Solubilization of Escherichia coli nitrate reductase by a membrane-bound protease

Nitrate reductase extracted from the membrane of Escherichia coli by alkaline heat treatment was purified to homogeneity and used to prepare specific antibody. Nitrate reductase, precipitated by this antibody from Triton extracts of the membrane, contained a third subunit not present in the purified enzyme used to prepare the antibody. Nitrate reductase precipitated by antibody from alkaline heat extracts was composed of peptide fragments of various sizes. These fragments were produced by a membrane-bound protease which was activated by alkaline pH and heat. It is the action of this protease that releases the enzyme from the membrane, as shown by the observations that protease inhibitors decreased the amount of solubilization of the enzyme, and the enzyme remaining in the membrane after heating showed much less proteolytic cleavage than that which was released.