthesis as well as PC-PLC (Amtmann, 1996). These additional effects must be taken into account in interpretation of results obtained with D609 and limit its utility as a pharmacological tool to characterize actions of PC-PLC and supports the notion that D609 is a non-specific PC-PLC inhibitor. PC-PLC hydrolyzes PC to form DAG and phosphocholine. DAG stimulates A-SMase and release of ceramide from SM. SM synthase transfers the phosphocholine head group from PC to ceramide to form SM and DAG. CPT synthesizes PC from CDP-choline and DAG. Since addition of CDP-choline + DAG also did not rescue the cells, in all likelihood D609 inhibits CPT (Anthony et al., 1999, Wright et al., 2001, Ng et al., 2004, 2004). PC: phosphatidylcholine; SM: sphingomyelin; A-SMase: acidic sphingomyelinase; PC-PLC: PC-phospholipase C; DAG: 1,2-diacylglycerol; CDP-C: CDP-choline; CPT: CDP-choline:DAG cholinephosphotransferase. In summary, D609 probably inhibits CPT and SM synthase as well as PC-PLC. The abrupt loss of cell viability also suggests that D609 might be causing loss of ATP followed by mitochondrial dysfunction (loss of cardiolipin) and increase in [Ca2+]i. These probable actions of D609 have been presented in the composite scheme.