Detailed Characterization of a High‐molecular‐weight Glycoprotein Secreted by Lung Cancer Cells

A cancer‐associated, high‐molecular‐weight glycoprotein antigen (6B3·Ag) recognized by monoclonal antibody 6B3 was purified from culture medium of human large cell lung carcinoma cell line (HLC‐2) and characterized biochemically and immunochemically. The 6B3·Ag was purified more than 1,200‐fold with a yield of 30% by salting out, precipitation by acidification at pH 4.5, and chromatographies on Sepharose 4B and concanavalin A‐Sepharose. The molecular weight of 6B3·Ag is approximately 1,000,000 and the molecule is a homodecamer of 94,000 subnnits. The 6B3·Ag is a glycoprotein containing 22.9% sugars, consisting of both N‐ and O‐glycoside chains. The N‐terminal 19 amino acids were determined and only 4 out of 19 amino acid residues were different from those of an antigen, L3, secreted by lung carcinoma cell line Calu‐1. The serum level of 6B3·Ag was determined in normal adults as well as patients with various diseases by enzyme‐linked immunosorbent assay. The mean serum level of 6B3‐Ag was 3.1 μg/ml, ranging from 1.6 to 6.2 μg/ml in 131 healthy adults. When the cut‐off value was set at 6.2 μg/ml, the incidence of positive values in the sera was elevated not only in malignant diseases such as hepatoma (73%) and leukemia (62%), but also in benign diseases such as chronic hepatitis (42%) and liver cirrhosis (63%). While the incidence of positive values was elevated in advanced liver diseases, namely, chronic hepatitis, liver cirrhosis and hepatoma, the cancer specificity of 6B3·Ag did not appear to be high.