The properties of two isoforms, or and fi, of rabbit antithrombin III (ATIII) were compared in the presence of undamaged or de-endothelialized rabbit aortic wall. Similar quantities of ATIII-tt and ATUI-fi bound to and rapidly saturated the endothelium in vitro, but the rate of transendothelial passage of ATIII-/3 exceeded that of ATIII-a by 22%. Furthermore, ATIII-0 was adsorbed approximately twice as rapidly as ATlII-a by the subendothelium of the de-endothelialized aorta. Binding of both isoforms was decreased (ATIII-/3 more than Al'lU-or) by pretreating the subendothelial surface with heparitinase. Also, subendothelium-bound ATIII-/J was desorbed more readily than bound ATlII-aby thrombin. In vivo, the rate of uptake of iodine-131-labeled ATI1I-/3 from the circulation by the aortic wall and the major organs was 30-50% faster than that of iodine-125-labeled ATIII-o: In contrast, the uptake of m I-ATIII-0 by the de-endothelialized aorta in vivo was three times faster than that of '"I-ATlII-a. By these criteria, ATIII-^ is the more active of the two isoforms. We surmise that plasma and, consequently, vessel wall levels of ATIII-/3 may be vital for controlling thrombogenic events caused by injury to the vascular wall. (Arteriosclerosis and Thrombosis 1991;ll:530-539) F rom recent reports, two forms of the glycoprotein antithrombin III (ATIII) have been isolated from rabbit 1 and human 2 plasma. Both the rabbit and human isoforms were separated by gradient elution from a heparin-Sepharose affinity column, the major human isoform, ATIII-a, eluting at 0.9-1.0 M NaCl, and the minor isoform, ATIII-ft at 1.4 M NaCl.2 Human ATIII-0 was shown to contain approximately 25-30% less carbohydrate than ATIII-a.2 More recently, Brennan et al 3 have reported that, unlike the human ATIII-a isoform, which has four biantennary /V-glycans (at asparagine [Asn] 96, 135, 155, and 172), the ATIII-0 isoform possesses only three 7V-glycans, lacking that at Asn 135. Can-ell et al 4 have suggested that the absence of the N-glycan at Asn 135 may extend the binding site for heparin and, consequently, increase the affinity of ATIII-/3 for heparin.
