ABCD
The ABCD subfamily comprises four human genes, and ABCD1, ABCD2, ABCD3 encode proteins located in the peroxisome. The ABCD proteins are all half-transporters and, consequently, function as homo- or heterodimers, and two of these (ABCD1, ABCD2) take part in the regulation of very-long-chain fatty acid transport (Dean, 2002).
The mammalian peroxisomal ABC transporters are the adrenoleukodystrophy protein (ALDP/ABCD1), ALDP-related protein (ALDRP/ABCD2), and a 70-kDa peroxisomal membrane protein (PMP70/ABCD3). X-linked adrenoleukodystrophy (X-ALD) is associated with variants in ABCD1 (Mosser et al., 1993) . X-ALD is characterized by the accumulation of very long-chain fatty acids in the peroxisome that is an outcome of impaired beta-oxidation. Patients with X-ALD display progressive demyelination of the neurons in the central nervous system, testicular malfunction, and adrenal insufficiency (Smith et al., 1999). ABCD2 may be involved in the metabolic transport of VLCFA’s (Morita & Imanaka, 2012). The function of ABCD3 consists of the transportation of branched-chain acyl-CoA into peroxisomes. ABCD4 resides in the mitochondria and the lysosomes. In the lysosomes, ABCD4 is involved in the transport of vitamin B12 from lysosomes into the cytosol. Mutations in ABCD4 cause an inborn error of vitamin B12 metabolism, resulting in the lysosomes failing to release cobalamin resulting in symptoms mimicking cobalamin deficiency (Kawaguchi & Morita, 2016).