Properties and Conformation of the Histidine Residues at the Active Site of Ribonuclease

The formation, isolation, and characterizat,ion of two isomeric, inactive derivatives of ribonuclease, l-carboxymethylhistidinc-119-and 3-carboxymethylhistidine-12-ribonuclease,1were dcscribed in the preceding paper (2).Studies on the course of the alkylation reaction and on some of the properties of the two alkylated derivatives are the subject of the present communication.It has been found that activity can be restored to mixtures of the inactive derivatives by effecting suitable conformational changes, notably by the formation of aggregates.These studies support the conclusion that the histidine residues at positions 119 and 12 both form part of the active site of the enzyme and provide evidence on the spatial relationship of these residues to one another. EXPERIMENTAL PROCICI)URE:The materials, analytical methods, and general I)roccdurc for alkylation of ribonuclease were the same as those used in the preceding paper (2).In experiments designed to study the effects of salts or of alcohol on the alkylation reaction, water was replaced by the solution to be tested.Chromatography Aggregates of /lllc?/latedRibonucleases-Aggregates were formed by lyophilization from 50y0 acetic acid in the manner previously