A SYNTHETIC STUDY OF THE EFFECT OF TYROSINE AT POSITION 120 OF RIBONUCLEASE

[Tyr¹²⁰]-RNase 111–124 and [Ala¹²⁰]-RNase 111–124 were synthesized and purified to chromatographic and electrophoretic homogeneity. The peptides were mixed noncovalently with RNase 1–118 that had been prepared by enzymatic degradation of native bovine pancreatic ribonuclease A. The activities of the resulting complexes were compared with that of the corresponding complex containing the natural peptide sequence, [Phe¹²⁰]-RNase 111–124, and with native RNase, using yeast RNA, C > p and U > p as substrates. The Tyr¹²⁰ and Phe¹²⁰ complexes were equally active against RNA and C > p, but the Tyr¹²⁰ complex was twice as active as the Phe¹²⁰ complex against U > p. The complex with alanine at position 120 was only 1% as active toward C > p as the complex containing phenylalanine. The prediction that giraffe RNase, which contains tyrosine at position 120, would be relatively more active toward U > p than C > p compared with bovine RNase was verified.