Mort J S, Recklies A D
Biochem J. 1986 Jan 1;233(1):57-63. doi: 10.1042/bj2330057.
Two high-Mr forms of cathepsin B have been described previously, both of which are stable at alkaline pH, in contrast with the lysosomal proteinase. One form is latent and activated by pepsin treatment; the other form is active as measured with synthetic substrates. In the present study it was shown that the two forms are indistinguishable on the basis of molecular size as determined by gel-filtration chromatography or sodium dodecyl sulphate/polyacrylamide-gel electrophoresis followed by immunoblotting. Both forms lose their alkali-stability upon exposure to Hg2+, and after Hg2+ treatment the latent form becomes immuneprecipitable by an antiserum that reacts only with denatured cathepsin B. Lysosomal cathepsin B is bound by the plasma proteinase inhibitor alpha 2-macroglobulin, a process that requires proteolytic cleavage of the inhibitor. In contrast, the stable active form of cathepsin B is not bound by this inhibitor unless this enzyme is first destabilized by Hg2+ treatment. These results indicate that cathepsin B exists in three different states of activity, completely latent, partially active and fully proteolytically active. To exhibit true endopeptidase activity it seems that the enzyme must be in an alkali-unstable form.
先前已描述过组织蛋白酶B的两种高分子量形式，与溶酶体蛋白酶不同，这两种形式在碱性pH下均稳定。一种形式是潜伏性的，经胃蛋白酶处理后被激活；另一种形式在用合成底物测定时具有活性。在本研究中发现，通过凝胶过滤色谱法或十二烷基硫酸钠/聚丙烯酰胺凝胶电泳随后进行免疫印迹法测定，这两种形式在分子大小上无法区分。两种形式在暴露于Hg2+后均失去其碱稳定性，并且在Hg2+处理后，潜伏形式可被仅与变性组织蛋白酶B反应的抗血清免疫沉淀。溶酶体组织蛋白酶B与血浆蛋白酶抑制剂α2-巨球蛋白结合，这一过程需要对该抑制剂进行蛋白水解切割。相反，组织蛋白酶B的稳定活性形式不会被该抑制剂结合，除非该酶首先通过Hg2+处理而失稳。这些结果表明，组织蛋白酶B以三种不同的活性状态存在，即完全潜伏、部分活性和完全蛋白水解活性。为了表现出真正的内肽酶活性，该酶似乎必须处于碱不稳定形式。
