Acyl-carnitines

Glutamate aspartate exchange Acyl-carnitine exchange Calcium/sodium exchange Calcium... 

Defects of mitochondrial transport interfere with the movement of molecules across the inner mitochondrial membrane, which is tightly regulated by specific translocation systems. The carnitine cycle is shown in Figure 42-2 and is responsible for the translocation of acyl-CoA thioesters from the cytosol into the mitochondrial matrix. The carnitine cycle involves four elements the plasma membrane carnitine transporter system, CPT I, the carnitine-acyl carnitine translocase system in the inner mitochondrial membrane and CPT II. Genetic defects have been described for each of these four steps, as discussed previously [4,8,9]. 

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. 

Mardones, C., Vizioli, N., Carducci, C., Rios, A., and Valcarcel, M. (1999). Separation and determination of carnitine and acyl-carnitines by capillary electrophoresis with indirect UV detection. Anal. Chim. Acta 382, 23—31. 

Figure 7.11 Mechanism of transport of long-chain fatty adds across the inner mitochondrial membrane as fatty acyl carnitine. CRT is the abbreviation for carnitine palmitoyl transferase. CPT-I resides on the outer surface of the inner membrane, whereas CPT-II resides on the inner side of the inner membrane of the mitochondria. Transport across the inner membrane is achieved by a carrier protein known as a translocase. FACN - fatty acyl carnitine, CN - carnitine. Despite the name, CRT reacts with long-chain fatty acids other than palmitate. CN is transported out of the mitochondria by the same translocase.

Fatty acyl carnitine is transported via a translocase that transports acylcarnitine into and carnitine out of the mitochondrion (Chapter 7). 

After uptake by the cell, fatty acids are activated by conversion into their CoA derivatives—acyl CoA is formed. This uses up two energy-rich anhydride bonds of ATP per fatty acid (see p. 162). For channeling into the mitochondria, the acyl residues are first transferred to carnitine and then transported across the inner membrane as acyl carnitine (see B). 

The inner mitochondrial membrane has a group-specific transport system for fatty acids. In the cytoplasm, the acyl groups of activated fatty acids are transferred to carnitine by carnitine acyltransferase [1 ]. They are then channeled into the matrix by an acylcar-nitine/carnitine antiport as acyl carnitine, in exchange for free carnitine. In the matrix, the mitochondrial enzyme carnitine acyltransferase catalyzes the return transfer of the acyl residue to CoA. 

Carnitine deficiency leads to impaired carnitine shuttle activity the resulting decreased LCFA metabolism and accumulation of LCFAs In tissues and wasting of acyl-carnitine in urine can produce cardiomyopathy, skeletal muscle myopathy, encephalopathy, and impaired liver function. 

Figure 8-3. The carnitine shuttle. A long-chain fatty acyl CoA (LCFA CoA) can diffuse across the outer mitochondrial membrane but must be carried across the inner membrane as acyl-carnitine. The active sites of CPT-I and CPT-II are oriented toward the interiors of their respective membranes. CPT, carnitine palmitoyltransferase.

Carnitine and its esters are present in all biological fluids, and depending on the enzyme defect, a particular acylcarnitine pattern becomes apparent where those acyl-carnitine species serving as direct substrates for the defective enzyme accumulate disproportionately to the down- and upstream metabolites (Table 3.2.1). 

Fig. 3.2.3 Profiles of acylcarnitines as their butyl esters in plasma (precursor of m/z 85 scan) of a normal control (a) and three patients with elevated Gi-acyl carnitine (m/z 288 peak 4) that represents primarily butyrylcarnitine in a patient with short-chain acyl-CoA dehydrogenase (SCAD) deficiency (b), isobutyrylcarnitine in a patient with isobutyryl-CoA dehydrogenase (IBDH) deficiency (c), and a natural isotope of formiminoglutamate (FIGLU m/z 287 peak 3) in a patient with glutamate formimino-transferase deficiency (d). Peak 1 free carnitine (m/z 218), peak 2 acetylcarnitine (C2 m/z 260). The asterisks represent the internal standards (from left to right) d3-acetylcarnitine (C2 m/z 263), d3-propionylcarnitine (C3 m/z 277), d3-butyrylcarnitine (C4 m/z 291), d3-octanoylcarnitine (C8 m/z 347), d3-dodecanoylcarnitine (Ci, m/z 403), and d3-pal-mitoylcarnitine (Ci6 m/z 459)...

Vianey-Saban C, Boyer S, Levrat V, et al (2004) Interference of Cefotaxime in plasma acyl-carnitine profile mimicking an increase of 3-hydroxypalmitoleylcarnitine (C16 l-OH) using butyl esters. J Inherit Metab Dis 27 94... 

In metabolism, more specifically newborn screening for amino acids and acyl-carnitines, we use two other types of MS/MS analyses the precursor and neutral loss scans. In both types of scan, the question is asked where did I come from In other words, a fragment of a molecule is detected that is unique or common to a particular molecule or family of structurally similar molecules. Because the instruments are linked in space rather than time, when a particular product is detected in the second mass spectrometer (MS2), the computer software knows what mass was being scanned in the first mass spectrometer (MSI). The difference between a precursor ion and neutral loss scan is the nature of the product. With a precursor ion scan, that product of interest is also an ion and it can be detected. With a neutral loss... 

FIGURE17-6 Fatty acid entry into mitochondria via the acyl-carnitine/ carnitine transporter. After fatty acyl-carnitine is formed at the outer membrane or in the intermembrane space, it moves into the matrix by facilitated diffusion through the transporter in the inner membrane. In the matrix, the acyl group istransferred to mitochondrial coenzyme... 

Figure 17-2 Some specific defects in proteins of P oxidation and acyl-carnitine transport causing cardiomyopathy are indicated by the green asterisks. CPT I and CPT II are carnitine palmitoyltransferases I and II. After Kelly and Strauss.48...

Acyl-CoA is not transported across the inner membrane of the mitochondrion. Instead, the acyl-CoA reacts with carnitine to yield the acyl-carnitine derivative. This reaction is catalyzed by carnitine acyltransferase I, which is located on the outer mitochondrial membrane. The acyl-carnitine is transported across the inner membrane by a specific carrier protein. Once inside the matrix of the mitochondrion, the acyl-carnitine is converted back to its acyl-CoA... 

This reaction is catalyzed by carnitine acyltransferase I on the outer membrane (fig. 18.21). A protein carrier in the inner mitochondrial membrane transfers the acyl-carnitine derivatives across the membrane. Once inside the mitochondria, the reaction is reversed by carnitine acyltransferase II to yield a fatty acyl-CoA (see fig. 18.21). Thus, at least two distinct pools of acyl-CoA occur in the cell, one in the cytosol and the other in the mitochondrion. 

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]...

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