Isolation and characterization of the three fractions (DE‐I, DE‐II and DE‐III) of rat‐liver Z‐protein and the complete primary structure of DE‐II

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Abstract
Three fractions (DE-I, DE-II and DE-III) of Z-protein (fatty acid binding protein) were isolated from rat liver cytosol by DEAE-cellulose chromatography and characterized. They had the same MW of 14,000 and essentially identical amino acid composition. However, compositions of endogenous fatty acids differed strikingly from one another. Long-chain fatty acids detected in DE-II were palmitic, stearic, oleic, linoleic and arachidonic acids. In contrast to DE-II, DE-II contained mainly arachidonic acid. Molar ratios of endogenous long-chain fatty acids to both DE-II and DE-III were estimated to be around 1.0. Unlike the latter 2 fractions, DE-I was virtually lipid-free. Analyses of the 3 fractions by polyacrylamide gel electrophoresis, electrofocusing and DEAE-cellulose chromatography before and after delipidation suggested that the difference between DE-I and DE-II was in part due to fatty acids bound to DE-II. In contrast, DE-III appeared to be somewhat different from these forms in its protein structure, though tryptic peptide mappings of the 3 fractions did not reveal clear differences among them. Analysis of the primary structure was made on the most abundant fraction, DE-II, to investigate the relationship among the 3 fractions and to other proteins. The protein was a single chain consisting of 127 amino acid residues and had a mostly acetylated NH2 terminus and a free SH group. The complete sequence of Z-protein showed striking homology to cellular retinoid binding proteins and peripheral nerve myelin P2 protein, which indicated the presence of a new family of cellular lipid-binding proteins diverged from a common ancestor. A possible intragenic duplication of Z-protein was also suggested.