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Transacylation of lyso platelet-activating factor and other lysophospholipids by macrophage microsomes. Distinct
Abstract:
Rabbit alveolar macrophage microsomes were found to acylate 1-[3H]alkyl-glycero-3-phosphocholine (GPC) (lyso platelet-activating factor) in the absence of any cofactors, indicating the presence of transacylation activity. The transacylation activity was comparable to the activity of acyl-CoA:1-alkyl-GPC acyltransferase. The fatty acyl moieties introduced into 1-[3H]alkyl-GPC from membrane lipids by microsomes were mainly 20:4 (n-6). A very similar acylation profile was observed for the acylation of 1-[3H]alkyl-GPC in intact macrophages, suggesting that the CoA-independent transacylation system plays a very important part in the acylation of 1-[3H]alkyl-GPC in cells. We also confirmed that 14C-labeled 20:4(n-6), 20:5(n-3), 22:4(n-6), and 22:6(n-3) were transferred well from diacyl-GPC to 1-alkyl-GPC in a CoA-independent manner. The transfer rates for 16:0, 18:0, and 18:1 from diacyl-GPC to 1-alkyl-GPC were very low in the presence and absence of CoA. On the other hand, the transfer of 20:4 from diacyl-GPE or diacyl-GPI to 1-alkyl-GPC or 1-acyl-GPC was markedly increased by the addition of CoA. The above results indicate that the transacylation system exhibits distinct donor and acceptor selectivities and CoA dependency. These transacylation reactions could be very important in the regulation of the levels and the availability of lysophospholipids, including lyso platelet-activating factor, and C20 and C22 polyunsaturated fatty acids in living cells.
Insights
Rabbit macrophages possess transacylase activity, enabling lipid transfer to lyso platelet-activating factor without cofactors. This CoA-independent pathway is crucial for regulating lipid levels in cells.
Area of Science:
- Biochemistry
- Cell Biology
- Lipid Metabolism
Background:
- Platelet-activating factor (PAF) is a potent lipid mediator involved in inflammation and immunity.
- Lyso PAF, a precursor to PAF, can be acylated to form PAF.
- The acylation of lyso PAF is a critical step in PAF biosynthesis and regulation.
Purpose of the Study:
- To investigate the presence and characteristics of transacylation activity in rabbit alveolar macrophages.
- To determine the role of CoA-independent transacylation in the acylation of lyso PAF.
- To elucidate the substrate specificity and cofactor dependency of macrophage transacylases.
Main Methods:
- Incubation of rabbit alveolar macrophage microsomes with 1-[3H]alkyl-glycero-3-phosphocholine (lyso PAF).
- Analysis of fatty acyl moieties transferred to lyso PAF using various lipid donors.
- Comparison of CoA-independent transacylation with acyl-CoA:1-alkyl-GPC acyltransferase activity.
Main Results:
- Rabbit alveolar macrophage microsomes exhibit significant CoA-independent transacylation activity, comparable to acyl-CoA:1-alkyl-GPC acyltransferase.
- The primary fatty acyl moiety transferred to lyso PAF was 20:4 (n-6), originating from membrane lipids.
- CoA-independent transfer of polyunsaturated fatty acids (20:4, 20:5, 22:4, 22:6) from diacyl-GPC was efficient, while saturated and monounsaturated fatty acids transferred poorly.
- CoA addition enhanced the transfer of 20:4 from diacyl-GPE or diacyl-GPI, indicating distinct donor/acceptor selectivities and CoA dependency.
Conclusions:
- A CoA-independent transacylation system plays a significant role in the acylation of lyso PAF in macrophages.
- This system demonstrates selectivity for polyunsaturated fatty acids and specific lipid donors.
- Macrophage transacylation reactions are important for regulating lysophospholipid levels and polyunsaturated fatty acid availability in cells.