Fluorescence studies on the interaction of adenine with ricin A-chain.

Ricin A-chain, an N-glycosidase that attacks 28S rRNA at a highly conserved adenine residue, has a unique tryptophan (Trp-211) in the putative active site cleft. Fluorescence spectroscopy revealed that specific binding of adenine to the A-chain caused a large enhancement of Trp-211 fluorescence (70%) and a concomitant red shift of the emission spectrum (8 nm). A Scatchard plot of the fluorescence enhancement data was not linear, indicating that the environment of Trp-211 was altered by heterogeneous binding of adenines. These results, taken together with the protective effect of adenine on the ribosome-inactivation by ricin A-chain, suggest that at least two adenines bind to the active site cleft.