Inborn errors of P-oxidation

One of the functions of hepatic P-oxidation is to provide ketone bodies, acetoac-etate and p-hydroxybutyrate, to the peripheral circulation. These can then be utilized by peripheral tissues such as brain and heart. Beta-oxidation itself produces acetyl-CoA which then has three possible fates entry to the Krebs cycle via citrate S5mthase keto-genesis or transesterification to acetyl-carnitine by the action of carnitine acetyltrans-ferase (CAT). Intramitochondrial acetyl-carnitine then equilibrates with plasma via the carnitine acyl-camitine translocase and presumably via the plasma membrane carnitine transporter. Human studies have shown that acetyl-carnitine may provide up to 5% of the circulating carbon product from fatty acids and can be taker and utilized by muscle and possibly brain." In addition, acyl-camitines are of important with regard to the diagnosis of inborn errors of P- oxidation. For these reasons, we wished to examine the production of acetyl-carnitine and other acyl-camitine esters by neonatal rat hepatocytes. 

The metabolism of phenylalanine will now be considered in some detail, as two inborn errors of metabolism are known that affect this pathway. Phenylalanine is first hydroxylated by phenylalanine hydroxylase to form another aromatic amino acid tyrosine (Fig. 8). The coenzyme for this reaction is the reductant tetrahydrobiopterin which is oxidized to dihydrobiopterin. Phenylalanine hydroxylase is classified as a monooxygenase as one of the atoms of 02 appears in the product and the other in HzO. The tyrosine is then trans-aminated to p-hydroxyphenylpyruvate, which is in turn converted into homogentisate by p-hydroxyphenylpyruvate hydroxylase. This hydroxylase is an example of a dioxygenase, as both atoms of 02 become incorporated into the product (Fig. 8). The homogentisate is then cleaved by homogentisate oxidase, another dioxygenase, before fumarate and acetoacetate are produced... 

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. 

The known human inborn errors affecting auxiliary enzymes of P-oxidation consists of one identified case of 2,4-dienoyl-CoA reductase deficiency and a possible case of peroxisomal MFE-1 deficiency. Once the auxiliary enzymes required for flie metabolism on unsaturated fatty acids have been identified and characterized also at molecular level, it may be possible to identify more inborn errors belonging to this cat ory. 

Bennett MJ, Rinaldo P, and Strauss AW (2000) Inborn errors of mitochondrial fatty acid oxidation. Critical Reviews in Clinical Laboratory Sciences 37 1—44. 

Altered cellular levels of riboflavin and derived flavins impacts on flavo-protein function. This is particularly relevant in the context of inborn errors of metabolism affecting p-oxidation and amino acid catabolism enzymes which are frequently a result of missense mutations and result in protein misfolding or catalytic impairment. 

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