Acylcarnitine long-chain

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

Figure 22-1. Role of carnitine in the transport of long-chain fatty acids through the inner mitochondrial membrane. Long-chain acyl-CoA cannot pass through the inner mitochondrial membrane, but its metabolic product, acylcarnitine, can. 

A few patients have been described with a defect involving the carnitine-acylcarnitine translocase system, which facilitates the movement of long-chain acylcarnitine esters across the inner membrane of the mitochondrion (Fig. 42-2). These patients have extremely low carnitine concentrations and minimal dicarboxylic aciduria [4]. 

Carnitine is a vitamin-like quaternary ammonium salt, playing an important role in the human energy metabolism by facilitating the transport of long-chained fatty acids across the mitochondrial membranes. An easy, fast, and convenient procedure for the separation of the enantiomers of carnitine and 0-acylcarnitines has been reported on a lab-made teicoplanin-containing CSP [61]. The enantioresolution of carnitine and acetyl carnitine was enhanced when tested on a TAG CSP, prepared in an identical way [45]. Higher a values were reached also in the case of A-40,926 CSP [41]. 

Fischbach PS, Corr PB, Yamada KA Long-chain acylcarnitine increases intracellular calcium and induces afterdepolarizations in adult ventricular myocytes (abstracts). Circulation 1992 86(suppl I) 748. 

Wu J, Corr PB Influence of long chain acylcarnitines on the voltage-dependent calcium current in adult ventricular myocytes. Am J Phyisol 1992 263 H410-H417. 

Wu J, McHowat J, Saffitz JE, Yamada KA, Corr PB Inhibition of gap junctional conductance by long chain acylcarnitines and their preferential accumulation in junctional sarcolemma during hypoxia. Circ Res 1993 72 879-889. 

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. 

The long-chain FAO disorders of CACT deficiency and CPT-II deficiency can not be differentiated because both cause accumulation of the same long-chain acylcarnitine species, which is explained by the fact that neither enzyme is involved in the chain-shortening action of FAO (Table 3.2.1 Fig. 3.2.6e and f). Isolated long-chain... 

Another antibiotic that may cause problems in the interpretation of butylated acylcarnitines is cefotaxime (Fig. 3.2.5d) [63]. This antibiotic, or metabolites thereof, reveals itself by acylcarnitine analysis at m/z 470, which is otherwise considered to represent the monounsaturated form of 3-hydroxy hexadecenoylcarnitine (Ci i-OH). In poorly resolved scans this may be difficult to differentiate from m/z 472, which is a marker for LCHAD and TFP deficiencies. However, whereas m/z 472 (C16-OH) is more abundant than C16 1-OH in these FAO disorders, the profile of a patient treated with cefotaxime usually reveals an m/z 470 to m/z 472 ratio that is greater than 1. Furthermore, and in contrast to cefotaxime treatment, both LCHAD and TFP deficiencies are usually accompanied by elevations of other long-chain species (Table 3.2.1) [57]. 

Shen JJ, Matern D, Millington DS, et al (2000) Acylcarnitines in fibroblasts of patients with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency and other fatty acid oxidation disorders. J Inherit Metab Dis 23 27-44... 

Browning MF, Larson C, Strauss A, Marsden DL (2005) Normal acylcarnitine levels during confirmation of abnormal newborn screening in long-chain fatty acid oxidation defects. J Inherit Metab Dis 28 545-550... 

Roe DS, Yang BZ, Vianey-Saban C, Struys E, Sweetman L, Roe CR (2006) Differentiation of long-chain fatty acid oxidation disorders using alternative precursors and acylcarnitine profiling in fibroblasts. Mol Genet Metab 87 40-47... 

Tissues contain not only long-chain acylcarnitines but also acetylcarnitine and other short-chain acylcarnitines, some with branched chains.41 By accepting acetyl groups from acetyl-CoA, carnitine causes the release of free coenzyme A which can then be reused. 

LeCluyse EL, Sutton SC, Fix JA (1993) In vitro effects of long-chain acylcarnitines on the permeability, transepithelial electrical resistance and morphology of rat colonic mucosa. J Pharmacol Exp Ther 265 955-962... 

Figure 2.7. The complex pathways and processes involved in fat catabolism in vertebrate tissues such as cardiac and skeletal muscles. FFAs arrive at the cell boundary either via VLDL or albumin-associated and enter the cell either by simple diffusion or through transporters. In the cytosol, FFAs are bound by FABPs, which increase the rate and amount of FFA that can be transferred to sites of utilization. Shorter chain FFAs are converted to acetylCoA in peroxisomes longer chain FFAs are directly transferred to mitochondria (via a complex system involving acylcarnitines) as long-chain acylCoA derivatives these enter the /6-oxidation spiral and are released as acetylCoA for entrance into the Krebs or citric acid cycle in the mitochondrial matrix. Fatty acid receptors (FARs) in the nucleus bind to fatty acid response elements (FAREs) and in turn regulate the production of enzymes in their own metabolism. (Modified from Veerkamp and Maatman, 1995.)...

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.

Carnitine palmitoyltransferases I and II catalyze the transfer of long-chain acyl coenzyme A into mitochondria. The I isozyme is located on the cytosol side of the inner membrane and catalyzes the formation acylcarnitine from acyl-CoA and carnitine. After acylcarnitine crosses the inner membrane, it is converted back to acyl-CoA by the action of the II isozyme. This assay measures the activity of carnitine palmitoyltransferase I in intact mitochondria. 

These findings are consistent with impaired fatty-acid oxidation reduced mitochondrial entry of long-chain acylcarnitine esters due to inhibition of the transport protein (carnitine palmityl transferase 1) and failure of the respiratory chain at complex II. Another previously reported abnormality of the respiratory chain in propofol-infusion syndrome is a reduction in cytochrome C oxidase activity, with reduced complex IV activity and a reduced cytochrome oxidase ratio of 0.004. Propofol can also impair the mitochondrial electron transport system in isolated heart preparations. 

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... 

Figure 55-14 Plasma profiles of plasma acylcarnitine butyl-ester derivatives. A, Normal control. B, Propionic acidemia. C, Short-chain acyl-CoA dehydrogenase deficiency. D, Isovaleric acidemia. E, Medium-chain acyl-CoA dehydrogenase deficiency. F, Very long-chain acyl-CoA dehydrogenase deficiency. G, Long-chain L-3-hydroxy acyl-CoA dehydrogenase deficiency.The symbol marks internal standards [ Hjj-acetylcarnitine (m/z 263) [ HaJ-propionylcarnitine (m/z 277) fH ]-butyrylcarnitlne (m/z 295) pHal-octanoylcarnitine (m/z 347) [ Haj-dodecanoylcarnltine (m/z 403) [ Haj-palmitoy I carnitine (m/z 459).

The answer is b. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) A deficiency in carnitine, carnitine acyl-transferase 1, carnitine acyltransferase 11, or acylcarnitine translocase can lead to an inability to oxidize long-chain fatty acids. This occurs because all of these components are needed to translocate activated long-chain (>10 carbons long) fatty acyl CoA across mitochondrial inner membrane into the matrix where P oxidation takes place. Once long-chain fatty acids are coupled to the sulfur atom of CoA on the outer mitochondrial membrane, they can be transferred to carnitine by the enzyme carnitine acyltransferase I, which is located on the cytosolic side of the inner mitochondrial membrane. Acyl carnitine is transferred across the inner membrane to the matrix surface by translocase. At this point the acyl group is reattached to a CoA sulfhydryl by the carnitine acyltransferase 11 located on the matrix face of the inner mitochondrial membrane. 

The fact that the mitochondrial inner membrane is virtually impermeable to long-chain fatty acyl-CoA, while the fatty acid oxidative machinery is located inside the mitochondrial matrix, a space enclosed by the inner membrane, might create a serious problem for cellular energy production. The problem is solved by the development of a transmembrane carnitine-dependent transport system for the long-chain acyl residue of acyl-CoA. Catalyzed by carnitine acyltransferase I (CAT-I), which is attached to the inner surface of the mitochondrial outer membrane, fatty acyl-CoA is converted to fatty acyl-carnitine by replacing the CoA residue with carnitine (Figure 3). Fatty acyl-carnitine is transported across the mitochondrial inner membrane in exchange for a molecule of free carnitine by carnitine-acylcarnitine translocase. After arrival in the mitochondrial matrix, fatty acyl-carnitine is converted back to acyl-CoA by carnitine acyltransferase II (CAT-II), an enzyme located on the inner surface of the mitochondrial inner membrane. 

Su X., Han X., Mancuso D.J., Abendschein D.R., Gross R.W. Accumulation of long-chain acylcarnitine and 3-hydroxy acylcarnitine molecular species in diabetic myocardium identification of alterations in mitochondrial fatty acid processing in diabetic myocardium by shotgun lipidomics. Biochemistry 44 (2005) 5234-5245. 

Many of these disorders are associated with the urinary excretion of acylcarnitines, acyl conjugates of glycine, and dicarboxylic acids that are characteristic of the metabolic block. A general conclusion derived from studies of these disorders is that an impairment of p-oxidation makes fatty acids available for microsomal (o-oxidation by which fatty acids are oxidized at their terminal (to) methyl group or at their penultimate (o) - 1) carbon atom. Molecular oxygen is required for this oxidation and the hydroxylated fatty acids are further oxidized to dicarboxylic acids. Long-chain dicarboxylic acids can be... 

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