A membrane-bound protease in microsomes of spinach callus.

A di-isopropyl phosphorofluoridate-sensitive endopeptidase activity against some minor components of heat-denatured alpha-casein was detected in the endoplasmic reticulum and Golgi body-rich fraction of spinach callus. The activity was not solubilized with 0.05% sodium deoxycholate, but with 0.5% sodium cholate. The activity was strongly inhibited by deoxycholate (0.2-0.5%), di-isopropyl phosphorofluoridate, p-chloro-mercuric benzoate, o-phenanthroline, NiCl(2), CuCl(2), and ZnSO(4), and moderately by phenylmethylsulfonyl fluoride, l-1-tosylamide-2-phenylethyl chloromethyl ketone, iodoacetic acid, ethylenediaminetetraacetate, and FeSO(4), and slightly by chymostatin. The inhibitory effect of o-phenanthroline was partially recovered with the addition of FeSO(4) and ZnSO(4).