Substrate specificity of rat liver mitochondrial carnitine palmitoyl transferase I: evidence against α‐oxidation of phytanic acid in rat liver mitochondria

Abstract

The two branched chain fatty acids pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) were converted to coenzyme A thioesters by rat liver mitochondrial outer membranes. However, these branched chain fatty acids could not be converted to pristanoyl and phytanoyl carnitines, respectively, by mitochondrial outer membranes. As expected, the unbranched long chain fatty acids, stearic acid and palmitic acid, were rapidly converted to stearoyl and palmitoyl carnitines, respectively, by mitochondrial outer membranes. These observations indicate that the branched chain fatty acids could not be transported into mitochondria. The data presented strongly suggest that in rat liver, c~-oxidation of phytanic acid occurs in organelles other than mitochondria.demonstrated that CPTI activity is reversibly inhibited by malonyl-CoA. Recently, it has been shown that malonyl-CoA inhibits peroxisomal, microsomal and plasma membrane carnitine acyl transferases [8,10,13]. Therefore, in order to understand the role of CPTI in fatty acid oxidation, CPTI must be separated from other carnitine acyl transferases. In view of the difficulties in isolating CPTI in the active state, we decided to investigate CPTI activity in highly purified mitochondrial outer membranes from rat liver. We present evidence that branched chain fatty acids with ~-or fl-methyl groups are poor substrates for CPTI, suggesting that these branched chain fatty acids cannot be transported into mitochondria via the carnitine/ acylcarnitine transport system.

Defective peroxisomal catabolism of branched fatty acyl coenzyme A in mice lacking the sterol carrier protein-2/sterol carrier protein-x gene function

“…Catabolism of phytanic acid proceeds by way of ␣-oxidation, yielding the (n − 1) lower homolog pristanic acid (C19:0), which is further catabolized by way of ␤-oxidation in peroxisomes (Singh et al 1994;Singh and Poulos 1995;Steinberg 1995). To discriminate between defective phytanoyl-CoA ␣-oxidation and pristanoyl-CoA ␤-oxidation, we then quantified pristanic acid.…”

“…Finally, 3-ketopristanoyl-CoA is substrate for thiolytic cleavage, catalyzed by a 3-ketopristanoyl-CoA thiolase, which yields the (n-3) lower homolog of pristanoyl-CoA (4,8,12-trimethyltridecanoyl-CoA) and propionyl-CoA (for review, see Steinberg 1995). Although studies on mice are not available, it now appears that this pathway operates in a similar way in rats and humans (Watkins et al 1994;Singh and Poulos 1995).…”

Acylcarnitines: Nomenclature, Biomarkers, Therapeutic Potential, Drug Targets, and Clinical Trials

“…The metabolism of phytanic and pristanic acid occurs mainly in the liver and kidney tissue peroxisomes where peroxisomal a-oxidation of phytanoyl-CoA yields pristanoyl-CoA that can be further b-oxidized (Wierzbicki, 2007). It is hypothesized that the synthesis of these long-chain acylcarnitines is catalyzed by peroxisomal CrOT (Herzog et al, 2017), as phytanic and pristanic acids are poor substrates for carnitine palmitoyltransferase 1 (CPT1, EC:2.3.1.21) (Singh and Poulos, 1995).…”

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

“…Intracellular levels of free phytanic acid are normally kept low by binding to fatty acid binding protein (FABP) and SCP2, both abundantly present (0.2-0.4 mM) ( 93 ), and a large portion will be converted into phytanoylCoA. Effects of phytanoyl-CoA on intra-mitochondrial processes ( 94 ) are questionable, because the CoA-ester is not recognized by the carnitine-palmitoyl transferase/translocation machinery ( 95 ).…”