Detection of the homology among proteins by immunochemical cross-reactivity between denatured antigens. Application to the threonine and methionine regulated aspartokinases-homoserine dehydrogenases from Escherichia coli K 12

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Abstract
The 2 isofunctional enzymes aspartokinases-homoserine dehydrogenases I and II from E. coli K 12 are comparred using immunochemical techniques. The antibodies raised against 1 of these 2 proteins when in its native state can only recognize the homologous antigen, whether it is native or denatured. Contrarily, the antibodies raised against 1 of these 2 proteins when in its denatured state can recognize both the homologous and heterologous denatured antigens. The existence of this cross-reaction only between the 2 denatured aspartokinases-homoserine dehydrogenases suggests that these two enzymes have some similarity since such a reaction is not detected with several other denatured proteins. The regions involved in this similarity are buried inside the native proteins, and become exposed only upon denaturation. The same result, the existence of a cross-reaction between denatured species and none between the native ones, is obtained with proteolytic fragments derived from these 2 proteins and endowed with homoserine dehydrogenase activity. This resemblance between the 2 aspartokinases-homoserine dehydrogenases suggests that these proteins derive from a common ancestor. Apparently, such a cross-reaction between 2 denatured proteins is evidence for an homology between their amino acid sequences, and the use of denatured proteins as both immunogens and antigens could be useful in detecting sequence homologies.