Acid proteases and histologic correlations in experimental ulceration in vitamin A deficient rabbit corneas.

Although xerophthalmia due to severe vitamin A deficiency is the leading cause of childhood blindness in the underdeveloped countries, little is known about the proteases (other than collagenase) that are involved in the degradative mechanism. The degree of cellular autolysis and stromal degradation observed histologically in early stages of xerophthalmia and in ulcerating corneas in vitamin A deficient rabbits in this study were, in general, proportional to the levels of the proteases studied. The only major histologic and ultrastructural alteration observed in early xerophthalmic corneas was autolysis of superficial epithelial and stromal cells. In contrast, in the ulcerating corneas the stroma was infiltrated heavily with inflammatory cells and extensive stromal degradation was observed in the central necrotic region of the lesions. Maximal proteolytic activity toward hemoglobin was observed at pH 3.3 for corneal extracts from normal (N) and pair-fed control (C) rabbits and rabbits with early xerophthalmia (X) and ulcerating xerophthalmia (U) corneas. This activity was a cathepsin D-like enzyme per cornea that had a ratio of 1:1:3:16 in the N, C, X, and U corneas. The ratio of cathepsin B-like activity per cornea for N, C, X, and U corneas was 1:2:2:10.