RNA cleavage and inhibition of protein synthesis by bleomycin

Bleomycin is a clinically used antitumor antibiotic long thought to function therapeutically at the level of DNA cleavage. Recently, it has become clear that bleomycin can also cleave selected members of all major classes of RNA. Using the computer program COMPARE to search the database established by the Anticancer Drug Screening Program of the National Cancer Institute, a possible mechanism-based correlation was found between onconase, an antitumor ribonuclease currently being evaluated in phase III clinical trials, and the chemotherapeutic agent bleomycin. Following these observations, experimentation revealed that bleomycin caused tRNA cleavage and DNA-independent protein synthesis inhibition in rabbit reticulocyte lysate and when microinjected into Xenopus oocytes. The correlation of protein synthesis inhibition to the previously reported site-specific RNA cleavage caused by bleomycin supports the thesis that RNA cleavage may constitute an important element of the mechanism of action of bleomycin.

DNA-damaging agents from Crypteronia paniculata

A survey of crude plant extracts using a new yeast strain designed to identify DNA-damaging agents resulted in the identification of an extract prepared from Crypteronia paniculata. Bioassay-guided fractionation resulted in the isolation of three active compounds. Two of these were ellagic acid derivatives, namely, 3,3'-di-O-methylellagic acid 4'-O-beta-d-xylopyranoside (1) and 3'-O-methyl-3,4-methylenedioxyellagic acid 4'-O-beta-d-glucopyranoside (2). The third was identified as kaempferol-3-O-alpha-l-rhamnoside (3). The three principles exhibited strong, selective cytotoxity toward the RAD52 repair-deficient yeast strain.

Solid-phase synthesis of bleomycin A(5) and three monosaccharide analogues: exploring the role of the carbohydrate moiety in RNA cleavage

The solid-phase synthesis of bleomycin A5 (BLM A5) and three monosaccharide analogues is presented. The monosaccharide analogues incorporated alpha-d-mannose, alpha-l-gulose, and alpha-l-rhamnose moieties in lieu of the disaccharide normally present in BLM A5. Also explored were the abilities of each of the monosaccharide congeners to cleave a 53-nt RNA. The elaboration of these carbohydrate-modified bleomycin analogues helps to define the role of the disaccharide moiety during the RNA cleavage event. The relatively facile solid-phase synthesis of bleomycin A5 and each of the carbohydrate analogues constitutes an important advance in the continuing mechanistic studies of bleomycin.

Site-specific incorporation of (aminooxy)acetic acid into proteins

By employing a general biosynthetic method for the elaboration of proteins containing unnatural amino acid analogues, we incorporated (aminooxy)acetic acid into positions 10 and 27 of Escherichia coli dihydrofolate reductase. Introduction of the modified amino acid into DHFR was accomplished in an in vitro protein biosynthesizing system by readthrough of a nonsense (UAG) codon with a suppressor tRNA that had been activated with (aminooxy)acetic acid. Incorporation of the amino acid proceeded with reasonable efficiency at codon position 10 but less well at position 27. (Aminooxy)acetic acid was also incorporated into position 72 of DNA polymerase beta. Peptides containing (aminooxy)acetic acid have been shown to adopt a preferred conformation involving an eight-membered ring that resembles a gamma-turn. Accordingly, the present study may facilitate the elaboration of proteins containing conformationally biased peptidomimetic motifs at predetermined sites. The present results further extend the examples of ribosomally mediated formation of peptide bond analogues of altered connectivity and provide a conformationally biased linkage at a predetermined site. It has also been shown that the elaborated protein can be cleaved chemically at the site containing the modified amino acid.

Fluorescence resonance energy transfer between unnatural amino acids in a structurally modified dihydrofolate reductase

The cleavage of a substrate protein by HIV-1 protease has been monitored in real time by the use of a dihydrofolate reductase fusion protein in which a fluorescence donor and a fluorescence acceptor were introduced into sites flanking the HIV-1 protease cleavage site. The amino acids 7-azatryptophan and dabcyl-1,2-diaminopropionic acid were introduced into specific sites of the DHFR fusion protein in an in vitro protein biosynthesizing system using two misacylated suppressor tRNAs, each of which recognized a specific, unique codon introduced into the mRNA. Excitation of the fluorescence acceptor in the initially expressed protein afforded no light production, consistent with quenching by fluorescence resonance energy transfer. Treatment of the elaborated protein with HIV-1 protease cleaved the protein between the fluorescence donor and acceptor, affording a time-dependent increase in fluorescence that was equal in magnitude to that produced by admixture of a stoichiometric amount of free 7-azatryptophan to the solution containing the intact protein.

Total synthesis of deamido bleomycin a(2), the major catabolite of the antitumor agent bleomycin

Metabolic inactivation of the antitumor antibiotic bleomycin is believed to be mediated exclusively via the action of bleomycin hydrolase, a cysteine proteinase that is widely distributed in nature. While the spectrum of antitumor activity exhibited by the bleomycins is believed to reflect the anatomical distribution of bleomycin hydrolase within the host, little has been done to characterize the product of the putative inactivation at a chemical or biochemical level. The present report describes the synthesis of deamidobleomycin demethyl A(2) (3) and deamido bleomycin A(2) (4), as well as the respective aglycones. These compounds were all accessible via the key intermediate N(alpha)-Boc-N(beta)-[1-amino-3(S)-(4-amino-6-carboxy-5-methylpyrimidin-2-yl)propion-3-yl]-(S)-beta-aminoalanine tert-butyl ester (16). Synthetic deamido bleomycin A(2) was shown to be identical to the product formed by treatment of bleomycin A(2) with human bleomycin hydrolase, as judged by reversed-phase HPLC analysis and (1)H NMR spectroscopy. Deamido bleomycin A(2) was found to retain significant DNA cleavage activity in DNA plasmid relaxation assays and had the same sequence selectivity of DNA cleavage as bleomycin A(2). The most significant alteration of function noted in this study was a reduction in the ability of deamido bleomycin A(2) to mediate double-strand DNA cleavage, relative to that produced by BLM A(2).