Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within t he lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg2+ Incubating these fused microsomes with tri[H-3] oleoylglycer ol and [C-14]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [3H]:[14C] ratio close to 2:1, The da ta suggest that the precursor tri[H-3]oleoylglycerol was hydrolyzed by micr osomal lipase to membrane-bound di[3H]oleoylglycerol and subsequently re-es terified with luminal [C-14]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was ina ctive, is consistent with the existence of a latent diacylglycerol acyltran sferase (DGAT II) within the microsomal lumen. Moreover, because luminal sy nthesis of TAG was carnitine-dependent and markedly reduced by glybenclamid e, a potent carnitine acyltransferase inhibitor, microsomal carnitine acylt ransferase appears to be essential for trafficking the [C-14]oleoyl CoA int o the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymati c process leading to the synthesis of luminal triacylglycerol, which is a m ajor component of very low density lipoproteins.