Differential substrate specificities of human ABCD1 and ABCD2 in peroxisomal fatty acid β-oxidation

The gene mutated in X-linked adrenoleukodystrophy (X-ALD) codes for the HsABCD1 protein, also named ALDP, which is a member of the superfamily of ATP-binding cassette (ABC) transporters and required for fatty acid transport across the peroxisomal membrane. Although a defective HsABCD1 results in the accumulation of very long-chain fatty acids in plasma of X-ALD patients, there is still no direct biochemical evidence that HsABCD1 actually transports very long-chain fatty acids. We used the yeast Saccharomyces cerevisiae to study the transport of fatty acids across the peroxisomal membrane. Our earlier work showed that in yeast the uptake of fatty acids into peroxisomes may occur via two routes, either as (1.) free fatty acid or as (2.) acyl-CoA ester. The latter route involves the two peroxisomal half-ABC transporters, Pxa1p and Pxa2p, which form a heterodimeric complex in the peroxisomal membrane. We here report that the phenotype of the pxa1/pxa2Δ yeast mutant, i.e. impaired growth on oleate containing medium and deficient oxidation of oleic acid, cannot only be partially rescued by human ABCD1, but also by human ABCD2 (ALDRP), which indicates that HsABCD1 and HsABCD2 can both function as homodimers. Fatty acid oxidation studies in the pxa1/pxa2Δ mutant transformed with either HsABCD1 or HsABCD2 revealed clear differences suggesting that HsABCD1 and HsABCD2 have distinct substrate specificities. Indeed, full rescue of beta-oxidation activity in cells expressing human ABCD2 was observed with C22:0 and different unsaturated very long-chain fatty acids including C24:6 and especially C22:6 whereas in cells expressing HsABCD1 rescue of beta-oxidation activity was best with C24:0 and C26:0 as substrates.