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Arginine synthesis and nitrogen excretion in the myxomycete Physarum polycephalum
Abstract:
The nitrogen excretory metabolism of the myxomycete Physarum polycephalum was studied. When cultured in partially defined broth medium or on agar, the principal excretory product was ammonia nitrogen. A small, variable quantity of urea was excreted in liquid culture. No uric acid or other purines were detected in the cultures. When microplasmodia were incubated with sodium [14C]bicarbonate, radioisotope was incorporated into citrulline, arginine, and urea. Incubation with L-[carbamoyl-14C]citrulline yielded labelled arginine, urea, and CO2. Substantial urease activity was found in extracts of the microplasmodia. These results, in conjunction with the lack of an absolute nutritional requirement for arginine, provide evidence that Physarum has a functional arginine biosynthetic pathway, an arginase, and a urease.
Insights
Physarum polycephalum primarily excretes ammonia nitrogen. This study reveals the myxomycete possesses a functional arginine biosynthesis pathway, arginase, and urease, crucial for nitrogen metabolism.
Area of Science:
- Biochemistry
- Mycology
- Cellular Metabolism
Background:
- The nitrogen excretory metabolism of Physarum polycephalum, a model myxomycete, is not fully elucidated.
- Understanding nitrogenous waste products is key to comprehending cellular physiology and metabolic pathways.
Purpose of the Study:
- To investigate the nitrogen excretory products of Physarum polycephalum.
- To determine the presence and function of key enzymes involved in nitrogen metabolism, specifically related to arginine and urea cycling.
Main Methods:
- Culturing Physarum polycephalum in defined media.
- Analyzing excretory products (ammonia, urea, purines) using biochemical assays.
- Utilizing radiolabeled substrates (sodium [14C]bicarbonate, L-[carbamoyl-14C]citrulline) to trace metabolic pathways.
- Assessing enzyme activity (urease) in microplasmodial extracts.
Main Results:
- Ammonia nitrogen was the principal excretory product in both broth and agar cultures.
- Urea was excreted in variable amounts in liquid culture; uric acid and other purines were undetected.
- Radioisotope incorporation confirmed the presence of pathways leading to citrulline, arginine, and urea.
- Significant urease activity was detected in microplasmodial extracts.
Conclusions:
- Physarum polycephalum exhibits a functional arginine biosynthetic pathway.
- The presence of arginase and urease activities supports a role in nitrogen metabolism and urea cycling.
- These findings clarify the nitrogen excretory metabolism in this important model organism.