QUANTIFICATION OF ENDOTHELIAL PLASMINOGEN ACTIVATOR INHIBITOR (PAI-1) ANTIGEN BY AN ENZYME LINKED IMMUNOSORBENT ASSAY (ELISA)

An ELISA to measure PAI-1 antigen has been developed using PAI-1 purified from human endothelial cell conditioned medium and a monospecific antiserum raised against it in the rabbit. Test and standard samples diluted in assay buffer containing non-immune rabbit serum were incubated in microplate wells coated with anti-PAI-1 IgG. Then biotinylated anti-PAI-1 IgG was added to the wells followed by a streptavidin-biotinylated horseradish peroxidase complex. Tetramethylbenzidine was used as substrate and the optimised ELISA had a detection limit of 0.2 ng PAI-1 ml-1 sample, using purified PAI-1 of known concentration as a standard for the assay.PAI-1 antigen was readily detectable in human plasmas and higher concentrations were invariably detected in the corresponding whole blood sera.α2antiplasmin and antithrombin III which have been shown to have high degrees of sequence homology with PAI-J were undetectable in the ELISA at concentrations of 10 ug ml-1 . Sera from baboon and Rhesus and Cynomologus monkeys exhibited partial cross reactivity in the ELISA while no crossreactivities were observed with a wide range of non-primate sera that included dog, goat, cow, horse, pig and rat.A variety of human cell cultures were assayed for PAI-1 antigen. Endothelial cells from umbilical cord veins and arteries and from adult sapgenous veins secreted, typically,1-2 ug PAI-1 24-1 hr per 10 cells. This represented approximately 2-4% of the total secreted protein. Detectable but considerably lower levels of PAI-1 were secreted by keratinocytes and fibroblasts as well as a lung carcinoma cell line A549.No PAI-1 was detected in media conditioned by two melanoma cell lines, a breast carcinoma cell line MCF7 or a monocytic leukaemia JIII. The ELISA proved suitable for monitoring altered cellular PAI-1 metabolism, and in conjunction with functional assays, for investigating changes in PAI-1 specific activity.