Pr. Gardner et al., Nitric-oxide dioxygenase activity and function of flavohemoglobins - Sensitivity to nitric oxide and carbon monoxide inhibition, J BIOL CHEM, 275(41), 2000, pp. 31581-31587
Widely distributed flavohemoglobins (flavoHbs) function as NO dioxygenases
and confer upon cells a resistance to NO toxicity. FlavoHbs from Saccharomy
ces cerevisiae, Alcaligenes eutrophus, and Escherichia coli share similar s
pectra, O-2, NO, and CO binding kinetics, and steady-state NO dioxygenation
kinetics. Turnover numbers (V-max) for S. cerevisiae, A. eutrophus, and E.
coli flavoHbs are 112, 290, and 365 NO heme(-1) s(-1), respectively, at 37
degrees C with 200 mu M O-2. The K-M values for NO are low and range from
0.1 to 0.25 mu M. V-max/K-M(NO) ratios of 900-2900 mu M-1 s(-1) indicate an
extremely efficient dioxygenation mechanism. Approximate K-M values for O-
2 range from 60 to 90 mu M. NO inhibits the dioxygenases at NO:O-2 ratios o
f greater than or equal to 1:100 and makes true K-M(O-2) values difficult t
o determine, High and roughly equal second order rate constants for O-2 and
NO association with the reduced flavoHbs (17-50 mu M-1 s(-1)) and small NO
dissociation rate constants suggest that NO inhibits the dioxygenase react
ion by forming inactive flavoHbNO complexes. Carbon monoxide also binds red
uced flavoHbs with high affinity and competitively inhibits NO dioxygenases
with respect to O-2 (K-I(CO) = similar to 1 mu M). These results suggest t
hat flavoHbs and related hemoglobins evolved as NO detoxifying components o
f nitrogen metabolism capable of discriminating O-2 from inhibitory NO and
CO.