Carnitine palmitoyltransferase I activity monitoring in fibroblasts and leukocytes using electrospray ionization mass spectrometry

An improved enzyme assay for carnitine palmitoyl transferase I in fibroblasts using tandem mass spectrometry

Carnitine palmitoyl transferase I (CPTI), which converts acyl-CoA and carnitine into acyl-carnitine and free CoASH, is the rate limiting enzyme of hepatic mitochondrial beta-oxidation. CPTI-deficiency is a severe disorder characterized by Reye-like attacks with hypoketotic hypoglycemia, hepatomegaly, elevated liver enzymes and hyperammonemia. We developed a simple tandem-MS-based assay to measure CPTI activity in human fibroblasts. Surprisingly, a large part of the palmitoyl-carnitine formed in our assay by CPTI was degraded into C14- to C2-acyl-carnitines. Degradation of the product of CPTI leads to under estimation of the CPTI activity. When we used potassium cyanide to inhibit enzymes downstream of CPTI and thereby degradation of the product, we measured four times more CPTI activity than the previous methods. This inhibition is essential for correct calculation of CPTI activity. In fibroblasts of CPTI-deficient patients, CPTI activity was not detectable and this assay can be used for the diagnosis of CPTI-deficiency.

Tandem mass spectrometric assay for the determination of carnitine palmitoyltransferase II activity in muscle tissue

Carnitine palmitoyltransferase II (CPT-II) mediates the import of long-chain fatty acids into the mitochondrial matrix for subsequent beta-oxidation. Defects of CPT-II manifest as a severe neonatal hepatocardiomuscular form or as a mild muscular phenotype in early infancy or adolescence. CPT-II deficiency is diagnosed by the determination of enzyme activity in tissues involving the time-dependent conversion of radiolabeled CPT-II substrates (isotope-exchange assays) or the formation of chromogenic reaction products. We have established a mass spectrometric assay (MS/MS) for the determination of CPT-II activity based on the stoichiometric formation of acetylcarnitine in a coupled reaction system. In this single-tube reaction system palmitoylcarnitine is converted by CPT-II to free carnitine, which is subsequently esterified to acetylcarnitine by carnitine acetyltransferase. The formation of acetylcarnitine directly correlates with the CPT-II activity. Comparison of the MS/MS method (y) with our routine spectrophotometric assay (x) revealed a linear regression of y = 0.58x + 0.12 (r = 0.8369). Both assays allow one to unambiguously detect patients with the muscular form of CPT-II deficiency. However, the higher specificity and sensitivity as well as the avoidance of the drawbacks inherent in the use of radiolabeled substrates make this mass spectrometric method most suitable for the determination of CPT-II activity.

The utilization of alanine, glutamic acid, and serine as amino acid substrates for glycine N-acyltransferase

The conjugation of benzoyl-CoA with the aliphatic and acidic amino acids by glycine N-acyltransferase, as well as the amides of the latter group, was investigated. Bovine and human liver benzoyl-amino acid conjugation were investigated using electrospray ionization tandem mass spectrometry (ESI-MS-MS). Bovine glycine N-acyltransferase catalyzed conjugation of benzoyl-CoA with Gly (Km(Gly) = 6.2 mM), Asn (Km(Asn) = 129 mM), Gln (Km(Gln) = 353 mM), Ala (Km(Ala) = 1573 mM), Glu (Km(Glu) = 1148 mM) as well as Ser in a sequential mechanism. In the case of the human form, conjugation with Gly (Km(Gly) = 6.4 mM), Ala (Km(Ala) = 997 mM), and Glu was detected. The presence of these alternative conjugates did not inhibit bovine glycine N-acyltransferase activity significantly. Considering the relatively low levels at which these conjugates are formed, it is unlikely that they will have a significant contribution to acyl-amino acid conjugation under normal conditions in vivo. However, their cumulative contribution to acyl-amino acid conjugation under metabolic disease states may prove to have a useful contribution to detoxification of elevated acyl-CoAs.