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#phospholipid transport

3 public questions tagged with this topic.

Properties of Plasma Membrane

Which enzyme is responsible for maintaining lipid asymmetry by moving phospholipids to the inner leaflet?

Enzymatic control of transbilayer phospholipid distribution involves three distinct activities defined directionally. Flippase originally term for inward moving ATP dependent P4-ATPases ATP8A1 present in liver canaliculi ATP8B1 deficient in familial intrahepatic cholestasis, ATP11C required lymphocyte development, translocate aminophospholipids phosphatidylserine and phosphatidylethanolamine from exoplasmic to cytosolic leaflet maintaining negative inner surface important for vesicle formation and apoptosis prevention. Floppase ABC transporters ABCB1 multidrug resistance P-glycoprotein ABCB4 phosphatidylcholine translocator essential bile formation ABCC1 move lipids outward ATP driven supporting excretion and lipidation of apolipoproteins via ABCA1. Scramblase phospholipid scramblase 1 TMEM16F anoctamin 6 calcium activated provides nonselective bidirectional mixing without ATP dissipating asymmetry during platelet activation exposing PS for coagulation factor assembly triggering thrombin generation and during apoptosis enabling phagocytosis. Lipase hydrolyzes ester bond producing lysophospholipid and fatty acid not intact transport. Coordinated regulation maintains asymmetry required for membrane curvature budding and signaling. Flippase specifically inward ATP dependent activity. Detailed kinetic studies show flippase specificity for aminophospholipids while floppase prefers choline phospholipids ensuring directional gradient generation and maintenance.

Ref: Pomorski et al., Lipid Transport by P4-ATPase Flippases, J Lipid Res Review.

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Membrane Transportation

Which ABC transporter plays a role in cholesterol and phospholipid transport in hepatocytes?

Row labeled Q30 appears truncated but query addresses which ABC transporter handles cholesterol and phospholipid transport in hepatocytes. In canalicular membrane ABCB4 MDR3 is established phosphatidylcholine floppase essential for biliary phospholipid secretion, pairing with bile salt export via ABCB11 and cholesterol export via ABCG5/G8. Its ATP-driven flipping of phosphatidylcholine to outer leaflet allows bile acids to extract lipid into mixed micelles that solubilize cholesterol and buffer bile acid detergent toxicity. At sinusoidal membrane, ABCA1 effluxes cholesterol and phospholipid to apolipoprotein A1 forming nascent HDL, contributing to reverse cholesterol transport. ABCB1 MDR1 exports amphipathic drugs xenobiotics rather than bulk biliary lipids, while SGLT1 and SGLT2 are sodium-glucose cotransporters secondary active members of SLC5 family irrelevant to lipid secretion. Clinically ABCB4 defects produce low-phospholipid bile, cholestasis, cholelithiasis and progressive liver injury, highlighting distinct partitioning of lipid transport functions among hepatic ABC proteins for bile formation and systemic lipoprotein metabolism. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Borst et al., Annu Rev Biochem 2000, ABC transporters in lipid transport; Alberts, Chapter 11.

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Membrane Transportation

Which ABC transporter plays a role in cholesterol and phospholipid transport in hepatocytes?

Cholesterol and phospholipid homeostasis in hepatocytes involves coordinated action of several ABC transporters at canalicular membrane. ABCB4 MDR3 flips phosphatidylcholine from inner to outer leaflet making it available for bile salt extraction forming mixed micelles that protect biliary epithelium and solubilize cholesterol exported by ABCG5/G8 heterodimer. ABCB1 MDR1 primarily exports hydrophobic xenobiotics and drugs, not bulk phospholipid for bile formation, though it can translocate short-chain lipids in vitro. ABCA1, not listed among options but relevant, mediates phospholipid and cholesterol efflux to apolipoprotein A1 for nascent HDL biogenesis at basolateral side. SGLT family members SLC5A1 and SLC5A2 are sodium-glucose symporters secondary active, unrelated to lipid transport. Mutations in ABCB4 cause progressive familial intrahepatic cholestasis type 3 with low biliary phospholipid, cholangitis and cholelithiasis, while ABCG5/G8 defects cause sitosterolemia, demonstrating specific partitioning of cholesterol and phosphatidylcholine pathways among distinct ABC transporters in liver physiology and disease. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Oude Elferink & Paulusma 2007, Hepatic lipid transporters; Nicolaou et al., J Hepatol 2012.

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