Giardia lamblia trophozoites are flagellated protozoa that inhabit the human small intestine, where they are exposed to various dietary
lipids and
fatty acids. It is believed that G. lamblia, which colonizes a
lipid-rich environment of the human small intestine, is unable to synthesize
phospholipids, long-chain
fatty acids, and
sterols de novo. Therefore, it is possible that this protozoan has developed a special process for acquiring
lipids from its host. We have previously shown that G. lamblia can take up
saturated fatty acids and incorporate them into
phosphatidylglycerol (PG) and other
glycerol-based
phospholipids (Stevens et al., Experimental Parasitology, 86, 133-143, 1997). In the present study, an attempt has been made to investigate the underlying mechanisms of transesterification and interesterification reactions of giardial
phospholipids by free and conjugated
fatty acids. Results show that exogenously supplied,
unsaturated, fatty acids were taken up by Giardia and incorporated into various
phosphoglycerides, including PG. To test whether this intestinal pathogen can utilize conjugated
fatty acids, live trophozoites were exposed to either [3]H;cbphosphatidylcholine (PC), where the
fatty acid was 3H-labeled at its sn2 position, or to [14C]
lyso-PC (
fatty acid was 14C-labeled at the sn1 position) for 90 min, followed by
phospholipid analysis using thin-layer chromatography. The results suggest that conjugated
fatty acids, like
free fatty acids, were incorporated into PG. It was also observed that
aristolochic acid, an inhibitor of Ca2+-
ionophore-stimulated
phospholipase A2, decreased the transfer of
fatty acids from [3H]PC to PG, indicating that giardial
phospholipases were involved in these esterification reactions. Additional experiments, which include culturing trophozoites in serum-supplemented and serum-deprived medium, along with numerous biochemical analyses suggest that (i) PG is a major transesterified and interesterified product, (ii) it is likely that giardial
phospholipases are involved in esterification reactions, (iii) in G. lamblia, PG is localized in perinuclear membranes, as well as intracellularly, but not in the plasma membrane, and (iv) various synthetic analogs of PG inhibit the growth of the parasite in vitro. These studies suggest that PG is an important
phospholipid of Giardia and a potential target for
lipid-based
chemotherapy against
giardiasis.