Carnitine esters

The physiological functions of carboxylesterases are still partly obscure but these enzymes are probably essential, since their genetic codes have been preserved throughout evolution [84] [96], There is some evidence that microsomal carboxylesterases play an important role in lipid metabolism in the endoplasmic reticulum. Indeed, they are able to hydrolyze acylcamitines, pal-mitoyl-CoA, and mono- and diacylglycerols [74a] [77] [97]. It has been speculated that these hydrolytic activities may facilitate the transfer of fatty acids across the endoplasmic reticulum and/or prevent the accumulation of mem-branolytic natural detergents such as carnitine esters and lysophospholipids. Plasma esterases are possibly also involved in fat absorption. In the rat, an increase in dietary fats was associated with a pronounced increase in the activity of ESI. In the mouse, the infusion of lipids into the duodenum decreased ESI levels in both lymph and serum, whereas an increase in ES2 levels was observed. In the lymph, the levels of ES2 paralleled triglyceride concentrations [92] [98],... 

Wang J, Guo X, Xu Y, Barron L, Szoka EC Jr. Synthesis and characterization of long chain alkyl acyl carnitine esters. Potentially biodegradable cationic lipids for use in gene delivery. J Med Chem 1998 41(13) 2207-2215. 

P-oxidation continues only as far as a medium-chain CoA derivative, which is transported out of the peroxisome as its carnitine ester. It then enters the mitochondria where P-oxidation continues to produce acetyl-CoA. The acetyl-CoA, which is produced in the peroxisome, is also transported out for oxidation in the mitochondria (Appendix 7.4). 

Quantitation of urinary carnitine esters in a patient with medium-chain acyl-coenzyme A dehydrogenase deficiency effect of metabolic state and L-camitine therapy. 

Carnitine, L-3-hydroxy-4-(trimethylammonium)butyrate, is a water-soluble, tri-methylammonium derivative of y-amino-jS-hydroxybutyric acid, which is formed from trimethyllysine via y-butyrobetaine [40]. About 75% of carnitine is obtained from dietary intake of meat, fish, and dairy products containing proteins with trimethyllysine residues. Under normal conditions, endogenous synthesis from lysine and methionine plays a minor role, but can be stimulated by a diet low in carnitine. Carnitine is not further metabolized and is excreted in urine and bile as free carnitine or as conjugated carnitine esters [1, 41, 42]. Adequate intracellular levels of carnitine are therefore maintained by mechanisms that modulate dietary intake, endogenous synthesis, reabsorption, and cellular uptake. 

Schmidt-Sommerfeld E, Bobrowski PJ, Penn D, Rhead WJ, Wanders RJA, Bennet MJ (1998) Analysis of carnitine esters by radio-high performance liquid chromatography in cultured skin fibroblasts from patients with mitochondrial fatty acid oxidation disorders. Pediatr Res 44 210-214... 

Hamilton JJ, Hahn P (1987) Carnitine and carnitine esters in rat bile and human duodenal fluid. Can J Physiol Pharmacol 65 1816-1820... 

This three-step process for transferring fatty acids into the mitochondrion—esterification to CoA, transesterification to carnitine followed by transport, and transesterification back to CoA—links two separate pools of coenzyme A and of fatty acyl-CoA, one in the cytosol, the other in mitochondria These pools have different functions. Coenzyme A in the mitochondrial matrix is largely used in oxidative degradation of pyruvate, fatty acids, and some amino acids, whereas cytosolic coenzyme A is used in the biosynthesis of fatty acids (see Fig. 21-10). Fatty acyl-CoA in the cytosolic pool can be used for membrane lipid synthesis or can be moved into the mitochondrial matrix for oxidation and ATP production. Conversion to the carnitine ester commits the fatty acyl moiety to the oxidative fate. 

The helper effects of DOPE and cholesterol appear to be hydrocarbon chain-specific. This is demonstrated in studies of their mixtures with a series of alkyl acyl carnitine esters (alkyl 3-acyloxy-4-trimethylammonium butyrate chloride) tested with CV-1 cell culture (monkey fibroblast) [127]. The influence of the aliphatic chain length (n - 12-18) on transfection in vitro was determined using cationic liposomes prepared from these lipids and their mixtures with the helper lipids DOPE and cholesterol (Fig. 30). Both helper lipids provided for significant transfection enhancements in an apparently chain-specific manner, with the highest effects found for short-chain lipids with diC12 0 and diC14 0 chains in 1 1 mixtures with the respective helper lipid. 

Fig. 30 Transfection activity of lipoplexes consisting of alkyl/acyl carnitine esters, alone and with helper lipid (DOPE or cholesterol), on [1-galactosidase expression in CV-1 cell culture (monkey fibroblast) cationic lipid/DNA charge ratio 4 1 [127]...

Figure 9-1- Role of carnitine in fatty acid oxidation. Long-chain fatty acids are activated as the thioester of CoA on the cytoplasmic side of the mitochondrial membrane. The fatty acyl group is then transferred to form the corresponding carnitine ester in a reaction catalyzed by carnitine palmitoyltransferase I (CPT ]) The acylcarnitine then enters the mitochondrial matrix in exchange for carnitine via the carnitine-acylcarnitine translocase. The acyl group is transferred back to CoA in the matrix by carnitine palmitoyltransferase II (CPT II). The intramitochondrial acyl-CoA can then undergo P-oxidation.

Carititine (3-hydroxy,4-AT-trimethylaminobutyric acid) has a central role in the transport of fatty acids across the mitochondrial membrane for p -oxidation. At the outer face of the outer mitochondrial membrane, carnitine acyltransferase I catalyzes the reaction shown in Figure 14.1, the transfer of fatty acids from coenzyme A (CoA) to form acyl carnitine esters that cross into the mitochondrial matrix. At the timer face of the inner mitochondrial membrane, carnitine acyltransferase II catalyzes the reverse reaction. 

The total body content of carnitine is about 100 mmol, and about 5% of this turns over daily. Plasma total carnitine is between 36 to 83 /rmol per L in men and 28 to 75 /rmol per L in women, mainly as free carnitine. Although both free carnitine and acyl carnitine esters are excreted in the urine, much is oxidized to trimethylamine and trimethylamine oxide. It is not known whether the formation of trimethylamine and trimethylamine oxide is caused by endogenous enzymes or intestinal bacterial metabolism of carnitine. 

Total urinary excretion of carnitine is between 300 to 530 /rmol (men) or 200 to 320 /rmol (women) 30% to 50% of this is free carnitine the remainder is a variety of acyl carnitine esters. Acyl carnitine esters are readily cleared in... 

Urinary excretion of acyl carnitine esters increases considerably in a variety of conditions involving organic aciduria carnitine acts to spare CoA and pantothenic acid (Section 12.2), by releasing the coenzyme from otherwise nonmetabolizable esters that would trap the coenzyme and cause functional pantothenic acid deficiency. 

Lipids with imidazole derivative l-[2-[9-(Z) -octadecenoyloxy ] ethyl] ] - 2- [8] (Z) -heptade-cenyl]-3-[hydroxyethyl]imidazolinium chloride (DOTIM) (129) or non-glycerol-based hpids with long-chain alkylacyl carnitine ester (130). 

Carnitine serves as the carrier that transports activated long chain fatty acyl groups across the inner mitochondrial membrane (Fig. 23.4). Carnitine acyl transferases are able to reversibly transfer an activated fatty acyl group from CoA to the hydroxyl group of carnitine to form an acylcamitine ester. The reaction is reversible, so that the fatty acyl CoA derivative can be regenerated from the carnitine ester. 

The pool-size of intramitochondrial carnitine and carnitine esters can be altered by a leak reaction in which uncompensated carnitine flux across the mitochondrial membrane occurs. This leak is presumably mediated by the carnitine translocator and is 100-200 times slower than the exchange rate [22,87,89]. 

Before long-chain fatty acids can enter the mitochondria and get access to the P-oxidation pathway, they must first be activated to acyl-CoA in a reaction that requires ATP and coenzyme-A. The acyl-CoA still cannot cross the mitochondrial inner membrane and must react with carnitine to form the corresponding carnitine ester. This reaction is catalyzed by the enzyme carnitine palmitoyltransferase (CPT). The acylcamitine itself is also unable to diffuse into the mitochondrial matrix so that the transport is achieved by a specific protein, the carnitine acylcamitine translocase. Following transport across the mitochondrial inner membrane, acylcamitines are converted back to the corresponding acyl-CoA and carnitine. This reaction is catalyzed by another carnitine palmitoyltransferase which is a different enzyme than that involved in the formation of the acylcamitine outside the mitochondria. Hence, there are two CPTs, one associated with the inner aspect of the mitochondrial inner membrane, CPT-lP and one that lies... 

Lopez-Cardozo, M., Klazinga, W. van den Bergh, S.G. (1978) E wr. J. Biochem. 83, 629-634. Accumulation of carnitine esters of beta-oxidation intermediates during palmitate oxidation in rat liver mitochondria. 

Holland, P. Sherratt, H. (1973) Biochem. J. 173, 157-171. Biochemical effects of the hypoglycaemic compound pent-4-enoic add and related nai-hypoglycaemic fatty adds. Effects of the free adds and their carnitine esters on coenzyme A-dependent oxidations in rat liver mitochondria. 

PRODUCTION AND EXPORT OF ACYL-CARNITINE ESTERS BY NEONATAL RAT HEPATOCYTES... 

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