Nitrite reduction by Co(II) and Mn(II) substituted myoglobins: towards understanding necessary components of Mb nitrite reductase activity.

Nitrite reduction to nitric oxide by heme proteins is drawing increasing attention as a protective mechanism to hypoxic injury in mammalian physiology. Here we probe the nitrite reductase (NiR) activities of manganese(II)- and cobalt(II)-substituted myoglobins, and compare with data obtained previously for the iron(II) analog wt Mb(II). Both Mn(II)Mb and Co(II)Mb displayed NiR activity, and it was shown that the kinetics are first order each in [protein], [nitrite], and [H(+)], as previously determined for the Fe(II) analog wt Mb(II). The second order rate constants (k(2)) at pH 7.4 and T=25 °C, were 0.0066 and 0.015 M(-1)s(-1) for Co(II)Mb and Mn(II)Mb, respectively, both orders of magnitude slower than the k(2) (6M(-1)s(-1)) for wt Mb(II). The final reaction products for Mn(II)Mb consisted of a mixture of the nitrosyl Mn(II)Mb(NO) and Mn(III)Mb, similar to the products from the analogous NiR reaction by wt Mb. In contrast, the products of NiR by Co(II)Mb were found to be the nitrito complex Co(III)Mb(ONO(-)) plus roughly an equivalent of free NO. The differences can be attributed in part to the stronger coordination of inorganic nitrite to Co(III)Mb as reflected in the respective M(III)Mb(ONO(-)) formation constants K(nitrite): 2100 M(-1) (Co(III)) and <~0.4M(-1) (Mn(III)). We also report the formation constants (3.7 and 30 M(-1), respectively) for the nitrite complexes of the mutant metmyoglobins H64V Mb(III)(NO(2)(-)) and H64V/V67R Mb(III)(ONO(-)) and a K(nitrite) revised value (120 M(-1)) for the nitrite complex of wt metMb. The respective K(nitrite) values for the three ferric proteins emphasize the importance of a H-bonding residue, such as His64 in the Mb(III) distal pocket or the Arg67 in H64V/V67R Mb(III), in stabilizing nitrite coordination. Notably, the NiR activities of the corresponding ferrous Mbs follow a similar sequence suggesting that nitrite binding to these centers are analogously affected by the H-bonding residues.