title: Phosphorylation of cytochrome-P-450-dependent monooxygenase components.
abstract: Most chemical carcinogens require activation by polysubstrate monooxygenase. The phosphorylation of essential components of this cytochrome P-450 monooxygenase system, isolated from rabbit liver microsomes, cytochrome P-450 (LM2) and cytochrome reductase, was tested using two different protein kinases. One of the kinases, a cyclic AMP-independent phosvitin kinase (kinase P), was inactive in all systems tested. However, the catalytic subunit of a cyclic AMP-dependent protein kinase (kinase C) catalyzed phosphoryl group transfer to both proteins, but to different extents. Cytochrome P-450 was phosphorylated when added as sole component and also when in the presence of P-450 reductase and phosphatidylcholine. In contrast, the weak phosphorylation of P-450 reductase was reduced considerably in a complete reconstituted system containing P-450 and phosphatidylcholine. The inclusion of kinase P did not alter these results which excludes the possibility that these kinases participate in a sequential phosphorylation mechanism. The monooxygenase constituents themselves were without kinase activity. When hepatic microsomes were isolated in presence of the phosphatase inhibitor sodium fluoride no significant change in monooxygenase (7-ethoxycoumarin O-deethylation) activity was observed, whilst after preincubation with either acid or alkaline phosphatase a significant reduction in monooxygenase activity was measured. Thus, cytochrome P-450 (LM2) is phosphorylatable by protein kinase C and the catalytic activity of polysubstrate monooxygenase decreases after preincubation of microsomes with phosphatases.
mesh_terms: D000135:Acid Phosphatase / Q000494:pharmacology; D000469:Alkaline Phosphatase / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003579:Cytochrome Reductases / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism; D010105:Oxygenases / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000494:pharmacology; D012969:Sodium Fluoride / Q000494:pharmacology
chemical_list: D012969:Sodium Fluoride; D003577:Cytochrome P-450 Enzyme System; D010105:Oxygenases; D003579:Cytochrome Reductases; D011494:Protein Kinases; D000469:Alkaline Phosphatase; D000135:Acid Phosphatase