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First-trimester screening for trisomies 21 and 18 on the basis of maternal age, maternal levels of free beta human chorionic gonadotropin and pregnancy-associated plasma protein A, and measurement of fetal nuchal translucency has good sensitivity at an acceptable false positive rate.

The residues that are indispensable for the ribonucleolytic activity of angiogenin are also known to be essential for its angiogenic activity. We now demonstrate that residues in another region of the protein, devoid of catalytic residues, are additionally required for angiogenesis. Endoproteinase Lys-C or a baby hamster kidney cell protease cleaves angiogenin at the peptide bond either between Lys-60 and Asn-61 or between Glu-67 and Asn-68, respectively. The two polypeptide fragments resulting from either cleavage remain linked by disulfide bonds. These two derivatives and des-(Asn"'-Glu 7)-angiogenin-n which both bonds are cleavedretain their ribonucleolytic activities toward tRNA, 18S and 28S rRNA, and dinucleoside phosphates but are no longer angiogenic on the chicken embryo chorioallantoic membrane. C and A2 (12-14), (ii) the highaffinity, specific binding of angiogenin to endothelial cells (15), (iii) the isolation of an endothelial cell surface protein that binds angiogenin (16), and (iv) the capacity of certain enzymatically inactive angiogenin mutants to inhibit angiogenesis (9). The last item suggests in addition that the receptor-binding region of angiogenin is, at least in part, distinct from the catalytic site.Ifa region of the angiogenin molecule functions exclusively in receptor binding, it might be possible to alter that region selectively, thereby affecting angiogenesis but without altering enzymatic activity. We have been able to achieve precisely this dissociation of activities by specific proteolysis of the peptide bond either between Lys-60 and Asn-61 or between Glu-67 and Asn-68 in angiogenin to generate angiogenin K and angiogenin E, respectively. Angiogenin E and K, as well as des-(Asn61Glu67)-angiogenin in which both bonds are cleaved, retain enzymatic activity yet are neither angiogenic nor capable of activating second messenger-regulated pathways in endothelial cells. Moreover, all three derivatives fail to compete with angiogenin in the angiogenesis assay. This indicates that the region from Lys-60 to Asn-68 encompassing the two cleavage sites may form part of a cell-surface receptor binding site separate and distinct from the enzymatic active site of angiogenin. MATERIALS AND METHODSAngiogenin was obtained from recombinant expression systems in either Escherichia coli (17) or baby hamster kidney (BHK) cells (18). In the latter case, purification to homogeneity was achieved by CM-52 cation-exchange chromatography followed by Mono S (Pharmacia) HPLC. In some instances, the final preparation obtained had been partially cleaved by a BHK cell protease (see Results). The cleaved derivative, denoted angiogenin E, was purified by C18 HPLC using a Synchropak RP-P column (250 x 4.6 mm; SynChrome, Lafayette, IN). Elution was achieved with a 54-min linear gradient from 30o to 42% solvent B at 0.8 ml/min at ambient temperature, where solvent A was 0.1% CF3COOH in water and solvent B was 2-propanol/acetonitrile/water, 3:2:2 (vol/vol) containing 0.08% CF3COOH. Angiogenin K (19) was prepare...

First-trimester screening for Down syndrome and trisomy 18 is effective and offers substantial benefits to clinicians and patients.

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