Human pancreatic ribonuclease (HPR) and bovine seminal ribonuclease (BS-RNase) exhibit significantly higher activity against double stranded RNA (dsRNA), compared to RNase A. The high dsRNA cleavage activity of BS-RNase, in part, has been attributed to glycine residues at positions 38 and 111. HPR possesses a glycine residue at position 38, whereas it has a glutamic acid at position 111. To understand the mechanism of dsRNA degradation by the single strand preferring HPR, we have generated HPR variants containing mutations at positions 38 and 111. Our study shows that Glycine 38 is crucial for the full catalytic activity of the human enzyme on duplex RNA as its substitution with aspartate or alanine results in a drastic reduction in the dsRNA cleavage activity of HPR. The substitution of Glutamate 111 with glycine also resulted in the reduction of the dsRNA cleavage activity of HPR, indicating that a glycine residue at 111 is not a requirement for the ribonucleolytic activity on double stranded substrate. (C) 2002 Elsevier Science (USA). All rights reserved.