“…The amino acids valine, leucine, and isoleucine are precursors to isobutyryl-CoA, isovaleryl-CoA, and 2-methylbutyryl-CoA, which are generated by the bacterial enzyme ␣-keto acid dehydrogenase and incorporated into Chlamydia-specific branched-chain fatty acids by 3-oxoacyl-ACP synthase III (FabH). Thus, lipids synthesized by C. trachomatis can be detected by incubation of Chlamydia-infected cells with radiolabeled valine, leucine, or isoleucine followed by total lipid extraction and monitoring of the increase in radioactivity in this fraction compared to that for control uninfected cells (11,25,26). Of these three amino acids, isoleucine has previously been reported to show the most extensive incorporation into Chlamydia branched-chain fatty acids (25).…”