Two antigenic sites of tissue transglutaminase

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Abstract
The immunization of rabbits with purified guinea pig liver transglutaminase resulted in the appearance of 2 antibody populations against the enzyme: one which reacted only with the Ca2+-enzyme complex and another which reacted with the intact as well as the Ca2+-enzyme. The Ca2+-induced conformational change of the enzyme molecule exposed a new antigenic determinant which initiated the production of a specific antibody population. When the glutamine substrate of the enzyme was a dipeptide, the result of the interaction of the Ca2+-enzyme and its isolated specific antibody was an apparent activation of the catalytic activity. When protein substrates were used, an inhibition was observed. The characterization of the mechanism of the activation and the inhibition has led to the conclusion that the consequence of the interaction of the Ca2+-enzyme and its specific antibody was not only a limited steric hindrance of the active center but, also a stabilization of the otherwise labile Ca2+-enzyme. The other antibody population reacted with both forms of the enzyme and its inhibitory effect, which was observed in each assay, could be due to a prevention of the Ca2+-induced formation of the active enzyme.