Thielavin B

We devised two screening systems to detect cell wall transglycosylation inhibitors. One screen utilizes a mutant of Enterococcus faecalis strain A256 that is dependent on vancomycin or moenomycin for growth. In the absence of transglycosylation inhibitors the strain fails to grow, while in the presence of inhibitors, cells are rescued. A second screening organism E. faecalis strain MDD212 utilizes a translational fusion of the lacZ gene to the vanH promoter in a derivative of E. faecalis that contains a vancomycin resistance determinant. Induction of beta-galactosidase occurs when cells are exposed to inhibitors of transglycosylation. Our natural products drug source of fungal fermentations was tested with these screens. Several cultures that produced the same family of compounds, called the thielavins, were detected. Thielavin B inhibited the formation of peptidoglycan in an in vitro assay, suggesting that these screening systems can detect compounds that interfere with cell wall transglycosylation.

Two potent inhibitors of prostaglandin biosynthesis, thielavin A (C31H34O10) and B (C29H30O10), were isolated from cultures of Thielavia terricola. Both of these compounds were shown to be structurally related to depsides, thus consisting of three hydroxybenzoic acid groups. Concentrations required for 50% inhibition of the conversion of 14C-arachidonic acid into prostaglandins F2 alpha plus E2 by microsomes of ram seminal vesicles were 12 microM for thielavin A and 9 microM for thielavin B, respectively. Of the enzymatic steps involved in prostaglandin synthesis, thielavin A specifically inhibited the conversion of arachidonic acid into prostaglandin H2, while prostaglandin E2 synthesis from the endoperoxide was the most sensitive to thielavin B. Thromboxane A2 synthesis from prostaglandin H2 in bovine platelet microsomes were inhibited by 50% at concentrations of 150 and 350 microM of thielavin A and B, respectively. Thielavin B was significantly effective on carrageenan-induced oedema of rats when administered intravenously but on on oral administration. The anti-inflammatory activity was not detectable with thielavin A either on intravenous injection or on oral administration.

As part of our ongoing investigation of filamentous fungi for anticancer leads, an active fungal extract was identified from the Mycosynthetix library (MSX 55526; from the Order Sordariales). Bioactivity-directed fractionation yielded the known ergosterol peroxide (2) and 5α,8α-epidioxyergosta-6,9(11),22-trien-3β-ol(3), and a new benzoate trimer, termed thielavin B methyl ester (1). The structure elucidation of 1 was facilitated by the use of HRMS coupled to an APPI (atmospheric pressure photoionization) source. Compound 1 proved to be moderately active against a panel of three cancer cell lines.