Effect of lipid-depletion on the different forms of monoamine oxidase in rat liver mitochondria

Abstract

In order to study the possible role of phospholipids in the genesis of different forms (‘A’ and ‘B’) of monoamine oxidase (MAO), rat liver mitochondrial monoamine oxidase was compared in mitochondria before and after lipid-depletion by extraction with aqueous methyl ethyl ketone with respect to substrate specificity and inhibitor sensitivity. With serotonin (substrate for the ‘A form’ of the enzyme) 7 per cent of the activity in the mitochondrial preparation was recovered after extraction, while 80 per cent was recovered with β-phenylethylamine (substrate for the ‘B form’ of the enzyme) and 3 per cent with tyramine (which is supposed to be a substrate for both forms). A comparison of the sensitivity to the inhibitors clorgyline (‘A form’ inhibiting) and deprenil (‘B form’ inhibiting) before and after extraction also showed that the ‘B form’ of the enzyme was almost exclusively recovered in the lipid-depleted residue. From extraction experiments performed on mitochondria with either the ‘A’ or the ‘B form’ of the enzyme selectively inhibited with clorgyline or deprenil, respectively, it could be concluded that no transformation of the ‘A form’ into ‘B form’ occurred as a result of the extraction. After extraction of mitochondria in which both forms of monoamine oxidase had been labelled with the irreversible inhibitor [ 14C]pargyline most of the radioactivity was found in the lipidzdepleted residue. This indicates that the ‘A form’ was not liberated from the membranes by the extraction, but was still present in the membrane residues in an inactivated state. The results do not support the hypothesis that the multiple functional forms of monoamine oxidase are explained by the binding of different amounts of membrane material to one single enzyme species.