Effects of salts on the aspartate transcarbamylase of a halophilic eubacterium, Vibrio costicola

The aspartate transcarbamylase (ATCase) in cell-free extracts of the moderately halophilic eubacterium, Vibrio costicola, was stable in 1.5 M NaCl, but not in 0.5 M NaCl on prolonged storage at 4 °C in concentrated extracts. At lower salt concentrations, activity was lost rapidly. ATCase activity was optimal at about 1.5 M NaCl or 1.0 M KCl, although high activity was detected at 0.15 M NaCl. In the presence of 0.03 M aspartate both succinate and maleate inhibited ATCase activity. CTP inhibited the activity of the enzyme at low salt concentrations (0.15 to 0.3 M). Much less inhibition occurred at higher salt concentrations. Precipitating the enzyme with ammonium sulphate resulted in loss of CTP inhibition. The ATCase of V. costicola differs from those of a nonhalophile (Saccharomyces cerevisiae) and an extremely halophilic archaebacterium (Halobacterium cutirubrum) in its salt-response patterns of activity and regulation.Key words: halophilic, aspartate transcarbamylase, Vibrio costicola.