Radioimmunoassay and chemical properties of glucose-6-phosphate dehydrogenase and of a specific NADP(H) binding protein (FX) from human erythrocytes

Two sensitive radioimmunoassays, based on a double-antibody technique, were developed which allow detection of nanogram amounts of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and of a so far unknown NADP(H)-binding protein present in human erythrocytes (designated FX). The two proteins isolated in homogeneous form from human erythrocytes were iodinated with 125I by means of lactoperoxidase. Antisera to both purified proteins were raised in rabbits and sequentially adsorbed on human erythrocytes and on human serum before use. No cross-reaction between the two proteins was apparent. Hemolysates from normal as well as from glucose-6-phosphate dehydrogenase-deficient subjects were investigated for their content in both immunoreactive proteins using the two radioimmunoassay methods. This preliminary study showed significantly lowered levels of immunoreactive glucose-6-phosphate dehydrogenase in erythrocytes from subjects carrying the Mediterranean variant of this enzyme (characterized by severe deficiency of catalytic activity), compared with normal subjects. This figure was reversed as concerns the content of immunoreactive FX which was found to be twice as high in glucose-6-phosphate dehydrogenase Mediterranean erythrocytes as in normal ones. The two purified proteins were submitted to a comparative analysis of their chemical properties including NH2-terminal residues, CNBr peptides and tryptic fingerprints. These studies revealed significant differences in the primary structures of the two proteins and therefore tend to exclude FX's being a discrete product arising from degradation of native glucose-6-phosphate dehydrogenase. Moreover, amino acid analysis and tryptic fingerprints indicated that FX, as well as glucose-6-phosphatase dehydrogenase, is composed of very similar and possibly identical polypeptide chains.