Hemipalmitoylcarnitinium a strong competitive inhibitor of purified hepatic carnitine palmitoyltransferase

Abstract We have synthesized (2 S ,6 R :2 R ,6 S )-6-carboxymethyl-2-hydroxy-2-pentadecyl-4,4-dimethylmorpholinium bromide (hemipalmitoylcarnitinium, HPC), which is a conformationally restricted analog inhibitor of carnitine palmitoyltransferase (CPT; EC 2.3.1.21). rac-HPC inhibits catalytic activity in purified rat liver CPT. In the forward reaction, HPC competes with both ( R )-carnitine ( K i (app) = 5.1 ± 0.7 μ M) and palmitoyl-CoA ( K i (app = 21.5 ± 4.9 μ M). In the reverse reaction, inhibition by HPC is competitive with palmitoyl-( R )-carnitine ( K i (app = 1.6 ± 0.6 μ M), but inhibition is uncompetitive with CoA. The forward reaction is also competitively inhibited by its product, palmitoyl-( R )-carnitine, K i (app)'s 14.2 ± 2.1 μ m relative to ( R )-carnitine and 8.7 ± 2.6 μ m relative to palmitoyl-CoA. rac-HPC is the most potent synthetic reversible inhibitor of purified CPT. HPC fails to inhibit carnitine acetyltransferase (CAT; EC 2.3.1.7). Palmitoylcholine also inhibits CPT in the forward reaction, competing with ( R )-carnitine ( K i (app = 18.6 ± 4.5 μ M) and with palmitoyl CoA ( K i (app = 10.4 ± 2.5 μ M). Choline is not an effective CPT inhibitor. We have shown [R. D. Gandour et al. (1986) Biochem. Biophys. Res. Commun. 138 , 735–741] that hemiacetylcarnitinium inhibits CAT but not CPT. The combined data demonstrate further differences between the carnitine recognition sites in CPT and CAT.