Involvement of recognition and interaction of carnitine transporter in the decrease of L-carnitine concentration induced by pivalic acid and valproic acid

Purpose. Prodrugs with pivalic acid and valproic acid decrease L-carnitine concentration in plasma and tissues by urinary excretion of acylcarnitine as pivaloylcarnitine (PC) and valproylcarnitine (VC), respectively. We investigated the role of the Na+/L-carnitine cotransporter in the porcine kidney epithelial cell line, LLC-PK1 for the decrease of L-carnitine concentration. Methods. The uptake of L-[H-3] carnitine, acetyl-L-[H-3] carnitine (AC), L-[H-3] PC and L-[H-3] VC were investigated in LLC-PK1 cells seeded in a 6-well culture plate. Results. L-Carnitine and AC uptake in LLC-PK1 cells exhibited Na+ dependency. The Km values for L-carnitine and AC uptake were 11.0 and 8.18 mu M, respectively. These results indicated expression of Na+/L-carnitine cotransporter in LLC-PK1 cells. PC and VC inhibited Na+/L-carnitine cotransporter in the competitive (K-i = 90.4 mM) and noncompetitive (K-i = 41.6 mu M) manners, respectively. PC and VC uptake by Na+/L-carnitine cotransporter were not observed in LLC-PK1 cells. Conclusions. These data suggested that PC and VC formed in the body could not be reabsorbed in the kidney, resulting in the decrease of L- carnitine concentration. In addition, inhibition of L-carnitine reabsorption by VC with lower Ki value could induce the decrease of L- carnitine concentration. Collectively, the recognition and interaction of Na+/L-carnitine cotransporter are important factors for carnitine homeostasis.