The activities of thiol proteases in the rat visceral yolk sac increase during late gestation.

While the rat YVS has been shown to possess an active lysosomal proteolytic system, there are no published reports on the identity of these proteases nor on their changes in activity during the latter half of gestation. We have used specific synthetic substrates to show that cathepsins B, L and H are present in this organ from days 12.5 to 20.5 of gestation. Cathepsins B and L exhibit a marked increase in activity beginning on day 15.5 of gestation. By days 19.5-20.5, cathepsin B activity is increased tenfold over that observed on day 12.5. The activity of cathepsin L may be elevated on day 12.5, decreases more than half by day 14.5 and then increases fourfold by day 20.5. The activity of cathepsin H does not change throughout this period nor do the cathepsins exhibit marked changes in activity in the placenta during this same period or in the PYS from days 12.5 to 14.5 of gestation. These results indicate a specific increase in VYS cathepsin B and L activities late in gestation. These enzymes may be involved in meeting the nutritional needs of the embryo and/or in the degenerative changes which may occur in the VYS and PYS prior to parturition. Studies on the degradation of rat serum albumin by extracts of day 19.5 VYS indicate that cathepsin L may be the quantitatively most important protease in late gestation.