Glycinebetaine as an osmoregulant and compatible solute in the marine cyanobacterium Spirulina subsalsa

Glycinebetaine was found to be the major organic substrate accumulating under hypersaline growth conditions in the halotolerant cyanobacterium Spirulina subsalsa. In addition to its proposed role as osmolite, glycinebetaine is shown to specifically protect enzymatic activity. Glucose-6-phosphate dehydrogenase from S. subsalsa retained full activity in the presence of NaCl at concentrations as high as 1.5 m, provided that comparable concentrations of glycinebetaine were also present in the reaction mixture. A kinetic analysis indicated that glycinebetaine protected the enzyme against both NaCl-induced decrease in V_max and reduction in affinity to glucose 6-phosphate. The alternative osmolites, glycerol and proline, protected the enzyme against the reduction in V_max but not against the reduction in affinity to glucose 6-phosphate.