Protein oxidation and proteolytic degradation. General aspects and relationship to cataract formation.

1. Intracellular proteins are subject to oxidative and photooxidative denaturation. 2) Proteolytic systems recognize and selectively degrade oxidatively denatured, and photooxidatively denatured proteins. By degrading mildly denatured proteins these proteolytic systems prevent further oxidative/photooxidative damage which could otherwise result in the formation of cross-linked (undigestible) proteins, or protein fragments with toxic biological activities. Proteolytic systems also provide amino acids for the synthesis of new (replacement) proteins. 3) A 700,000 dalton neutral endoproteinase, which we have called macroxyproteinase or M.O.P., appears to be mostly responsible for the degradation of oxidatively denatured proteins. M.O.P. has been shown to function in red blood cells and in the eye lens, and appears to also exist in many other mammalian cell types. 4) Cataract is a disease associated with aging, and with photooxidative denaturation (and cross-linking) of lens crystallins and other proteins. 5) Both cataract and aging of lens cells are associated with declining proteolytic capacity and diminished antioxidant protection. 6) Lens aging and in vivo photooxidative stress can cause opacity ("cataract"), cross-linking of crystallins, and diminished proteolytic capacity. 7) High levels of dietary ascorbate increase ascorbate concentrations in lens tissue, and are associated with greater resistance of lens proteins and lens proteolytic enzymes to oxidative/photooxidative stress in vitro.