Kinetics, pharmacokinetics, and regulation of L-carnitine and acetyl-L-carnitine metabolism.
In mammals, the carnitine pool consists of nonesterified L-carnitine and many acylcarnitine esters. Of these esters, acetyl-L-carnitine is quantitatively and functionally the most significant. Carnitine homeostasis is maintained by absorption from diet, a modest rate of synthesis, and efficient renal reabsorption. Dietary L-carnitine is absorbed by active and passive transfer across enterocyte membranes. Bioavailability of dietary L-carnitine is 54-87% and is dependent on the amount of L-carnitine in the meal. Absorption of L-carnitine dietary supplements (0.5-6 g) is primarily passive; bioavailability is 14-18% of dose. Unabsorbed L-carnitine is mostly degraded by microorganisms in the large intestine. Circulating L-carnitine is distributed to two kinetically defined compartments: one large and slow-turnover (presumably muscle), and another relatively small and rapid-turnover (presumably liver, kidney, and other tissues). At normal dietary L-carnitine intake, whole-body turnover time in humans is 38-119 h. In vitro experiments suggest that acetyl-L-carnitine is partially hydrolyzed in enterocytes during absorption. In vivo, circulating acetyl-L-carnitine concentration was increased 43% after oral acetyl-L-carnitine supplements of 2 g/day, indicating that acetyl-L-carnitine is absorbed at least partially without hydrolysis. After single-dose intravenous administration (0.5 g), acetyl-L-carnitine is rapidly, but not completely hydrolyzed, and acetyl-L-carnitine and L-carnitine concentrations return to baseline within 12 h. At normal circulating l-carnitine concentrations, renal l-carnitine reabsorption is highly efficient (90-99% of filtered load; clearance, 1-3 mL/min), but displays saturation kinetics. Thus, as circulating L-carnitine concentration increases (as after high-dose intravenous or oral administration of L-carnitine), efficiency of reabsorption decreases and clearance increases, resulting in rapid decline of circulating L-carnitine concentration to baseline. Elimination kinetics for acetyl-L-carnitine are similar to those for L-carnitine. There is evidence for renal tubular secretion of both L-carnitine and acetyl-L-carnitine. Future research should address the correlation of supplement dosage, changes and maintenance of tissue L-carnitine and acetyl-L-carnitine concentrations, and metabolic and functional changes and outcomes.
Rebouche
Charles J
CJ
Department of Pediatrics, University of Iowa, Iowa City, Iowa, USA. charles-rebouche@uiowa.edu
D000108:Acetylcarnitine / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics; D001682:Biological Availability; D002331:Carnitine / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics; D004032:Diet; D019587:Dietary Supplements; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007700:Kinetics; D008657:Metabolic Clearance Rate; D013997:Time Factors; D014018:Tissue Distribution
D000108:Acetylcarnitine; D002331:Carnitine