. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 74 J. E. DOELLER AND D. W. KRAUS times will be displayed by hemoglobins with low dissoci- ation rate constants. For example, the hemoglobin ofAs- caris lumbricoides would require more than 15 min to deoxygenate, given its rate constant ( s_1) deter- mined by stopped-flow techniques (Gibson and Smith, 1965). The possibility that neurohemoglobin may possess low molecular dissociation rates was investigated by us- ing CO or dithionite to deoxygenate the neurohemoglo- bin. Because neurohemoglobin exhibits a much higher affi
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 74 J. E. DOELLER AND D. W. KRAUS times will be displayed by hemoglobins with low dissoci- ation rate constants. For example, the hemoglobin ofAs- caris lumbricoides would require more than 15 min to deoxygenate, given its rate constant ( s_1) deter- mined by stopped-flow techniques (Gibson and Smith, 1965). The possibility that neurohemoglobin may possess low molecular dissociation rates was investigated by us- ing CO or dithionite to deoxygenate the neurohemoglo- bin. Because neurohemoglobin exhibits a much higher affinity for CO than oxygen (Table I), CO preferentially binds to any heme iron exposed by oxygen dissociation. Oxygen reassociation is thus prevented and the rate of oxygen dissociation in situ can be approached, limited mostly by the rate of CO diffusion into the connective. Under these conditions, deoxygenation times were greatly reduced (see Fig. 5A, curve c). Dithionite, which scavenges oxygen, also greatly reduced deoxygenation times. The neurohemoglobin. therefore, probably does not have an exceptionally low molecular dissociation rate, and oxygen unloading from the connective is not reaction-limited. The unloading time of a diffusion-limited reaction is controlled by the combined effect of the diffusion rate of oxygen out of the region plus the number of dissociations and reassociations that take place before oxygen reaches the exterior (Colacino et ai, 1987). The diffusion rate can be limited by a specific diffusion boundary located between the hemoglobin and the exterior. Moreover, the reassociation of oxygen to deoxyhemoglobin is more likely when hemoglobin concentration is high and when oxygen permeability of the diffusion boundary is low. These factors will lengthen oxygen unloading times. Pre- liminary data indicate that unloading times are shorter when neurohemoglobin concentration is effectively low- ered by partial inactivation with CO. The time for oxygen unloadin
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
