Carnitine acetyltransferase

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 ...

Two studies were located in which rats received di- -octylphthalate dietary exposures of 250 or 500 mg/kg/day for either 10 or 26 weeks (Carter et al. 1992 DeAngelo et al. 1986). Five male rats were first initiated with a single subcarcinogenic intraperitoneal dose of diethylnitrosamine (30 mg/kg), followed by partial hepatectomy. Di-w-octylphthalatc caused substantial increases in gamma-glutamyltranspeptidase (GGT) positive liver foci when compared with the controls (e.g., from 3.5 to 20.8 foci/cm2) or in hepatic levels of marker enzymes for altered cellular foci (GGT and glutathione 5-transferase [GST]). Only a slight increase (threefold) was observed for carnitine acetyltransferase (CAT) activity, a marker for peroxisome... 

ATP. The presence of acetyl-carnitine in mitochondria, at a concentration 400-fold greater than that of acetyl-CoA, along with the enzyme carnitine acetyltransferase, acts as a buffer to protect against large changes in the concentration of acetyl-CoA ... 

CARNITINE ACETYLTRANSFERASE CARNITINE OCTANOYLTRANSFERASE CARNITINE PALMITOYLTRANSFERASE... 

Male Fischer 344 rats (body weight, 100-150 g) were fed 0.5-4% di(2-ethyl-hexyl) phthalate in the diet for one or four weeks and male Swiss Webster mice (20-30 g) were fed 2 or 4% di(2-ethylhexyl) phthalate in the diet for one or four weeks (Reddy etal., 1976). Di(2-ethylhexyl) phthalate increased relative liver weights and markedly induced hepatic carnitine acetyltransferase activity in both species (up to 25-fold in rats and 10-fold in mice). Some increase in hepatic catalase activity (approximately two-fold) was observed and subjective (non-morphometric) ultra-structural examination revealed marked peroxisome proliferation. This study also demonstrated that di(2-ethylhexyl) phthalate was a hypolipidaemic agent, as serum triglyceride levels were reduced to one seventh of control values in rats and one third of control values in mice. 

Hepatocytes isolated from male Wistar rats (180-250 g) were treated with 0.2 mM mono(2-ethylhexyl) phthalate or 1 mM 2-ethylhexanol for 48 h (Gray et al., 1982). Both di(2-ethylhexyl) phthalate metabolites increased carnitine acetyltransferase activity about nine-fold. In studies with hepatocytes from male Sprague-Dawley rats (180-220 g), treatment with 0.2 mM mono(2-ethylhexyl) phthalate and 1.0 mM 2-ethylhexanol for 48 h resulted in induction of carnitine acetyltransferase activity about 15-fold and six-fold, respectively (Gray et al., 1983). Mono(2-ethylhexyl) phthalate was also shown to induce cyanide-insensitive palmitoyl-CoA oxidation and, by ultra-structural examination, to increase numbers of peroxisomes. Hepatocytes were isolated from Wistar-derived rats (180-220 g) and treated for 72 h with 0-0.5 mM mono(2-ethylhexyl) phthalate and some mono(2-ethylhexyl) phthalate metabolites (Mitchell etal., 1985). Treatment with mono(2-ethylhexyl) phthalate and metabolites VI and IX (see Figure 1) resulted in a concentration-dependent induction of cyanide-insensitive palmitoyl-CoA oxidation. In addition, 0-0.5 mM mono(2-ethylhexyl) phthalate and 0-1.0 mM metabolite VI produced concentration-dependent increases in lauric acid hydroxylation. Treatment with metabolites I and V resulted in only small effects on the enzymatic markers of peroxisome proliferation. In another study with hepatocytes from Wistar-derived rats (180-220 g), metabolite VI was shown by subjective ultrastructural examination to cause proliferation of peroxisomes (Elcombe Mitchell, 1986). 

Hepatocytes were isolated from male Fischer 344 rats and from two human liver samples (liver surgery patients). Treatment with 200 pM mono(2-ethylhexyl) phthalate for either 48 or 72 h induced carnitine acetyltransferase activity in cultured rat but not human hepatocytes (Butterworth et al, 1989). 

Hepatocytes were isolated from male Wistar rats, two dogs (age, breed and sex not stated) and two human subjects (69-71 years of age, sex not stated) (Hildebrand et al., 1999). In collagen sandwich cultures, the rat hepatocytes responded to di(2-ethylhexyl) phthalate in the culture medium with slightly increased carnitine acetyltransferase activity, while dog and human hepatocytes did not respond. 

Reddy, J.K., Moody, D.E., Azamoff, D.L. Rao, M.S. (1976) Di-(2-ethylhexyl)phthalate an industrial plasticizer induces hypolipidemia and enhances hepatic catalase and carnitine acetyltransferase activities in rat and mice. Life Sci., 18, 941-945 Reddy, J.K., Reddy, M.K., Usman, M L, Lalwani, N.D. Rao, M S. (1986) Comparison of hepatic peroxisome proliferative effect and its implication for hepatocarcinogenicity of phthalate esters, di(2-ethylhexyl) phthalate, and di(2-ethylhexyl) adipate with a hypolipidemic drag. Environ. Health Perspect., 65, 317-327 Rexroat, M.A. Probst, GS. (1985) Mutation tests with Salmonella using the plate-incorporation assay. Prog. Mutat. Res., 5, 201-212 Rhodes, C., Orton, T.C., Pratt, I.S., Batten, P.L., Bratt, H., Jackson, S.J. Elcombe, C.R. (1986) Comparative pharmacokinetics and subacute toxicity of di(2-ethylhexyl) phthalate (DEHP) in rats and marmosets extrapolation of effects in rodents to man. Environ. Health Perspect., 65, 299-307... 

Hepatic Effects. No studies were located regarding hepatic effects in humans after oral exposure to DEHP. Limited information on hepatic effects in humans exposed to DEHP is available from studies of dialysis patients and cultured human hepatocytes. In one individual there was an increased number of liver peroxisomes after 1 year, but not after 1 month of treatment (Ganning et al. 1984, 1987). A serious limitation of this observation is that repeat biopsies were not obtained from the same patient, so that an appropriately controlled analysis is not possible. Additionally, analysis of liver biopsies from patients receiving other kinds of hypolipidemic drugs has not yielded any evidence for peroxisomal proliferation (Doull et al. 1999). Recognizing some limitations of using primary hepatocytes in vitro because of their tendency to lose some metabolic capabilities (Reid 1990), in cultured human hepatocytes there were no changes in the activities of peroxisomal palmitoyl-CoA oxidase and/or carnitine acetyltransferase when... 

Induction of peroxisome proliferation following treatment with DEHP is not due to the parent compound, but to DEHP metabolites. Studies with MEHP in vitro have demonstrated that the proximate peroxisome proliferators are mono(2-ethyl-5-oxohexyl) phthalate (metabolite VI) and mono(2-ethyl-5-hydroxyhexyl) phthalate, (metabolite IX) and that for 2-ethylhexanol, the proximate proliferator is 2-ethylhexanoic acid (Elcombe and Mitchell 1986 Mitchell et al. 1985a). Similar findings were observed by Maloney and Waxman (1999), who showed that MEHP (but not DEHP) activated mouse and human PPARa and PPARy, while 2-ethylhexanoic acid activated mouse and human PPARa only, and at much higher concentrations. Based on its potency to induce enzyme activities, such as the peroxisomal fatty acid (3-oxidation cycle and carnitine acetyltransferase, DEHP might be considered a relatively weak proliferator. 

Reddy JK, Moody DE, Azarnofif DL, et al. 1976. Di(2-ethylhexyl)phthalate An industrial plasticizer induces hypolipidemia and enhances hepatic catalase and carnitine acetyltransferase activities in rats and mice. Life Sci 18 941-946. 

Chase JFA, Tubbs PK. Conditions for self-catalysed inactivation of carnitine acetyltransferase-a novel form of enzyme inhibition. Biochem. J. 1969 111 225-235. 

L-carnitine Infant formulae UV-Vis 10-80 mg L-1 Bioreactor with immobilised enzyme/reaction with carnitine acetyltransferase coupled with acetyl coenzyme A and dithiobenzoate. [105]... 

The products of peroxisomal P-oxidation in animals are acetyl-CoA, NADH, and chain-shortened acyl-CoAs that are completely degraded in mitochondria. Chain-shortened acyl residues and acetyl groups are thought to leave peroxisomes as acylcamitines that can be formed from acyl-CoAs by peroxisomal carnitine octanoyltransferase and/or carnitine acetyltransferase [25,26]. The conversion of acyl-CoAs to acylcamitines regenerates CoA in peroxisomes as does the hydrolysis of acyl-CoAs by thioesterases. The recycling of cofactors in peroxisomes and the transport of substrates, cofactors, and metabolites across the peroxisomal membrane are aspects of p-oxidation that remain to be investigated. 

Carnitine acyltransferases (CATs) are a family of enzymes that transport fatty acyl-CoAs across membranes in a range of organelles so that the CoA-esters can be met -olized further. The CAT enzyme involved depends on the lengfli of the fatty acid moeity to be transported. Carnitine acetyltransferase acts with acetyl-CoA as substrate while mitochondrial carnitine palmitoyltransferase (CPT) I and CPT D, transport long-chain acyl groups into the mitochondrial matrix, where they undergo -oxidation. Peroxisomal carnitine octanoyltransferase (COT) transports medium-chain fatty acids across the membrane in peroxisomes in a similar way to CPT I in mitochondria. Peroxisomal P-oxi-dation may account for a significant proportion of all hver fatty acid oxidation and COT is involved in this process. ... 

The carnitine acyltransferases (CPT) catalyze the reversible acyl group transfer between the CoA and carnitine pools (reviewed in). Only carnitine acetyltransferase (CAT), the soluble peroxisomal COT and tbe mitochondrial inner carnitine palmitoyltransferase (CPT-II) are not regulated allosterieally. The mitochondrial outer carnitine... 

Colucci, W.J. Gandour, R.D. (1988) Bioorg. Chem. 16, 307-334. Carnitine acetyltransferase a review of its biology, enzymology, and bioorganic chemistry. 

Colucci, W.J., Gandour, R.D., Fronczek, F.R., Brady, P.S. Brady, LJ. (1987)/ Am. Chem. Soc. 109, 7915-7916. A siameso inhibitor chiral recognition of a prochiral bilaterally symmetric molecule by carnitine acetyltransferase. 

The medium-chain 3-ketoacyl-CoA thiolase and the 3-ketoacyl-CoA thiolase activity of the trifimctional protein are inhibited by acetyl-CoA. " We have also demonstrated the sensitivity of the trifunctional protein to [acetyl-CoA]/[CoA] but at a fixed [acetyl-CoA + CoASHj. This was carried out by incubating isolated trifunctional protein with 2-hexadecenoyl-CoA, NAD, CoASH, carnitine acetyltransferase and a range of concentrations of carnitine and acetyl-carnitine to keep the total [CoA] constant. Results from a typical titration are shown in Fig. 3. 

Bloisi, W., Colombo, 1., Garavaglia, B., Giardini, R., Finocchiaro, G. Didonato, S. (1990) Eiir. J. Biochem. 189, 539-546. Purification and properties of carnitine acetyltransferase from human hver. 

Searching sequence databases with PASTA turned up no other proteins homologous to CPTII except other carnitine acyl transferases such as carnitine acetyltransferase, choline acetyltransferase or carnitine octanoyltransferase. 

We crystallized carnitine acetyltransferase (studied in depth by Chase) and, by linking this to assays for CoA and acetyl-CoA, we found that the acylation state of both CoA and carnitine in rat hver depended on the dietary state the perfusion of hearts with... 

The role of carnitine in the transport of acyl chains into the mitochondria has been well studied. Two transferases, carnitine acetyltransferase (EC 2.3.1.7) and carnitine palmitoyltransferase, differing in their chain-length specificity, are involved. Both transferases act only on (—)-carnitine, the natural isomer, and catalyse freely reversible reactions. Part of the activity is tightly bound to the inner mitochondrial membrane. Carnitine has also been implicated in the oxidation of fatty acids by plant mitochondria (Thomas and McNeil, 1976) (but see remarks below). The role of carnitine in mammalian fatty acid oxidation was discussed by Greville and Tubbs (1968). 

After being transported across the inner mitochondrial membrane, acyl carnitine donates the acyl group to an endogenous CoA molecule in the mitochondrial matrix. After fatty-acid oxidation, the acetyl CoA transfers the acetyl group to carnitine in a reaction catalyzed by carnitine acetyltransferase (CAT). Acetylcar-nitine is transported out of the mitochondrial matrix and donates the acetyl group to exogenous CoA. 

BR-931 [4-chloro-6(2,3-xylidino)2-pyridinylthio-(N-P-hydroxyethyl)-acetamide], a potent inducer of Uver peroxisomes and of mitochondrial carnitine acetyltransferase, appeared to be a hypolipidemic... 

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