Structure-activity studies on synthetic peptides inhibiting herpes simplex virus ribonucleotide reductase

Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) ribonucleotide reductase is formed by the association of two nonidentical subunits. A peptide corresponding to the COOH terminus of the subunit 2, Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(7-15)), has been shown to completely inhibit the reductase activity (IC50 = 36 microM) without affecting the host isoenzyme. In order to study the relationship between chemical requirements and inhibitory potencies, a series of peptides, including fragments and analogs of H2-(7-15), were synthesized. The minimum active core can be assigned to the Val-Val-Asn-Asp-Leu sequence (IC50 = 760 microM). N alpha-Extended peptides, such as Ser-Thr-Ser-Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(4-15)) and Glu-Cys-Arg-Ser-Thr-Ser-Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(1-15)), respectively, have inhibitory potencies 2.1- and 1.4-fold greater than the nonapeptide H2-(7-15). N alpha-Deamination or acetylation of H2-(7-15) increases its potency by 1.8- and 3.0-fold, respectively, whereas amidation of the alpha-carboxylic function diminishes its activity by 3.2-fold. These results indicate that the alpha-amino group is not essential for maximum potency but suggest that a free carboxylic function is required. Substitution of Tyr7 or Ala8 by their respective D-isomer leads to a decrease of potency, suggesting that a specific conformation of the NH2-terminal portion is required to have a maximum activity. Monosubstitution in positions 11, 13, 14, and 15, by L-alanine completely abolishes activity stressing the importance of each amino acid residue contained in the minimum active core. Finally, nonapeptides corresponding to the COOH-terminal portion of the subunit 2 of Epstein-Barr and varicella-zoster virus ribonucleotide reductases also inhibit the HSV-1 reductase activity. The varicella-zoster virus nonapeptide is 4.0 times more potent than H2-(7-15), whereas the Epstein-Barr virus nonapeptide is 3.1 times less potent. These results should help us to design a new generation of potent inhibitors of herpes virus ribonucleotide reductases.