N-Acetylaspartate as an acetyl source in the nervous system

To understand the role of N-acetylaspartate (NAA) as an acetyl donor, we investigated the metabolism of NAA in brain and liver slice preparations. The tissue slices were incubated with [¹⁴C-acetyl]NAA (SA = 3 μCi/μmol) or [¹⁴C]acetate (SA = 3 μCi/μmol) for 2 h. The tissue was homogenized and was extracted using chloroform/methanol (2:1). The aqueous phase was initially analyzed using anion exchange HPLC while the lipid phase was analyzed using a two-dimensional TLC system. Further resolution of the NAA peak from the anion exchange HPLC was performed using a reverse phase HPLC system. The aqueous phase of both the liver and brain samples incubated with [¹⁴C-acetyl]NAA revealed similar patterns of three distinct radioactivity peaks corresponding to NAA, acetate and an early eluting unknown molecule. Further resolution of the NAA peak using reverse phase HPLC indicated that it corresponded to NAA and acetyl CoA. There was significant incorporation of radioactivity into various lipid components in both the brain and liver samples. Patterns similar to that observed with NAA were detected in the case of [¹⁴C]acetate in both the brain and liver slice preparations. These results demonstrate that NAA metabolism is not restricted to the nervous system, although its biosynthesis is. It is clear that acetyl moiety of NAA is incorporated into lipids and partially hydrolyzed to free acetate in both brain and liver preparations. Further, production of acetyl CoA from NAA indicates that the acetyl group of NAA is incorporated into lipids and perhaps other acetylated molecules via the acetyl CoA route. A working hypothesis on the metabolic role of NAA is presented.