Development of two assay systems of desialylated hCG specific for O-serine or N-asparagine linked carbohydrate chain and their clinical application

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Two specific and sensitive assays have been developed for the measurement of as-hCG in this study. These assay systems are immunoradiometric assays which utilize peanut lectin, Arachis hypogaea (PNA) or caster bean lectin, Ricin communis agglutinin (RCA-I), to selectively extract as-hCG and then directly quantify with purified and radiolabeled antiserum against hCG beta COOH terminal peptide (beta-CTP). PNA is highly specific for the terminal carbohydrate linkage Gal beta-(1----3)-GalNAc which is only exposed in hCG when the O-serine linked oligosaccharide of its unique beta-CTP region are desialylated. RCA-I is also specific for the terminal carbohydrate beta-D-Galp., especially the linkage of Gal beta-(1----4)-GlcNAc, which is exposed in hCG when the N-asparagine linked oligosaccharide of its alpha and beta subunits are desialylated. The assays (PNA-R525 and RCA-I-R525) are capable of reliably and accurately measuring as little as 0.01 and 0.002 pmoles as-hCG/ml without interference from hCG, respectively (crossreactivity: 0.1% and 0.03%). The urine concentrates of a patient with choriocarcinoma and a normal pregnant woman were fractionated by Sephadex G-100 gel filtration and total hCG and as-hCG levels were measured. When the fractions were analysed by beta-CTP RIA, there were two peaks of immunoreactive hCG in the urine of a patient with choriocarcinoma. The bigger molecular fractions were hCG of native size, and 16% of which was desialylated. The smaller molecular fraction was beta-CTP, and practically all of that was desialylated. Whereas, beta-CTP could not be observed, and only 1.5% of hCG of native size was desialylated in the urine of a normal pregnant woman. Although the absolute levels of as-hCG in the urine specimens increased as total hCG increased, the proportion of as-hCG decreased as total hCG concentration increased. The proportion of as-hCG in urine specimens from choriocarcinoma patients was higher than specimens from patients with hydatidiform mole (including invasive mole) in both these lectin immunoradiometric assay (LIRMA) systems. Also, the proportion of as-hCG in urine specimens from molar patients was higher than those from normal pregnant women using PNA as extracting reagent. Whereas, when RCA-I was used, there was no significant difference between specimens from molar patients and those from normal pregnant women.(ABSTRACT TRUNCATED AT 400 WORDS)