Properties and subcellular localization of elastase-like activities of arterial smooth muscle cells in culture

Abstract

The properties and subcellular localization of the elastase-like activities of smooth muscle cells cultured from pig aortas have been investigated. Homogenates of the cells hydrolysed N-succinyl-L-alanyl-L-alanyl-L-alanine-p-nitroanilide, a synthetic substrate for elastases, with a distinct pH optimum of 8.2 and hydrolysed insoluble elastin with a distinct pH optimum of 8.5. Both enzyme activities were directly proportional to the concentration of homogenate in the assay mixture. The activities toward both substrates were inhibited by phenylmethylsulphonyl fluoride and were therefore probably due to a serine peptidase(s). The activities were also inhibited by EDTA and, in a dose-related manner, by alpha 1-antiprotease. Pepstatin, which inhibits cathepsin D, and leupeptin, which inhibits cathepsin B, did not significantly inhibit the elastase-like activities in these cells. The cells were homogenized and a post-nuclear supernatant subjected to sucrose density gradient centrifugation. The distribution of elastase-like activity toward both substrates was similar to that of the plasma membrane marker 5'-nucleotidase, and distinct from those of marker enzymes for the other organelles. Cells were also homogenized with digitonin, which selectively increases the equilibrium density of the plasma membrane. The equilibrium densities of both 5'-nucleotidase and of the elastase-like activities were increased considerably, confirming the plasma membrane localization of the elastase-like activities. The subcellular localization of the elastase-like activities of arterial smooth muscle cells is therefore consistent with a role for them in the degradation of elastin in the normal arterial wall and in atherosclerotic lesions.