Photoaffinity labeling of a bacterial sialidase with an aryl azide derivative of sialic acid.

A photoreactive radioiodinatable derivative of 2-deoxy-2,3-didehydro-5-N-acetylneuraminic acid (NeuAc2en), 5-N-acetyl-9-(4-azidosalicoylamido)-2-deoxy-2,3-didehydroneuram inic acid (ASA-NeuAc2-en) has been synthesized and used to label the active site of Clostridium perfringens sialidase. Like NeuAc2en, its aryl azide derivative is a strong competitive inhibitor of sialidase (Ki approximately 15 microM). The absorbance spectrum of ASA-NeuAc2en shows a characteristic aryl azide peak, which disappears upon photolysis with UV light. When its radioiodinated counterpart 5-N-acetyl-9-(4-iodoazidosalicoylamido)-2-deoxy-2,3-didehydrone uraminic acid ([125I]IASA-NeuAc2en) was photolyzed in the presence of C. perfringens sialidase a 72-kDa protein was labeled. Labeling occurred specifically in the active site since it was inhibited in the presence of NeuAc2en. Chemical cleavage of the photoaffinity-labeled 72-kDa protein demonstrates that specifically labeled peptides involved in the formation of the active site can easily be determined. ASA-NeuAc2en is a valuable new tool for the identification and structural/functional analysis of sialidases and other proteins, recognizing this sialic acid derivative.