The enzyme primarily responsible for protein degradation in the stomach is

Secretions from the Oxyntic (Gastric) Glands:It is composed of three types of cells: (1) mucous neck cells, which secrete mainly mucus; (2) peptic (or chief) cells, which secrete large quantities of pepsinogen; and (3) parietal (or oxyntic) cells, which secrete hydrochloric acid and intrinsic factor. When pepsinogen is first secreted, it has no digestive activity. However, as soon as it comes in contact with hydrochloric acid, it is activated to form active pepsin.Pepsin functions as an active proteolytic enzyme in a highly acidic medium (optimum pH 1.8 to 3.5), but above a pH of about 5 it has almost no proteolytic activity and becomes completely inactivated in a sho time.Ref: Guyton; 13th edition; Chapter 65 Secretory Functions of the Alimentary Tract