Identification, Isolation, and Characterization of a 32-kDa Fatty Acid-Binding Protein Missing from Lymphocytes in Humans with Bietti Crystalline Dystrophy (BCD)

Abstract

Bietti crystalline dystrophy (BCD) is an autosomal recessive retinal degeneration characterized by intraretinal lipid inclusions with degeneration of the retina and sclerosis of the choroidal vessels, resulting clinically in progressive night blindness and constriction of the visual fields. Characterization of fatty acid metabolism in Bietti crystalline dystrophy suggested that BCD might result from abnormalities in lipid-binding proteins or one or more enzymes active in fatty acid elongation and desaturation. To further investigate the first possibility, the docosahexaenoic acid-binding proteins (DHABPs) of human lymphocytes from patients with Bietti crystalline dystrophy were studied and compared with those of normal controls. For fatty acid-binding protein (FABP) identification, lymphocyte cytosol was first subjected to Lipidex 1000 chromatography. FABPs were then cross-linked with [14C]22:6n-3 and identified by HPLC and SDS-PAGE. Ten major peaks corresponding to calculated molecular weights of 13, 14, 32, 43, 45, 50, 64, 96, 105, and 186 kDa exhibit high-affinity binding of fatty acids. Significantly, peaks corresponding to two fatty acid-binding proteins of 32 and 45 kDa present in age-matched controls are absent from lymphocytes of patients with BCD. The 32-kDa fatty acid-binding protein present in normal individuals but absent from patients with BCD was isolated from cultured control human lymphocytes, its fatty acid-binding properties were characterized, and its amino acid composition was analyzed. It shows specific binding of 3n-3 fatty acids, consistent with the pattern of abnormalities of lipid metabolism demonstrated in patients with BCD. These results suggest that the 32- and 43-kDa FABPs are reasonable candidates for causing BCD.