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D2 is a glycoprotein enriched in neuronal membranes and probably involved in intercellular adhesion. An immunochemical relationship between D2 and the neuronal cell adhesion molecule from chick has been demonstrated. Changes in D2 concentration in human body fluids correlate to certain neurological diseases. We here report the purification of the D2 membrane proteins from fetal and adult human brain and the demonstration of physicochemical differences between the two proteins. Enrichments of 133 times (fetal D2) and 350 times (adult D2) were found. Specific rabbit antisera against the purified D2 proteins were produced, and this enabled the setting up of an enzyme-linked immunosorbent assay for D2 quantification in human brain extracts, cerebrospinal fluids, sera, and amniotic fluids.

D2 is a glycoprotein existing in both membrane-bound and soluble forms. Employing a specific rabbit antibody against purified human brain D2, we developed an enzyme-linked immunosorbent assay (ELISA) for the quantification of D2 and applied it to amniotic fluids from 87 normal and 36 pathological pregnancies. With a cut-off point of 150 ng D2/ml, no false positive D2 values were obtained in any of the amniotic fluids from normal fetuses, although the alpha-fetoprotein concentrations were slightly increased in 13 cases. No false negative D2 values were found in any of the 18 investigated amniotic fluids from fetuses with anencephaly. Of 8 amniotic fluids from fetuses with spina bifida, 2 false negative D2 values were found. No false negative alpha-fetoprotein values were found in any of the cases with neural tube defects in this study. In 10 amniotic fluids from fetuses with other malformations, 5 samples showed raised D2 concentrations. The D2 level in sera from 10 women carrying normal fetuses and 16 women carrying malformed fetuses was also determined, but no statistically significant difference in D2 level was found in the pathological sera when compared with normal sera. It was concluded that the determination of D2 concentrations in amniotic fluid by means of the D2-ELISA may be used as an additional test in the screening of fetal malformations in early pregnancy.