Affinity Labeling of Pancreatic Ribonuclease

Pyridoxal phosphate can be firmly bound by reduction with sodium borohydride to many enzymes in which it occurs. Such treatment has been used to label specifically parts of peptide chains in the vicinity of the bound coenzyme. Pyridoxal phosphate may form specific complexes with many proteins for which it has no known coenzyme function. Bovine RNase is an enzyme for which it has no known function but which possesses a well characterized phosphate binding site. When RNase was treated with pyridoxal phosphate and sodium borohydride, the catalytic activity was lost. This loss is attributed to the binding of pyridoxal phosphate to a residue topologically adjacent to the active center. The special affinity of RNase for anions is sufficient to affect its specific labeling by pyridoxal phosphate.