alpha(2)-Macroglobulin (alpha M-2) is a proteinase inhibitor that functions by a trapping mechanism which has been exploited such that the receptor-recognized, activated form (alpha M-2*) can be employed to target antigens to antigen-presenting cells. Another potential use of alpha M-2* is as a drug delivery system. In this study we demonstrate that guanosine triphosphate, labeled with Texas red (GTP-TR) formed complexes with alpha M-2* following activation by proteolytic or non-proteolytic reactions. Optimal incorporation occurred with 20 mu M GTP-TR, pH 8.0 for 5 h at 50 degrees C. NaCl concentration (100 or 200 mM) had little effect on incorporation at this pH or temperature, but was significant at sub-optimum temperature and pH values. Maximum incorporation was 1.2 mol GTP-TR/mol alpha M-2*. PAGE showed that 70-90% of the GTP-TR is bound in a SDS/2-mereaptoethanol resistant manner. Guanosine, adenosine, and imidazole competed with GTP-TR to form complexes with alpha M-2*. (c) 2007 Elsevier Inc. All rights reserved.