Amino acid conjugation of fatty acids as a novel pathway in lipid metabolism

Abstract: Peroxisomes are cell organelles that play an important role in the metabolism of a variety of lipids including very long chain fatty acids, dicarboxylic acids, bile acids, and xenobiotic carboxylic acids to name a few. To accomplish these tasks peroxisomes contain many proteins involved in lipid metabolism including members of the Type I acyl-CoA thioesterase/acyltransferase protein family. We have identified two new peroxisomal members of this gene family, which we have named acyl-CoA:amino acid N-acyltransferase 1 (ACNAT1) and 2 (ACNAT2). In this study we have focused on establishing roles for ACNAT I and ACNAT2 in lipid metabolism. ACNAT I acts as an acyltransferase that efficiently conjugates long-chain and very long-chain fatty acids to taurine; these novel metabolites are known as Nacyltaurines (NATs) and may act to excrete fatty acids or have roles in cell signaling. While we have not yet established an activity for ACNAT2 we hypothesise that it also functions as an acyltransferase, but perhaps with a different substrate specificity such as polyunsaturated fatty acids or xenobiotics. The final step in de-novo bile acid synthesis occurs in peroxisomes, with the conjugation of primary bile acids to glycine and/or taurine. Bile acids deconjugated in the gut are recycled via enterohepatic circulation and reconjugated in liver cytosol. Therefore, two different pathways exist for the conjugation of bile acids, one in the cytosol and one in the peroxisomes. To date only the cytosolic bile acid conjugating enzyme, bile acid-CoA:amino acid N-acy ]transferase (BACAT) has been identified. We propose that ACNAT2 may function as the peroxisomal BACAT. The Type I acyl-CoA thioesterase/acyltransferase gene family contains nine members that display two different types of activities, one is hydrolysis of fatty acyl-CoAs and the other is the conjugation of bile acids or fatty acids to glycine and/or taurine. Using site directed mutagenesis we show how important the amino acid residues in the active site are for the optimal activity of three members of this gene family.