Utilization of intracellular acylcarnitine pools by mononuclear phagocytes

Carnitine is essential for the metabolism of long-chain fatty acids and has both direct and indirect roles in the metabolism of short-chain and medium-chain acyl-CoAs. The purpose of this study was to quantitate and identify the individual acylcarnitines that occur in human mononuclear phagocytes (MNP) after activating them with phorbol-12-myristate 13-acetate (PMA). Mononuclear phagocytes were isolated from healthy adults and the levels of free carnitine and individual acylcarnitines were determined in unactivated and activated cells. The degree of activation of MNP was assessed by following hydrogen peroxide production. In unactivated cells, acetyl-L-carnitine represented more than 80% of the total acylcarnitine pool. Small amounts of 3-carbon and 4-carbon acylcarnitines were present, with less than 10% of the carnitine pool being long-chain acylcarnitine. Free carnitine in unactivated cells represented 7% of the total carnitine pool, which remained essentially unchanged in unactivated cells when monitored for a period of 60 min. However, free carnitine rose to more than 50% of the total pool in PMA-activated cells. Similarly, after 1 h of activation, the acetylcarnitine level in activated cells decreased by more than 50%. These data suggest that acetylcarnitine plays a key metabolic role as MNP initiate an immune response. It was further shown that MNP contain both carnitine acetyltransferase and malonyl-CoA-sensitive carnitine palmitoyltransferase in mitochondrial-enriched fractions, as well as in post-mitochondrial supernatant fractions.