The clinical distinction between patients with a disorder of peroxisome assembly (e.g., Zellweger syndrome) and those with a defect in a peroxisomal fatty acid β-oxidation enzyme can be difficult. We studied 29 patients suspected of belonging to the latter group. Using complementation analysis, 24 were found to be deficient in enoylcoenzyme A hydratase/3- hydroxyacylcoenzyme A dehydrogenase bifunctional enzyme and 5 were deficient in acyl-CoA oxidase. Elevated plasma very long-chain fatty acids (VLCFA), impaired fibroblast VLCFA β-oxidation, decreased fibroblast phytanic acid oxidation, normal plasmalogen synthesis, normal plasma L-pipecolic acid level, and normal subcellular catalase distribution were characteristic findings in both disorders. The elevation in plasma VLCFA levels and impairment in fibroblast VLCFA β-oxidation were more severe in bifunctional- deficient than in oxidase-deficient patients. The clinical course in bifunctional deficiency (profound hypotonia, neonatal seizures, dysmorphic features, age at death ~9 months) was more severe than in oxidase deficiency (moderate hypotonia without dysmorphic features, development of a leukodystrophy, age at death ~4 yr). Based on these findings, accurate early diagnosis of these deficiencies of peroxisomal β-oxidation enzymes is possible.
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