MICROHETEROGENEITY OF HUMAN ALPHAFETOPROTEIN

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Alphafetoprotein (AFP), a glycoprotein, consists of two sub- fractions with respect to its affinity for the lectin concanavalin A (con A). The percentage of the con A non-reactive AFP has been shown to be significantly lower in amniotic fluid from pregnancies associated with neural tube defect. Human AFP has also been shown to exist as a series or family of charge isomers, each isomer differing in isoelectric point. Charge microheterogeneity, as well as differences in con A binding, of other glycoproteins has been attributed to subtle differences in carbohydrate structure. Because AFP exhibits carbohydrate-microheterogeneity, it is reasonable to assume that the differences observed in the proportion of con A non-reactive AFP as a result of neural tube defect will reflect changes in the isoelectric pattern of AFP. Aliquots of amniotic fluid samples obtained during routine diagnostic amniocentesis in the second trimester of pregnancy will be used. Careful attention will be paid to the gestational age at which the sample is obtained. The total concentration of AFP will be determined in each sample by specific radioimmunoassay (RIA). The charge microheterogeneity of AFP will be determined by subjecting aliquots of amniotic fluid to chromatofocusing, a technique which separates proteins according to pI. The AFP in each fraction of the column will be measured by RIA. The immunoreactive peaks, corresponding to the various charge isomers, will be pooled. The carbohydrate-microheterogeneity of the charge isomers as well as native AFP in amniotic fluid will be further assessed by con A column chromatography. Differences in the charge microheterogeneity of AFP between normal and abnormal pregnancies will be compared. The results of the proposed study should increase our knowledge of the biochemical properties of human AFP and improve our ability for prenatal diagnosis of birth defects.