Origin of plasma lysophosphatidylcholine: evidence for direct hepatic secretion in the rat

In the plasma, lysophosphatidylcholine (LPC) is formed by the action of lecithin-cholesterol acyltransferase (LCAT) when a fatty acid is removed from plasma phosphatidylcholine (PC) and transferred to cholesterol. To determine whether plasma LPC might also be generated by the hydrolysis of hepatic PC, we assessed phospholipid production by the isolated perfused rat liver. Bile duct-cannulated livers were perfused with bile salt and a recirculating, lipid-free medium containing albumin. We found that LPC accumulated in the perfusate to a greater extent than any other phospholipid, exceeding the accumulation of PC (the second most prevalent phospholipid) twofold. We further found that perfusate LPC was not formed by hydrolysis of PC in the perfusate and was not dependent on the presence of infused bile salt. LPC that accumulated in the perfusate was highly unsaturated and markedly dissimilar to the more saturated LPC that results from the activity of LCAT. Results thus indicate that the isolated liver directly secretes LPC, which is presumably generated from hydrolysis of hepatic PC. Because plasma LPC is to a great extent unsaturated in the live rat, these findings suggest that direct hepatic secretion is a quantitatively important source of plasma LPC.