Diminution of outer membrane permeability by Mg2+ in a marine pseudomonad.

Intact cells of the marine pseudomonad MB-45, in the presence of optimal Mg2+, exhibited little alkaline phosphatase activity as judged by the hydrolysis of p-nitrophenylphosphate. Sonic extracts, in contrast, were rich in this activity. Removal of the loosely bound outer layer did not diminish this crypticity of alkaline phosphatase, but decreasing the concentration of Mg2+ in the suspending medium progressively exposed the alkaline phosphatase. Since MB-45 did not liberate alkaline phosphatase into the surrounding medium even in the absence of Mg2+ and since this enzyme is localized in the periplasmic space, it can be concluded that the crypticity was due to the exclusion of p-nitrophenylphosphate by the outer membrane. Mg2+ is apparently essential for the full expression of this limited permeability.