Nitrite binds reversibly to the ferriheme pro-
teins metmyoglobin and methemoglobin in aqueous
bu er solution at a physiological pH of 7.4. The spectral
changes recorded for the formation of metMb2NO
2
)
di er signi®cantly from those observed for the nitrosy-
lation of metMb, which can be accounted for in terms of
the di erent reaction products. Nitric oxide binding to
metMb produces a nitrosyl product with Fe2II)-NO
+
character, whereas the reaction with nitrite produces an
Fe2III)-NO
2
±
complex. The kinetics of the binding and
release of nitrite by metMb and metHb were investigated
by stopped- ̄ow techniques at ambient and high pres-
sure. The kinetic traces recorded for the reaction of ni-
trite with metMb exhibit excellent single-exponential ®ts,
whereas nitrite binding to metHb is characterized by
double-exponential kinetics which were assigned to the
reactions of the
a
-and
b
-chains of metHb with NO
2
.
The rate constants for the binding of nitrite to metMb
and metHb were found to be much smaller than those
reported for the binding of NO, such that nitrite impu-
rities will not a ect the latter reaction. The activation
parameters 2
D
H
6
;
D
S
ne
;
D
V
6
) obtained from the tem-
perature and pressure dependence of the reactions sup-
port the operation of a dissociative mechanism for the binding and release of nitrite, similar to that found for
the binding and release of NO in metMb.