Carnitine conjugates

We conclude that we could selectively detect the carnitine conjugates by looking for the precursors of the 85 and 144 Th fragments, or by detecting the losses of 59 or 161 Da neutrals. 

An example of an application is given in Figure 4.13. The first spectrum is the FAB spectrum, sometimes called the source spectrum because it detects the ions formed in the source. The second spectrum is that obtained by selectively detecting the precursors of m/z 85. We see that the carnitine conjugates clearly dominate. 

Metabolite M4 was analyzed by thermospray LC/MS (Fig. 5a) and produced a protonated molecular ion at m/z 497, 499 (confirmed by fast atom bombardment mass spectral analysis). This mass spectrum, as well as proton NMR analysis (not shown) was consistent with the acyl carnitine conjugate of the parent. Fragmentation occurred via loss of the carnitine moiety to give m/z 371, 373 the fragment ion at m/z 455, 457 could correspond to an... 

Figure 5. Thermospray A) LC/MS and B) LC/MS/MS spectra of SK F 96148 biliary metabolite M4, the acyl carnitine conjugate.

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. 

Under physiologic conditions, carnitine is primarily required to shuttle long-chain fatty acids across the inner mitochondrial membrane for FAO and products of peroxisomal /1-oxidation to the mitochondria for further metabolism in the citric acid cycle [40, 43]. Acylcarnitines are formed by conjugating acyl-CoA moieties to carnitine, which in the case of activated long-chain fatty acids is accomplished by CPT type I (CPT-I) [8, 44]. The acyl-group of the activated fatty acid (fatty acyl-CoA) is transferred by CPT-I from the sulfur atom of CoA to the hydroxyl group of carnitine (Fig. 3.2.1). Carnitine acylcarnitine translocase (CACT) then transfers the long-chain acylcarnitines across the inner mitochondrial membrane, where CPT-II reverses the action of CPT-I by the formation of acyl-CoA and release of free un-esterified carnitine. 

Fatty acids are activated on the outer mitochondrial membrane, whereas they are oxidized in the mitochondrial matrix. A special transport mechanism is needed to carry long-chain acyl CoA molecules across the inner mitochondrial membrane. Activated long-chain fatty acids are transported across the membrane by conjugating them to carnitine, a zwitterionic alcohol. The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine to form acyl carnitine. This reaction is catalyzed by carnitine acyltransferase I (also called carnitine palmitoyl transferase I), which is bound to the outer mitochondrial membrane. 

The administration of pivaloyl-conjugated beta-lactam antibiotics to healthy volunteers for 54 days reduced mean serum carnitine 10-fold and muscle carnitine, as measured per non-collagen protein, more than 2-fold (62). Long-term treatment of children for 12-37 months to prevent urinary tract infection resulted in serum carnitine concentrations of 0.9-3.6 pmol/l (reference range 23-60 pmol/l). In four cases, muscle carnitine was 0.6-1.4 j,mol/g non-collagen protein (reference range 7.1-19) (63). 

In order to be metabolized, long-chain fatty acids must first undergo conjugation to carnitine for transport by the acylcarnitine-carnitine carrier across the mitochondrial inner membrane [139]. Short-chain fatty... 

T Kanazu, T Yamaguchi. Comparison of in vitro carnitine and glycine conjugation with branched-side chain and cyclic side chain carboxylic acids in rats. Drug Metab Dispos 25(2) 149—153, 1997. 

In order to be metabolized, long-chain fatty acids must first undergo conjugation to carnitine for transport by the acylcamitine-camitine carrier across the mitochondrial inner membrane [139]. Short-chain fatty acids enter the mitochondria through monocarboxylic acid transporters [139]. Studies were carried out to assess the effects cephaloridine, cephaloglydn and cephalexin on the mitochondrial oxidative metabolism of fatty adds such as butyrate and pahnitate [67]. 

The large CoA molecule renders complexes containing it unable to cross normal membranes. In Chap. 10, it was noted that fatty acids conjugated to CoA could not pass into the mitochondria they require transfer to carnitine before transport from the cytoplasm can take place. 

Fig. 3. Effects of c9,f11 -conjugated linoleic acid (CLA) and f10,c12-CLA on hepatic carnitine palmitoyltransferase (CPT) and fatty acid synthase (FAS) activities in OLETF rats. Rats were fed control diet (LA) or CLA diets (1 % c9,f11-CLA or flO,cl 2-CLA) for 2 wk. Values with different letters are significantly different, P< 0.05.

L-carnitine supplementation is routinely provided to patients with GA-1 as a way to reduce intramitochondrial glutaryl-CoA and provide extracellular release without the synthesis of glutaric acid and 3-hydroxyglutaric acid. L-camitine conjugates with coenzyme A esters to form acylcarnitines. The typical L-carnitine dose is 75-100 mg/kg/day or sufficient quantities to maintain free L-camitine concentrations within the normal range [4], Large doses of enteral L-carnitine may cause loose stools or diarrhea [11], In the hospitalized patient with acute illness, a continuous infusion of intravenous L-carnitine is preferably provided. 

Since L-camitine conjugates with toxic acyl-CoA metabolites produced in PROP and MM A, patients often develop a secondary carnitine deficiency. To prevent this, L-camitine in doses of 100-300 mg/ kg/day is prescribed on a routine basis [5, 11]. Some medical foods designed for PROP and MMA contain L-camitine, and this needs to be considered when determining the amount of supplement to prescribe. The IV form of L-camitine is often used during hospitalization for acute illness [3]. 

Isovaleryl-CoA dehydrogenase (EC 1.3.99.10). Defective conversion of isovaleryl-CoA to methylcro-tonyl-CoA (see Leudne). Elevated isovalerate in plasma and urine also increased urinary isovaleryl-glydne, isovalerylcamitine and sometimes 3-hydro-xyisovalerate. Ketoacidotic crises, sometimes with fatal coma. Slight mental retardation in survivors. Treated with low leucine diet and supplements of glycine and/or carnitine to increase excretion of isovaler-yl conjugates. Peritoneal dialysis in crises. 

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