A study of rat liver guanylate cyclase activation by peroxides of fatty acids, carbonyl compounds and biogenic amines

Abstract

(1) Preincubation of rat liver microsomes at 37°C results in an elevation of the level of malonaldehyde, indicating that the process of lipoperoxidation in microsomal membranes is underway. A joint incubation of the preparation of rat liver ‘soluble’ guanylate cyclase with rat liver microsomes preincubated at different times leads to a 2–3-fold activation of the soluble enzyme. An enhancement of the peroxidation rate in microsomes (by administration of Fe 2+) contributes to a still greater increase in the activity (7–8-fold) of the soluble form of guanylate cyclase as revealed by joint incubation of this preparation with Fe 2+ preincubated microsomes. A reduction of lipoperoxidation in liver microsomes of those rats whose diet contained an increased content of α-tocopherol resulted in a decrease in activation of soluble guanylate cyclase on its joint incubation with microsomes. The accumulation of malonaldehyde in microsomes and activation of guanylate cyclase could be prevented by administration of dithiothreitol. (2) Both the soluble and membrane-bound forms of rat liver guanylate cyclase are activated by hydroperoxides of fatty acids at concentrations of not less than 10 −6 M. The destruction of fatty acid hydroperoxides (with the formation of carbonyl compounds) following incubation with the guanylate cyclase cofactor Mn(II) was noted. (3) Aliphatic carbonyl compounds (aldehydes and ketones) activate the soluble form of guanylate cyclase, the concentration resulting in half-maximum activation being 3–5ṡ10 −9 M. Carbonyl compounds with less than 9 carbon atoms are effective, i.e., acetaldehyde, isobutyraldehyde, heptanal and acetone, activate guanylate cyclase whilst nonanal and methyl heptyl ketone fail to activate this enzyme. Membrane-bound (microsomal) guanylate cyclase is also activated by carbonyl compounds, including nonanal and methyl heptyl ketone, with the concentration of carbonyl compounds resulting in half-maximum activation being 1–ṡ10 −8 M. (4) Blocking of SH groups within the preparation of soluble guanylate cyclase by means of N-ethylmaleimide completely interferes with the carbonyl activation of the enzyme, decreasing the activation by fatty acid hydroperoxide. This indicates the involvement of protein SH groups in the activation of guanylate cyclase by carbonyl compounds and fatt acid hydroperoxides. (5) It was found that activation of guanylate cyclase by biogenic amines (serotonin, dopamine) in mouse liver homogenates was observed after a lag period, the presence of membrane components of the cell being required for activation. Inhibitors of enzymic oxidative deamination of amines prevent activation. Stable products of deamination of amines do not influence the activity of guanylate cyclase. Presumably, activation occurs in a cell-free system under the action of monoamine oxidase. (6) The role of lipoperoxidation in the change of guanylate cyclase activity is discussed.