A B S T R A C T High levels of a novel vitamin B12-binding protein (hepatoma B112 BP) have been observed recently in plasma obtained from three adolescent patients with hepatocellular carcinoma. This protein has now been isolated in homogeneous form from the plasma and pleural fluid of two of these patients by the use of affinity chromatography with vitamin B12-Sepharose. The hepatoma Bu BP belongs to the R-type group of B12-binding proteins and is essentially indistinguishable from the recently isolated human milk and saliva R-type proteins in terms of: (a) immunologic properties based on immunodiffusion and immunoprecipitation assays; (b) amino acid composition; (c) molecular weight based on amino acid and carbohydrate content; and (d) absorption spectra. Both hepatoma B22 BPs contain more sialic acid and less fucose than the milk and saliva B12 BPs. All four proteins contain similar amounts of galactose, mannose, galactosamine, and glucosamine. Differences in sialic acid content appear to account for the differences in electrophoretic mobility that were observed among the four proteins. Differences in total carbohydrate content appear to account for the differences in apparent molecular weight that were observed with both gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis.Tumor tissue from one of the patients contained 10 times as much R-type protein as did normal liver tissue from the same patient. This suggests, although it does not prove, that synthesis by the tumor is the Received for publication 30 December 1974 and in revised form 8 July 1975. cause of the high levels of R-type protein found in the plasma of certain patients with hepatocellular carcinoma. Plasma survival studies performed with rabbits indicate that the hepatoma B,2 BP has a prolonged plasma survival and suggests that this parameter is also of importance.
INTRODUCTIONA recent report (1) described three adolescents who presented with hepatocellular carcinomas, normal leukocyte counts, extraordinary elevations of serum vitamin B, (B,2),' (15-53 ng/ml, normal 0.3-0.9 ng/ml), and serum unsaturated B12-binding capacity, (5-480 ng/ml, normal 0.8-1.4 ng/ml). Studies (2) using sera from two of these patients indicate that the elevations of serum B12 and unsaturated B,2-binding capacity are due to the presence of a B,2-binding protein (B12 BP) that belongs to the R-type' class of immunologically related Bl2 BPs that are normally present in a number of human tissues and body fluids (3). These studies also demonstrated that the hepatoma-related B12 BP differs from the R-type B,2 BP found in increased
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