A low molecular weight binding protein for organic anions (Z protein) from human hepatic cytosol: purification and quantitation.

Human Z protein from liver was purified to homogeneity. The protein has a molecular weight of 11,000 an an isoelectric point of pH 5.8. Circular dichroism spectra of Z protein-bilirubin (unconjugated and diglucuronide) complexes revealed two ellipticity extrema, a negative peak at 460 nm, and a positive peak at 410 nm. Human serum albumin had a higher affinity for bilirubin than did Z protein. Fluorescence studies showed the approximate association constants of this protein and bilirubin, bromosulfophthalein, and indocyanine green were 10(6) M-1, 10(5) M-1, and 10(6) M-1, respectively. Immunofluorescence studies revealed that Z protein was localized in the cytoplasm of hepatocytes, proximal tubular epithelium, and epithelial cells of the small intestine. Radioimmunoassay studies were done to assess the amount of Z protein in controls and in various liver diseases. The highest concentrations of Z protein were found in the liver, kidney, heart muscle, and small intestine, in that order. In cases of acute and chronic hepatitis, hepatic Z protein concentrations were generally decreased, whereas serum Z protein concentrations were increased. In contrast, both serum and hepatic Z protein concentrations were decreased in cases of constitutional hyperbilirubinemia.