Rabbit antisera to pepsin and pepsinogen were characterized by several immunological criteria. Both antisera inhibited the rennet activity of pepsin. Antipepsinogen protected pepsin from alkaline denaturation. Using antipepsinogen, precipitin analysis at pH 5.5 indicated that the native enzyme resembles the precursor more closely than did the denatured enzyme. However, all three proteins have some antigenic sites in common. Both antisera reacted more efficiently with their homologous antigens. When measured by C' fixation, the pepsinogen-antipepsinogen system was inhibited by pepsin and to a greater degree, by the activation mixture and the pepsin-inhibitor complex. Pepsin-antipepsin was inhibited by pepsinogen. The specificity of these two antibodies toward pepsin and pepsinogen conformation was used to measure the disappearance of pepsinogen and the concomitant appearance of pepsin during autocatalytic conversion at pH 4.6. The experimental results obtained during the conversion could be duplicated by using varying proportions of pepsin and pepsinogen in the model system. The potentialities of employing these antisera to detect conformational changes such as the unmasking of the pepsin moiety in pepsinogen molecules modified by physical or chemical reagents are discussed.

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