Acid proteinases from three species will be studied. Activation studies on commercially available pig pepsinogen will concentrate on testing the reversibility of the first three of the activation steps that have so far been identified (tau 1/2 less than or equal to 5 msec, equals 50 msec, equals 32 sec at pH2). Chicken pepsinogen is activated by cleavage in a different part of the molecule than is the pig zymogen. We will repeat on this zymogen several of the studies we previously performed on the pig zymogen: spin-labeling and observation of the leaving rate of the labeled peptide from the pepsin, and electrophoretic analysis of short-term activation mixtures to seek evidence for a new activation intermediate. These studies will list the generality among other aspartate proteinase zymogens of phenomena studies with pig pepsinogen. As we accumulate reasonable amounts of dog pepsinogens, we will determine their amino acid composition, amino terminal acid sequence, and the pH-dependence of their activation. They will be activated so as to generate the first activation peptides which will then be characterized by amino acid composition and as inhibitors of pepsin. The pepsins obtained after complete activation will be characterized by amino acid composition, specificity against defined peptide substrates, and by pH-dependence of proteolysis and of denaturation.