title: Effect of hypoxia and CO on a cytochrome P-450-mediated reaction in rabbit lungs.
abstract: Evidence indicates that the diffusion of O2 and CO in tissue may be facilitated by a carrier molecule having a P50 that approximates tissue O2 partial pressure (PO2; 1-15 Torr) and a much higher affinity for CO than for O2. To determine whether cytochrome P-450 in lung satisfies these criteria, we measured the effect of hypoxia and of CO on the rate of metabolism of the cytochrome P-450 mediated O-demethylation of p-nitroanisole in isolated perfused rabbit lungs. Metabolism was inhibited by 50% of a control at an estimated tissue PO2 fo 4 Torr (5.5 microM). When inspired CO2 was kept at 200 Torr and inspired CO partial pressure (PCO) varied an estimated tissue PCO/PO2 ratio of 0.025 reduced the reaction rate by 50% of control, but some metabolism persisted at PCO/PO2 ratios larger than one. The relationship between reaction rate and PCO/PO2 ratio could not be fit by a single value for Haldane constant for M (CO affinity/O2 affinity) but could be described with a two-component model in which metabolism was equally divided between a high-affinity cytochrome (M = 200) and a low-affinity cytochrome (M = 2). These findings suggest that cytochrome P-450 could act as a carrier for O2 and CO in tissue with low PO2's.
mesh_terms: D000818:Animals; D002248:Carbon Monoxide / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003640:Dealkylation; D000860:Hypoxia / Q000201:enzymology*; D066298:In Vitro Techniques; D007700:Kinetics; D008168:Lung / Q000201:enzymology*; D008297:Male; D009575:Nitroanisole O-Demethylase / Q000378:metabolism; D009596:Nitrophenols / Q000032:analysis; D010101:Oxygen Consumption; D011817:Rabbits; D013997:Time Factors
chemical_list: D009596:Nitrophenols; D002248:Carbon Monoxide; D003577:Cytochrome P-450 Enzyme System; D009575:Nitroanisole O-Demethylase; C024836:4-nitrophenol