Acylcarnitine transferases

Pharmacological approaches include the inhibition of release of arachidonic acid by inhibition of phospholipase A2 and the inhibition of acylcarnitine transferase I by and oxfenicine, the latter of which has been shown to prevent or at least delay ischemia-induced uncoupling. There are at present no data available on the possible effects of inhibitors of arachidonic acid release on ischemia-induced uncoupling. 

The hver produces ketone bodies by oxidizing free fatty acids to acetyl CoA, which then is converted to acetoacetate and /3-hydroxybutyrate. The initial step in fatty acid oxidation is transport of the fatty acids into the mitochondria. The essential enzyme in this process, acylcarnitine transferase, is inhibited by intramitochondrial malonyl CoA, one of the products of fatty acid synthesis. Normally, insulin inhibits Upolysis, stimulates fatty acid synthesis (thereby increasing the concentration of malonyl CoA), and decreases the hepatic concentration of carnitine these factors aU decrease the production of ketone bodies. Conversely, glucagon stimulates ketone body production by increasing fatty acid oxidation and decreasing concentrations of malonyl CoA. In patients with type 1 DM, insulin deficiency and glucagon in excess provide a hormonal milieu that favors ketogenesis and may lead to ketoacidosis. 

The octanoyl CoA that is the end-product of peroxisomal oxidation leaves the peroxisomes and the octanoyl group is transferred through the inner mitochondrial membrane by medium-chain-length acylcarnitine transferase. In the mitochondria, it enters the regular p-oxidation pathway, beginning with medium-chain-length acyl CoA dehydrogenase (MCAD). 

Because of the importance of these enzyme activities for active fatty acid oxidation, our laboratory has carried out investigations of fatty acid activation, acylcarnitine transferase activity and fatty acid oxidation itself in a variety of mammalian sepcies during development. 

The short chain activating enzyme, acetyl-CoA synthetase, showed a similar developmental increment. Palmitylcarnitine transferase activity of developing rat liver and heart homogenates increased from negligible levels at the time of birth to adult levels by thirty days of age (Fig. 4). These changes were associated with a large increase in the overall rate of fatty acid oxidation in the rat (Fig. 5). Thus, both fatty acid activation and acylcarnitine transferase activity appear to be of considerable importance for the development of fatty acid oxidation in the rat. Augenfeldt... 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

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)...

Formiminoglutamate (FIGLU), a marker for glutamate formimino-transferase deficiency, was recently also shown to be detectable by acylcarnitine analysis represented as a peak with m/z 287 (Fig. 3.2.3d) [64]. In poorly resolved acylcarnitine profiles, this peak may be confused with iso-/butyrylcarnitine (m/z 288). To avoid the incorrect interpretation of acylcarnitine profiles, we recommend performing the analysis in product scan mode as opposed to multiple reaction monitoring (MRM) mode. For example, the FIGLU peak at m/z 287 would not have been correctly identified in MRM mode because the transition of 287 to 85 is typically not selected. However, the 288/85 transition would reveal abnormal results, but in fact not represent either butyryl- or isobutyrylcarnitine, but another FIGLU related ion species. 

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. 

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. 

Within the peroxisome, the acetyl groups can be transferred from CoA to carnitine by an acetylcarnitine transferase, or they can enter the cytosol. A similar reaction converts medium-chain-length acyl CoAs and the short-chain butyryl CoA to acyl carnitine derivatives. These acylcarnitines diffuse from the peroxisome to the mitochondria, pass through the outer mitochondrial membrane, and are transported through the inner mitochondrial membrane via the carnitine translocase system. 

Hug, G Soukup, S. Berry, H. Bove, K.E. (1989). Pediatr. Res. 25, 115A (Abstract) Carnitine palmitoyl transferase (CPT) deficieney of CPT II but not of CPT I with reduced total and free carnitine but increased acylcarnitine. 

The mechanism whereby erucic acid inhibits oxidation rates of fatty acids in heart mitochondria has received considerable attention. Erucic acid, as its carnitine ester, may be slow in transferring across the inner mitochondrial membrane via carnitine acyl transferase to reach the mitochondrial matrix space where p-oxidation takes place. This would impair erucic acid oxidation. Evidence for delayed erucylcarnitine entry into heart mitochondria has been presented (Christophersen and Bremer, 1972). What is not explained is how erucylcarnitine inhibits the oxidation of other acylcarnitine esters such as palmitylcarnitine. Erucylcarnitine appears to Inhibit palmitylcarnitine oxidation in heart mitochondria while at the same time depressing the overall... 

Succinyl-CoA 3-oxoacid-CoA-transferase deficiency (SCOT, 14.13) was already described in the early seventies. More than 10 patients are known now they all had multiple episodes of severe ketoacidosis, similar to the patients with jff-ketothiolase deficiency (see Sect. 7.4). Whenever there is hypoglycemia, this is denoted hyperketotic. Urine organic acids and plasma acylcarnitines are non-informative. Various mutations have been reported [17]. Growth and development of the patients was reported to be normal. 

Acylcarnitines are essential compounds for the metabolism of fatty acids and represent intermediates of mitochondrial fatty acid p-oxidation. In this process, fatty acids are first activated to form acyl CoAs in the cytosol of cells (see Section 11.3), then the acyl moieties are transferred to carnitine by carnitine palmitoyl transferase I (CPT-I), which is located at the outer mitochondrial membrane. The formed acylcarnitines are largely and selectively transported into the mitochondria for fatty acid p-oxidation to generate ATP through coordinating activities of CPT-I and CPT-II. The latter is located at the inner mitochondrial membrane and converts acylcarnitines back to acyl CoAs. 

McGarry et al. (1971) showed that, unlike oleate, octanoate was oxidized to ketone bodies at similar rates in livers from fasted and diabetic rats. Furthermore, studies with isolated mitochondria are consistent with a limitation of fatty acid oxidation at the transferase step (Fritz and Marquis, 1965). Carnitine acyltransferase I is probably as important as transferase II. The physiolgocial significance of the transferase step becomes more apparent when one considers that the majority of the free fatty acids circulating in the plasma during starvation are of the long-chain variety. Their oxidation to CO2 or to ketone bodies within the mitochondria ultimately depends on passage of the acylcarnitine across the inner mitochondrial membrane. 

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