A method for quantitative acylcarnitine profiling in human skin fibroblasts using unlabelled palmitic acid: diagnosis of fatty acid oxidation disorders and differentiation between biochemical phenotypes of MCAD deficiency

Optimized spectrophotometric assay for the completely activated pyruvate dehydrogenase complex in fibroblasts

BACKGROUND

Analysis of the pyruvate dehydrogenase complex (PDHc) activity in human skin fibroblasts is hampered by low enzyme activity in the cells. The most commonly used radiochemical method detects the formation of (14)CO(2), an endproduct of the E1 component of PDHc, from [1-(14)C]pyruvate.

METHODS

We report a spectrophotometric method for the analysis of PDHc activity in fibroblasts based on detection of NADH formation via a p-iodonitrotetrazolium violet (INT)-coupled system. We investigated in detail the specific requirements of this assay, such as cofactor requirements and the effects of suggested stimulatory compounds and different cell disruption procedures. The reliability of the optimized assay was studied by investigation of patients previously diagnosed with PDHc deficiency and by comparison with results from the radiochemical method.

RESULTS

Mean (SD) total PDHc activities were 136 (31) and 58 (21) mU/U of citrate synthase in fibroblast homogenates from 10 healthy volunteers and 7 PDHc-deficient patients, respectively, by the spectrophotometric assay. Similar results were obtained in a mitochondrial fraction. Dithiothreitol (DTT) increased the nonspecific inhibitor-insensitive rate with less pronounced effect on the specific rate of PDHc activity. Administration of DTT increased PDHc activity to 193 (3)% of control activity (without DTT), but decreased the inhibitor-sensitive rate from 99 (0.3)% (without DTT) to 69 (2)% (with 0.3 mmol/L DTT).

CONCLUSION

The simple, optimized spectrophotometric assay for PDHc analysis allows reliable investigation of the enzyme complex in human skin fibroblasts.

Blood acylcarnitine levels in normal newborns and heterozygotes for medium-chain acyl-CoA dehydrogenase deficiency: a relationship between genotype and biochemical phenotype?

Patients with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency are unable to metabolize medium-chain fatty acids. Affected patients display a characteristic acylcarnitine profile when blood spots are collected after birth and analysed by tandem mass spectrometry. To determine the potential risk of metabolic decompensation in newborns with elevations of diagnostic metabolites (octanoylcarnitine>0.3, but <1 micromol/L), we investigated the relationship between octanoylcarnitine (C8) concentration in neonatal blood spots and the 985A>G MCAD genotype. Octanoylcarnitine values from 7140 newborns' blood spots were sorted. The highest C8 was approximately 0.7 micromol/L, which is below the range in classical MCAD deficiency. Samples with C8 levels above 0.25 micromol/L (group C) represented 1.4% of the total. Values between 0.05 and 0.25 micromol/L (group B) made up 87.8% of the total; 10.8% of the samples had C8 values less than 0.05 micromol/L (group A). One hundred samples from each group were selected at random and genomic DNA was amplified by PCR and analysed for the presence of the 985A>G mutation. The analysed samples from groups A and B were all homozygous normal. The 100 samples from group C contained 26 samples that were heterozygous for the 985A>G mutation. These findings indicated that the frequency distribution of heterozygotes is not random within this population. Group C was further divided into C1, the 26 heterozygotes, and C2, the remaining 74 newborns in group C. In group C1 only 2 (8%) were in the 'high-risk' group characterized by either low birth weight or requiring admission to the neonatal intensive care unit. In contrast, 28 (38%) from C2 had low birth weight or were in the neonatal intensive care unit. In our dataset, C8/C2 and C8/C12 ratios were also significantly elevated in both groups C1 and C2 compared to controls (group B). In contrast to what others have reported, the ratio of C8/C10 did not differentiate the group B controls from heterozygotes or other patients in metabolic distress (group C2), but were lower than those seen in classic MCAD or mild MCAD deficiency.

Use of tandem mass spectrometry for multianalyte screening of dried blood specimens from newborns

BACKGROUND

Over the past decade laboratories that test for metabolic disorders have introduced tandem mass spectrometry (MS/MS), which is more sensitive, specific, reliable, and comprehensive than traditional assays, into their newborn-screening programs. MS/MS is rapidly replacing these one-analysis, one-metabolite, one-disease classic screening techniques with a one-analysis, many-metabolites, many-diseases approach that also facilitates the ability to add new disorders to existing newborn-screening panels.

METHODS

During the past few years experts have authored many valuable articles describing various approaches to newborn metabolic screening by MS/MS. We attempted to document key developments in the introduction and validation of MS/MS screening for metabolic disorders. Our approach used the perspective of the metabolite and which diseases may be present from its detection rather than a more traditional approach of describing a disease and noting which metabolites are increased when it is present.

CONTENT

This review cites important historical developments in the introduction and validation of MS/MS screening for metabolic disorders. It also offers a basic technical understanding of MS/MS as it is applied to multianalyte metabolic screening and explains why MS/MS is well suited for analysis of amino acids and acylcarnitines in dried filter-paper blood specimens. It also describes amino acids and acylcarnitines as they are detected and measured by MS/MS and their significance to the identification of specific amino acid, fatty acid, and organic acid disorders.

CONCLUSIONS

Multianalyte technologies such as MS/MS are suitable for newborn screening and other mass screening programs because they improve the detection of many diseases in the current screening panel while enabling expansion to disorders that are now recognized as important and need to be identified in pediatric medicine.

A comparison of in vitro acylcarnitine profiling methods for the diagnosis of classical and variant short chain acyl-CoA dehydrogenase deficiency

BACKGROUND

Homozygosity and compound heterozygosity for the short chain acyl-CoA dehydrogenase (SCAD) gene sequence variants 625G-->A and 511C-->T are associated with ethylmalonic aciduria (EMA), a biochemical indicator of SCAD deficiency. The clinical and biochemical implications of these variants are not fully understood. The effect of these variants on the accumulation of butyrylcarnitine by fibroblasts in culture was studied.

METHODS

In vitro acylcarnitine profiling in fibroblasts was carried out using [U-13C]-labeled or unlabeled palmitate in the presence of excess L-carnitine, with or without a medium chain acyl-CoA dehydrogenase (MCAD) inhibitor. Acylcarnitines were analyzed using tandem mass spectrometry. 625G/625G (wild type), 625G/625A and 625A/625A (variant) control fibroblasts were compared with fibroblasts from patients homozygous for inactivating SCAD mutations (SCAD deficient) and from patients with EMA who were homozygous or compound heterozygous for the SCAD variants.

RESULTS

Variant control and patient fibroblasts accumulated moderate amounts of butyrylcarnitine compared with wild-type controls and in contrast to the significant amount of butyrylcarnitine accumulated by SCAD deficient fibroblasts, regardless of incubation conditions.

CONCLUSIONS

Moderately reduced SCAD activity associated with SCAD variants can be detected using in vitro acylcarnitine profiling methods, which may be used as an indirect measure of SCAD activity.