Proteolytic Release of a Histidinol Dehydrogenase Fragment from the Double Enzyme Histidinol Dehydrogenase-Imidazolylacetolphosphate:l-Glutamate Aminotransferase

A double enzyme of Salmonella typhimurium, containing histidinol dehydrogenase and imidazolylacetolphosphate:l-glutamate aminotransferase, was mildly proteolyzed in an attempt to release these activities from covalent linkage. This treatment releases an active histidinol dehydrogenase fragment larger than normal histidinol dehydrogenase, while completely destroying aminotransferase activity. The normal, unfused enzymes show comparable sensitivities to proteolysis. The active fragment from the double enzyme appears to be composed of histidinol dehydrogenase subunits of roughly normal size and noncovalently bound smaller polypeptides.