Evaluation of kinetic constants for mixed inhibitors of cholinesterase

Anticholinesterase activity of the fusarium metabolite visoltricin and its N-methyl derivative

Visoltricin is a bioactive 4,5-disubstituted N-methylimidazole produced by Fusarium tricinctum, which shows toxic activity towards Artemia salina larvae and miotic activity on rabbit eye. The anticholinesterase properties and kinetic parameters of visoltricin and N-methyl visoltricin were assessed. Visoltricin anticholinesterase activity was demonstrated on human serum [median inhibitory concentration (IC(50)) = 2.6 x 10(-4)m], bovine erythrocyte acetylcholinesterase EC 3.1.1.7 (IC(50) = 4 x 10(-4)m) and human serum cholinesterase EC 3.1.1.8 (IC(50) = 1.9 x 10(-4)m). The acetylcholinesterase inhibition constants (K(i)) of visoltricin and N-methyl visoltricin were 1.88 x 10(-4)m, respectively. Kinetic studies showed for both compounds a mixed-type inhibition, with partial binding of the inhibitor to the enzyme-substrate complex. Visoltricin was a reversible inhibitor of acetylcholinesterase, as it was not hydrolysed by the inhibited enzyme, and the inhibition in vitro was concentration dependent but not time dependent. The anticholinesterase activity could explain, at least in part, the miotic activity of visoltricin.

Visoltricin, a novel biologically active compound produced by Fusarium tricinctum

The major compound responsible for toxicity to Artemia salina of some Fusarium tricinctum strains has been isolated, and its structure has been elucidated by spectroscopical methods, i.e. UV, IR, MS, 1H-NMR and 13C-NMR. The novel compound, trivially named visoltricin, is the first imidazole derivative produced by Fusarium spp., and its structure has been established as the methyl ester of 3-[1-methyl-4-(3-methyl-2-butenyl)-imidazol-5yl]-2-propenoic acid (molecular formula C13H18N2O2; MW = 234.297). Visoltricin was toxic to A. salina larvae (LD50 = 8.5 x 10(-7) M), and inhibited the growth of six human tumour cell lines (out of 60 lines tested) at concentrations lower than 10(-5) M. Tested on rabbit eye it showed an interesting miotic activity similar to that of pilocarpine, a miotic agent largely used in the therapy of glaucoma. This biological activity could be explained in part by the anticholinesterase properties shown by visoltricin towards both human serum and pure enzymes (EC 3.1.1.7 and EC 3.1.1.8). Kinetics studies showed for visoltricin a mixed-type and reversible inhibition of the EC 3.1.1.7 enzyme with the competitive inhibition constant (Ki) = 1.9 x 10(-4) M.

Modification of brain acetylcholine release by morphine and its antagonists in normal and morphine-dependent rats

1 The spontaneous release of acetylcholine (ACh) from the cerebral cortex of control and morphine-dependent rats was investigated. The rate of resting output of ACh in morphine-dependent animals was lower than that in the control animals.2 Administration of naloxone and nalorphine to morphine-dependent rats was followed by a significant rise in the release of cortical ACh. In control rats no such increase in the release of ACh occurred after similar injections of narcotic antagonists.3 Injections of morphine produced a consistent decrease in the rate of spontaneous release of cortical ACh in the control rats, but similar injections in the dependent rats did not produce a decrease in the rate of cortical ACh release.4 The relevance of these results with regard to development of the narcotic abstinence syndrome is discussed.