MODIFICATION OF CARBOXYL GROUPS OF RIBONUCLEASE BY ATTACHMENT OF GLYCINE OR ALANYLGLYCINE

Bovine pancreatic ribonuclease (RNase) was modified by binding glycine or alanylglycine to the carboxyl functions of the enzyme, making use of the phthalimidomethyl group for the reversible blocking of the glycine or dipeptide carboxyl groups and of a water- soluble carbodiimide for the coupling reaction. In the resulting RNase-glycine derivative all the eleven carboxyl groups of RNase reacted with glycine, while in the RNase-AlaGly derivative only eight carboxyl groups reacted. In both cases the electrical charges due to the carboxylate ions have not been abolished, even though their steric positions were somewhat displaced. RNase- glycine had an ultraviolet absorption spectrum differing from that of native RNase, and on spectrophotometric titration all six tyrosine residues were available for ionization. This derivative was inactive on RNA but possessed almost all the activity of the native enzyme on cytidine 2[image],3[image]-cyclic phosphate. The activity was abolished upon full reduction, but was completely recovered after reoxidation of the derivative. The preparation modified by the attachment of the alanylglycine peptide was totally inactive.