Characterization of Putrescine Production in Nongrowing Vibrio parahaemolyticus Cells in Response to External Osmolality

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Abstract
Nongrowing Vibrio parahaemolyticus cells rapidly produced putrescine (Put) from added arginine when subjected to a low osmotic stress. This phenomenon was characterized in connection with a regulatory mechanism of the responsible enzymes, arginine decarboxylase (ADC) and agmatine ureohydrolase (AUH). NaCl, KCl, LiCl, sucrose, and glycerol were used as solutes to prepare the resuspending media with various osmolalities. Regardless of whether the solutes were electrolytes or non‐electrolytes, exposure of cells to low osmolality brought about instantaneous increases in both intra‐ and extracellular Put contents without significant changes in the contents of other polyamines. This acceleration in Put production was accompanied by no increases in the specific activities of ADC and AUH. On the other hand, when cells were exposed to the osmolality equivalent to 2 or 5% NaCl, all solutes except for glycerol did not cause a remarkable variation in the intracellular Put content, while the amount of Put in the medium varied depending on the solute used; sucrose and glycerol still greatly prompted Put production, as judged by high Put contents in the media, even at the osmolality equivalent to 5% NaCl. The cation efflux from cells, measured as the K+ release, was observed whenever the increase in Put production occurred. Furthermore, in vitro experiments showed that NaCl and KCl inhibited ADC to a similar extent, about 70% inhibition being observed at 200 mm. However, AUH was not affected by these compounds. These results suggest that the reduction in the concentrations of Na+ and K+ predominantly present in cells may cause the increase in activity of the preexisting ADC, which leads to the enhancement of Put production.