Acyl carnitine excretion

Carnitine deficiency complicates HMG-CoA lyase deficiency and other inborn errors of metabolism, which results in organic acidemia. L-Camitine or P-hydroxy-y-trimethylammonium butyrate is a carrier molecule that transports long-chain fatty acids across the inner mitochondrial membrane for subsequent P-oxi-dation. L-Carnitine also facilitates removal of toxic metabolic intermediates or xenobiotics via urinary excretion of their acyl carnitine derivatives. Indeed, individuals with HMG-CoA lyase deficiency have been shown to excrete 3-methylgluatarylcamitine (Roe et al., 1986). In the absence of ketogenesis, the formation of the acyl carnitine derivative of 3-hydroxy-3-methylglutarate from HMG-CoA also serves to regenerate free CoA in the mitochondria and permits continued P-oxidation of fatty acids. 

As a result of the reduced activity of the mutase in vitamin B12 deficiency, there is an accumulation of methyhnalonyl CoA, some of which is hydrolyzed to yield methylmalonic acid, which is excreted in the urine. As discussed in Section 10.10.3, this can be exploited as a means of assessing vitamin B12 nutritional status. There may also be some general metabolic acidosis, which has been attributed to depletion of CoA because of the accumulation of methyl-malonyl CoA. However, vitamin B12 deficiency seems to result in increased synthesis of CoA to maintain normal pools of metabolically useable coenzyme. Unlike coenzyme A and acetyl CoA, neither methylmalonyl CoA nor propionyl CoA (which also accumulates in vitamin B12 deficiency) inhibits pantothenate kinase (Section 12.2.1). Thus, as CoA is sequestered in these metabolic intermediates, there is relief of feedback inhibition of its de novo synthesis. At the same time, CoA may be spared by the formation of short-chain fatty acyl carnitine derivatives (Section 14.1.1), which are excreted in increased amounts in vitamin B12 deficiency. In vitamin Bi2-deficient rats, the urinary excretion of acyl carnitine increases from 10 to 11 nmol per day to 120nmolper day (Brass etal., 1990). 

The major functions of pantothenic acid are in CoA (Section 12.2.1) and as the prosthetic group for AGP in fatty acid synthesis (Section 12.2.3). In addition to its role in fatty acid oxidation, CoA is the major carrier of acyl groups for a wide variety of acyl transfer reactions. It is noteworthy that a wide variety of metabolic diseases in which there is defective metabolism of an acyl CoA derivative (e.g., the biotin-dependent carboxylase deficiencies Sections 11.2.2.1 and 11.2.3.1), CoA is spared by formation and excretion of acyl carnitine derivatives, possibly to such an extent that the capacity to synthesize carnitine is exceeded, resulting in functional carnitine deficiency (Section 14.1.2). 

The total body content of carnitine is about 100 mmol, and about 5% of this turns over daily. Plasma total carnitine is between 36 to 83 /rmol per L in men and 28 to 75 /rmol per L in women, mainly as free carnitine. Although both free carnitine and acyl carnitine esters are excreted in the urine, much is oxidized to trimethylamine and trimethylamine oxide. It is not known whether the formation of trimethylamine and trimethylamine oxide is caused by endogenous enzymes or intestinal bacterial metabolism of carnitine. 

Total urinary excretion of carnitine is between 300 to 530 /rmol (men) or 200 to 320 /rmol (women) 30% to 50% of this is free carnitine the remainder is a variety of acyl carnitine esters. Acyl carnitine esters are readily cleared in... 

Urinary excretion of acyl carnitine esters increases considerably in a variety of conditions involving organic aciduria carnitine acts to spare CoA and pantothenic acid (Section 12.2), by releasing the coenzyme from otherwise nonmetabolizable esters that would trap the coenzyme and cause functional pantothenic acid deficiency. 

One of the most frequent defects of fatty acid oxidation is deficiency of a mitochondrial acyl-CoA dehydrogenase.50 If the long-chain-specific enzyme is lacking, the rate of P oxidation of such substrates as octanoate is much less than normal and afflicted individuals excrete in their urine hexanedioic (adipic), octanedioic, and decanedioic acids, all products of co oxidation.54 Much more common is the lack of the mitochondrial medium-chain acyl-CoA dehydrogenase. Again, dicarboxylic acids, which are presumably generated by 0) oxidation in the peroxisomes, are present in blood and urine. Patients must avoid fasting and may benefit from extra carnitine. 

Mele B, Kemer J, Jaszai V, Bieber LL. Differential excretion of xenobiotic acyl-esters of carnitine due to administration of pivampicillin and valproate. Biochem Med Metab Biol (1990) 43, 30-8. 

Several diseases are known that result in elevations in tissues and fluids of various esters of carnitine and reduce the availability of free carnitine, which is normally synthesized by humans and is necessary for the transport of long-chain fatty acids into mitochondria for oxidation. In several disorders arising from acyl-CoA dehydrogenase deficiencies, the accumulation of the acyl-CoA substrate frequently sequesters coenzyme A and reduces its availability for other unrelated but important and otherwise competent pathways. Carnitine administration can displace and make available much of the coenzyme A that had been isolated, and stimulate the excretion of the accumulating acidic metabolites now esterified to carnitine. Detection of reduced levels of serum or urinary free carnitine and elevations of esterified carnitine is therefore useful for diagnosis of a variety of metabolic disorders, among them congenital inability to synthesize carnitine. In this disorder, carnitine must be supplied by a carnitine-enriched diet as it is, in effect, a vitamin. 

Carnitine and its esters (see [1]) cannot be introduced to the mass spectrometer by gas chromatography, as they incorporate quaternary amine functions and will decompose in the attempt. Fast atom bombardment (FAB) and electrospray ionization (ESI) can use the formal charge on the quaternary amine function to advantage, as carnitine and its esters are very easily desorbed from glycerol on the FAB probe and from aerosol sprays in ESI. Eigure 7A illustrates the use of EAB in the quantitation of carnitine and its esters excreted in the urine of a patient presenting with a severe dicarboxylic aciduria associated with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. 

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