Pepsin Inhibitor in Blood Sera of Some Mammals and Birds

The influence of blood sera of some mammals and birds on the rate of pepsin-catalyzed hydrolysis of a low molecular weight substrate pGlu-His-Phe-Phe-p-NA at the optimal pH value of the reaction was studied. Two types of serum samples were used: non-treated, and acidified to pH 2.0. In the presence of both types of serum, the rate of the pepsin reaction was significantly reduced (p < 0.001). The strongest effect was detected in the presence of acidified serum of ruminants and carnivora with pepsin activity decreased below 5 ± 3% (goat) and 4 ± 2% (dog) of its original value. The effectiveness of sera to block pepsin reaction being expressed as the concentration of the total serum protein causing 50% decrease of the enzyme activity, IC50, was compared to the effectiveness of a potent aspartic proteases inhibitor pepstatin A IC50. In order to elucidate the origin of inhibition, the influence of serum antiproteases α1-antitrypsin, and α2-macroglobulin on the rate of pepsin reaction was examined. The influence of albumin – the major transporting serum protein was examined as well. Neither tested antiproteases nor albumin inhibited pepsin-catalyzed hydrolysis of the substrate at the pH value 4.7. Instead, a thermoresistent substance able to block pepsin activity was detected in blood serum of each of the studied species. Its effect on pepsin was found to be essentially independent of pH from 2.3 to 5.6.