Oxidation of Fatty Acids in Cultured Fibroblasts: a Model System for the Detection and Study of Defects in Oxidation

Abstract

Summary: A number of recently described inherited disorders interfere with the oxidation of fatty acids. In these disorders at least three different metabolic steps may be affected: (1) transport of long chain fatty acids into the mitochondria as in carnitine deficiency and carnitine palmitoyl transferase deficiency (CPT); (2) multiple acyl CoA dehydrogenase deficiency or glutaric aciduria type II (GAII) due presumably to a defective common electron transfering flavoprotein or iron sulfur flavoprotein; (3) specific long or medium chain fatty acyl CoA dehydrogenase deficiency as in inherited dicarboxylic aciduria. In order to develop a system for the detection and the study of the consequences of defects such as these on the oxidation of fatty acids, we investigated the metabolism of oleate (18 carbons), octanoate (eight carbons) and butyrate (four carbons) in intact cultured fibroblasts from patients with CPT deficiency, GAII, and dicarboxylic aciduria. In CPT deficient cells there was a markedly deficient ability to oxidize [1-¹⁴C] and [U-¹⁴C] oleate (19 and 5% of normal, respectively), whereas oxidations of [1-¹⁴C] octanoate and [1,4-¹⁴C] succinate were significantly increased (150 and 222%, respectively), and [1-¹⁴C] butyrate oxidation was normal. GAII cells displayed a nearly complete defect in the oxidation of [1-¹⁴C] and [U-¹⁴C] oleate (8 and 1%, respectively), as well as of [1-¹⁴C] octanoate and [1-¹⁴C] butyrate (8 and 5% of normal, respectively). The oxidation of [1,4-¹⁴C] succinate by GAII cells was normal. Cells from a patient with dicarboxylic aciduria showed a significant reduction in [¹⁴CO₂] production from [U-¹⁴C] oleate (57%) and [1-¹⁴C] octanoate (31%) and a normal oxidation of [1-¹⁴C] oleate, [1-¹⁴C] butyrate, and [1,4-¹⁴C] succinate. These observations are consistent with available information on the normal metabolism of fatty acids in liver and muscle and also with the hypothesis about the molecular localization of the defects in GAII and inherited dicarboxylic aciduria. They demonstrate that intact cultured skin fibroblasts represent a reliable and convenient model for the investigation of fatty acid oxidation in man. Speculation: Many aspects of the human acyl CoA dehydrogenases and their physiologic functions remain unknown, among them the problem of their acyl chain length specificity. Studies in cultured fibroblasts from patients with presumed mutations affecting the metabolism of fatty acids provide a means for the elucidation of these defects and at the same time give information on normal metabolic functions. It appears likely that a number of previously unrecognized defects in this area of metabolism remain to be found. The availability of a model system for their study in cultured fibroblasts should facilitate their discovery.