Cytochrome P450-dependent metabolism of arachidonic acid.

The cytochrome P450-dependent metabolism of arachidonic acid, the mechanisms of regulation of stereo- and regiospecificity by cytochrome P450 isoenzymes, and the biological relevance of metabolites of the arachidonic acid cascade is discussed in this review.

Eicosapolyynoic acids, inhibitors of lipoxygenases, weaken the short-term plasticity of cholinoreceptors of neurons of the edible snail.

The influence of three polyacetylenic analogs of natural polyenoic acids which are inhibitors of their lipoxygenase oxidation on the dynamics of the extinction of the inward current induced by repeated iontophoretic applications of acetylcholine to the soma was investigated in identified RPa3 and LPa3 of the edible snail using the method of bielectrode recording of the potential on the membrane. It was found that eicosa-5,8,11,14-tetraynoic acid (30-60 mumole/liter) and eicosa-5,8,11,14,17-pentaynoic acid (4-50 mumole/liter) decrease the amplitude of the inward current induced by application of acetylcholine, and attenuate its extinction during repeated applications. The third analog, eicosa-8,11,14-triaynoic acid, does not exert a modulating influence on the magnitude of the current and its extinction. It was hypothesized that the lipoxygenase metabolites of the polyenoic acids regulate the plasticity of the cholinoreceptors of the neurons of the edible snail. Taking the nonidentical inhibition by the compounds used of the various lipoxygenases into account, the participation in the plasticity of the cholinoreceptors of those eicosanoids which are formed from arachidonic acid under the influence of 5-lipoxygenases is most probable. At the same time, a regulatory role of the eicosanoids formed under the influence of other lipoxygenases is not excluded.

Keto fatty acids not containing doubly allylic methylenes are lipoxygenase substrates.

The soybean lipoxygenase I oxygenates the unusual substrate 12-keto-(9Z)-octadecenoic acid methyl ester as indicated by oxygen uptake and spectral changes of the incubation mixture. The main oxygenation products have been isolated by HPLC and identified as 9,12-diketo-(10E)-octadecenoic acid methyl ester and 12-keto-(10E)-dodecenoic acid methyl ester by UV and IR spectroscopy, cochromatography with an authentic standard, gas chromatography/mass spectroscopy, and 1H NMR. In the formation of both compounds the oxygenase and hydroperoxidase activities of the enzyme appear to be involved. These data and the earlier results on the oxygenation of furanoic fatty acids (Boyer et al., 1979) indicate that the lipoxygenase reaction is not restricted to substrates containing a 1,4-pentadiene structure.

Inactivation of 15-lipoxygenases by acetylenic fatty acids.

The inactivation of soybean lipoxygenase-1 and of rabbit reticulocyte lipoxygenase by five selected acetylenic fatty acids was studied. In all cases the inactivation was time-consuming and depended on the concentration of the inactivator. The inactivation kinetics was measured and the data were fitted to a kinetic model based on the assumption of catalytic self-inactivation. The kinetic constants (Km-value and inactivation rate k2) calculated indicated that 7,10,13-eicosatrienoic acid was the most powerful inactivator for the soybean enzyme followed by 8,11,14-eicosatrienoic acid. The occurrence of an additional triple bond between C-4 and C-5 or between C-5 and C-6 strongly reduced the suicidal rate. With the reticulocyte enzyme, only small differences in the reactivities towards various acetylenic fatty acids have been observed.