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Hb encapsulation in vesicles retards the reaction with NO, but not CO, by intracellular diffusion barrier
Author(s) -
Sakai Hiromi,
Tsuchida Eishun
Publication year - 2008
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.22.1_supplement.749.1
Subject(s) - chemistry , intracellular , biophysics , vesicle , diffusion , extracellular , membrane , biochemistry , thermodynamics , biology , physics
NO‐binding of Hb is restricted by encapsulation in RBCs to prevent vasoconstriction. To clarify the mechanism, we analyzed Hb‐vesicles (HbV) with different intracellular Hb concentrations, [Hb] in , and different particle sizes using stopped‐flow spectrophotometry. The apparent NO‐binding rate constant, k′ NO , of HbV at [Hb] in = 1 g/dl was almost equal to k NO of molecular Hb, indicating that the lipid membrane is not a barrier. With increasing [Hb] in to 35 g/dl, k′ NO decreased to 1/3, which was further decreased to 1/5 with enlarging particle diameter (<450 nm). For CO‐binding, which is intrinsically much slower than NO‐binding, k′ CO did not change greatly with [Hb] in and the particle diameter. Results obtained using diffusion simulations coupled with elementary reactions concur with these tendencies and clarify that NO is trapped rapidly by Hb from the interior surface area to the core of HbV at a high [Hb] in , retarding NO‐diffusion toward the core of HbV. In contrast, slow CO‐binding allows time for further CO diffusion to the core. Simulations extrapolated to a large particle (8 μm) showing retardation even for CO‐binding. The obtained k′ NO and k′ CO yield similar values to those reported for RBCs. In summary, the intracellular, not extracellular, diffusion barrier is predominant owing to the rapid NO‐binding that induces a sink of NO from the interior surface to the core, retarding further NO‐diffusion and binding.