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Diminution of outer membrane permeability by Mg2+ in a marine pseudomonad
Abstract:
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.
Insights
Marine pseudomonad MB-45 showed low alkaline phosphatase activity due to outer membrane exclusion of the substrate. Magnesium ions (Mg2+) were essential for this membrane permeability, controlling enzyme activity.
Area of Science:
- Microbiology
- Enzymology
Background:
- Marine pseudomonads are important in oceanic ecosystems.
- Alkaline phosphatase plays a key role in nutrient cycling.
Purpose of the Study:
- To investigate the low alkaline phosphatase activity in intact marine pseudomonad MB-45 cells.
- To determine the factors influencing the enzyme's accessibility to its substrate.
Main Methods:
- Assessing alkaline phosphatase activity via p-nitrophenylphosphate hydrolysis in intact cells and sonic extracts.
- Investigating the effect of Mg2+ concentration and outer layer removal on enzyme activity.
Main Results:
- Intact cells showed minimal activity, while sonic extracts were highly active.
- Decreasing Mg2+ concentration progressively increased enzyme accessibility.
- Outer membrane exclusion of the substrate was identified as the cause of enzyme crypticity.
Conclusions:
- The outer membrane of MB-45 restricts substrate access to periplasmic alkaline phosphatase.
- Magnesium ions (Mg2+) are crucial for maintaining the outer membrane's selective permeability, thereby regulating enzyme activity.