Identification of low-Ca2+- and high-Ca2+-requiring neutral proteases in rat retina

Two low-Ca2+-requiring proteases (calpain I) and one high-Ca2+-requiring protease (calpain II) have been separated from the cytosol of rat retina by DEAE-cellulose column chromatography. Calpain I was half-maximally activated at 3 microM free Ca2+ and fully activated at 10 microM free Ca2+. Half-maximal activation of calpain II was at 0.4 mM free Ca2+ while full activation was observed at 0.8 mM free Ca2+. Calpain activity has also been demonstrated in sealed rod outer segments. The soluble fraction of the rod outer segments contained 89% of the enzyme activity. The possible role of calpain in retina is discussed.