Incorporation of ubiquitin into the rat brain mitochondria is accompanied by increased proteolytic digestibility of MAO.

Incubation of rat brain mitochondria with ubiquitin followed by mitochondria sedimentation was accompanied by reduction of ubiquitin content in the supernatant only when ATP was included into the incubation mixture. Subsequent incubation of resedimented mitochondria revealed higher sensitivity to trypsin of MAO-A in ubiquitin-incorporated mitochondria. In control mitochondria (initially incubated with ATP) 0.5 mg/ml trypsin caused a decrease of MAO-A activity by 32.2 +/- 4.2%, whereas in ubiquitin-incorporated mitochondria (initially incubated with ATP + ubiquitin) reduction of MAO-A activity was significantly higher (51.4 +/- 2.5%, p < 0.02). Activity of MAO-B was resistant to trypsinolysis and incorporation of ubiquitin did not influence sensitivity of MAO-B to trypsin. Although there is no direct evidence yet that mitochondrial MAO is a target for ubiquitination the increased sensitivity to trypsinolysis of MAO-A suggests that incorporation of ubiquitin into mitochondria may increase susceptibility of MAO to certain proteases involved into degradation of these enzymes.