A re-examination of the association of 'early pregnancy factor' activity with fractions of heterogeneous molecular weight distribution in pregnancy sera.

The association of 'early pregnancy factor' ('EPF') activity in early pregnancy sera with multiple fractions of heterogeneous molecular mass was re-examined to test whether previously perplexing observations could be explained by a new multi-factorial model of the serum components required for this activity expression. Gel permeation fractionation of human pregnancy sera revealed 'EPF' activity associated with fractions containing components ranging in MW from < or = 1 kDa to > or = 500 kDa. A significant activity peak was observed eluting on the total volume (Vt) of the column, indicating the presence of active molecules of very low molecular mass. Multiple activity peaks were also observed in the macromolecular fractionation region ranging in apparent molecular weight from 12 kDa, through 25, 70 and 250 to > or = 500 kDa. Analysis of these fractions revealed that they all contained thioredoxin and active moieties of low molecular mass, with the latter probably directly associated with the former. Adsorption with specific anti-thioredoxin antibodies removed from these fractions the capacity to display 'EPF' activity. Further analyses revealed that in these fractions thioredoxin played a permissive role allowing the low molecular mass active moieties to express activity in the bioassay in the presence of otherwise counteracting substances. The results of these studies are consistent with the proposal that 'EPF' activity expression in pregnancy sera is due to the presence of a multi-factorial system in which thioredoxin plays an essential permissive role in concert with active moieties of low molecular mass.