Detection and characterization of acetohydroxy acid synthase in Spirulina platensis

Abstract Growth of the cyanobacterium Spirulina platensis, like that of many prokaryotic and eukaryotic organisms, is inhibited by low concentrations of valine, one of the three end-products of the branched-chain amino acid biosynthetic pathway. We assayed and partially characterized the activity of acetyhydroxy acid synthase (AHAS), the first common enzyme of the branched pathway in cell-free extracts from axenic S. platensis cultures. Assays performed at various pH values showed two peaks of activity, both inhibited by valine. FAD was not required for enzyme activity but protected it during dialysis. We also investigated whether the three amino acids were able to cause repression of AHAS synthesis and a significant drop in the enzyme-specific activity could be seen only when cultures were grown in the presence of valine. Chromatography on hydroxylapatite showed one single peak of activity.