Bioactive Secondary Metabolites from Marine and Terrestrial Bacteria: Isoquinolinequinones, Bacterial Compounds with a Novel Pharmacophor

Abstract

Antibiotics are defined [5,6] as low-molecular weight (MW < 2000 Dalton) secondary metabolites from natural sources including their chemically or biosynthetically produced derivatives, which show inhibition of the growth of higher organisms (e.g. tumour cells) or pathogens (e.g. bacteria, fungi, viruses) at low concentration, and subsequently can be used to cure the infectious diseases. Not all natural products serve as antibiotics, many of them function as signalling substances between micro- 4Recently, chemists on a worldwide basis have paid attention towards the potential of marine microorganisms (e.g. bacteria, fungi, blue green algae, dinoflagellates etc.), as an alternative source for isolation of novel metabolites with interesting biological and pharmaceutical properties [20,21] . Most metabolites of bacterial origin come from one group, Gram-positive soil bacteria of the order Actinomycetes.1993 [4] .It was expected that marine bacteria might produce compounds, which differ basically from terrestrial products, as it is known from spongial metabolites. Many metabolites of marine microorganisms, especially those of Streptomycetes are, however, similar to or identical with those of terrestrial origin, and it is not easy to decide if a microorganism is truly marine. One of the principal difficulties appears to be therefore the definition of a marine microorganism. It was reported recently, that the marine microbial Picoplankton contains a high abundance of rare species, virtually none of which has ever been isolated and chemically investigated [27] . It can be expected that truly marine bacteria can be found in this or related sources. Fenical postulated that the newly described genus Salinospora is a rare Actinomycete restricted to the Pentabromopseudilin (9), an antibiotic and cytotoxic brominated marine natural product, was isolated from Alteromonas luteoviolaceus. 70 % of the compound's molecular weight consists of bromine [44][45][46] . Due to its highly toxic effects, it could not be used as medicine. The chloro analogue 10 was isolated in 1978 from the terrestrial Actinoplanes sp. ATCC 33002 [47] . A recent study carried out on 136 marine 19 1.6 The Phylogenetic Tree of Bacteria Bioactive compounds have been reported from marine bacteria belonging to the genus Pseudoalteromonas, Cytophaga, Alteromonas, Micrococcus, Bacillus, Acinetobacter, Agrobacterium, Pseudomonas [40] as well as Streptomycetes. Presently, there are two microbial phylogenetic hot spots known for the production of secondary metabolites: 24Zeeck and co-workers [69] have isolated 3-(4'-hydroxyphenyl)-4-phenylpyrrole-2, 5-dicarboxylic acid (25a), 3,4-di(4'-hydroxyphenyl)pyrrole-2,5-dicarboxylic acid (25b) and 7-hydroxy-2H-benzo [1,4]thiazin-3-one (26) from the culture broth of the North Sea strain RK377 fermented on MB medium with artificial sea water. From the same strain on SJ medium, two new imidazole and pyrimidin derivatives, namely glusun I ( 27) and glusun II (28), were isolated. Continuing these investigations, the same group...