Equal inhibition of HIV replication by stereoisomers of phosphatidyl-azidothymidine. Lack of stereospecificity of lysosomal phospholipase A1.

Glycerol-1-P and glycerol-3-P stereoisomers of dipalmitoylphosphatidylazidothymidine were synthesized and found to have equal antiretroviral activity in HIV-infected HT4-6C cells. It was anticipated that the glycerol-1-P isomer would be less active because of slow metabolic conversion by cellular phospholipases A and C, but the antiretroviral results suggested that the human cell line (HT4-6C) may have phospholipases capable of hydrolyzing 2,3-dipalmitoyl-sn-glycerol-1-phospho-5'-azidothymidine (AZT). To evaluate this possibility, we purified lysosomal phospholipase A1, an enzyme known to play a major role in cellular phospholipid catabolism. This enzyme rapidly hydrolyzed both the sn-1 and sn-3 isomers of dipalmitoylphosphatidyl-AZT. We synthesized sn-2,3-dipalmitoyl-glycero-1-phosphocholine and found that it is also hydrolyzed readily by lysosomal phospholipase A1 although the Vmax, 59 mumol mg-1 h-1, is slightly lower than that of the sn-1,2-dipalmitoyl-glycero-3-phosphocholine, 89 mumol mg-1 h-1. In conclusion, our studies show that sn-2,3-dipalmitoyl-glycerol-1-phospho-AZT is equal in antiviral activity to sn-1,2-dipalmitoyl-glycero-3-phospho-AZT in HIV-infected HT4-6C cells. This surprising result is due in part to the lack of stereospecificity of lysosomal phospholipase A1.