The physiological significance of the cooperativity of human hemoglobin (Hb) is considered from the viewpoint of the effectiveness of the Bohr shift at the sites of O2 release and uptake across the placental membrane. The effects of the Bohr shift was examined by changing the O2 saturation of Hb (S(pO2)) per unit change in P50, -dS(PO2)/dP50, where P50 is partial pressure of O2 at half saturation. The Bohr shift at the sites of O2 uptake and release was found to be highly effective in both fetal and maternal bloods at physiological degree of cooperativity (Hill's coefficient, n=2.65). From the results obtained in this paper, it is concluded that the positions of OECs of fetal and maternal Hbs are regulated to receive a maximal benefit from the Bohr shift, and that a relatively low n value of human tetrameric Hb is adequate for the O2 and CO2 exchange across the placental membrane.
INTRODUCTION
Hemoglobin (Hb) combines with O2 reversibly. The oxygen equilibrium curve (OEC) of Hb, that is a plot of O2 saturation (S) vs. partial pressure of O2 (PO2 or P), is characterized by its position and degree of sigmoidicity. The position or O2-affinity of the OEC is represented by the oxygen pressure at half saturation (P50), while the sigmoidicity or cooperativity can be expressed by Hill's coefficient (nmax) as the highst slope of the Hill plot of log(S/(1–S)) vs. logP (Hill, 1910).
The O2 affinity of Hb can be modified by carbon dioxide. An increase in the partial pressure of carbon dioxide (PCO2) with a concomitant decrease in pH reduces the O2 affinity of Hb. This modulation is known as the “classical Bohr effect” (Bohr et al., 1904). Changes in pH without those in PCO2can also modulate O2 affinity, and this effect is simply called the “Bohr effect”. The magnitude of the Bohr effect is given by the change in log P50 per unit change in pH (Δlog P50/ΔpH) as Bohr coefficient. In the physiological pH range, the Bohr coefficient for human Hb is −0.48 (Severinghaus, 1966). The rightward shift of the OEC upon the lowering of pH (the Bohr shift) facilitates the release of O2 into fetal tissues without any change in the ambient O2 pressure. The additional amount of O2 released from Hb by the Bohr shift depends not only on the magnitude of the Bohr coefficient, but also on the position and the steepness of the OEC. In our previous study, the Bohr shift-dependent additional amount of O2released from human adult Hb in the venous blood (at PO2=40 torr) was calculated in order to estimate the effectiveness of the Bohr shift (Itoh et al., 2001). As a result, the position of the OEC of human adult Hb was found to be optimized so as to receive a maximal benefit from the Bohr effect.
In the case of fetus, the PO2 condition of the blood is restricted to a very low and narrow range (from 35 to 15 torr) compared to that of the maternal blood. Therefore, the role of the Bohr effect in fetal respiration seems to be especially important. The lowering of pH caused by the production of CO2 in tissues induces a rightward shift of the OEC, which in turn facilitates the release of O2 from the Hb. The diffusion of CO2 from fetal blood into maternal blood across the placental membrane causes such a shift to the maternal blood OEC, promoting the release of O2 from maternal blood. On the other hand, a reduction in fetal blood PCO2 causes a leftward shift of the fetal blood OEC. The simultaneous rightward shift of the maternal blood OEC (with consequent decrease in O2 saturation) and leftward shift of the fetal OEC (with consequent increase in O2 saturation) facilitate the diffusion of O2 from maternal blood to fetal blood. This phenomenon is known as the “double Bohr effect”. It is thus important to examine the effectiveness of the Bohr shift at O2 uptake site in fetal blood and compare it with that at O2release site in maternal blood.
Under physiological conditions, the O2 affinity of Hb may be largely altered, but the cooperativity essentially remains constant. The physiological significance of the magnitude of cooperativity and its constancy in tetrameric Hb has not yet been fully explained. The aim of this paper is to present a theoretical treatment for the influence of cooperativity on the Bohr shift in O2 uptake and release process in the placenta, and to understand the physiological significance of the rather low cooperativity of the tetrameric Hb.
MATERIALS AND METHODS
All the OEC data used in this study were taken from the previously published data for human fetal and adult Hb solutions measured under various experimental conditions (Imai, 1982; Imai and Yonetani, 1975; Imaizumi et al., 1982; Tyuma et al., 1973). These OEC data can be described by Adair's intermediate compound theory (Adair, 1925). According to the theory, the O2 saturation of Hb is expressed as a function of p as follows:
where a1=4k1, a2=6k1k2, a3=4k1k2k3 and a4=k1k2k3k4 Here, ki (i=1 to 4) differs from Ki (i=1 to 4) defined by Adair in the sense that the former is corrected for the statistical factor (intrinsic Adair constant).
Hypothetical OECs with arbitrary P50 values were constructed from the published set of four Adair constant values which were varied by multiplication with a common factor, i.e. ki·constant. By doing this, the P50 value of the hypothetical OEC was varied without any change in shape.
The effect of changes in cooperativity (n) on the Bohr shift was investigated in a wide range of n value, using Hill's empirical equation:
where n is a conventional expression of cooperativity.
The effectiveness of the Bohr shift in maternal blood under physiological conditions was examined using the standard OEC for human whole blood with the P50 value of 26.7 torr and n value of 2.65. The Adair constant values for the standard OEC are: k1=0.0037 torr−1, k2=0.047 torr−1, k3=0.012 torr−1, and k4=1.1 torr−1 (Mohammed Mawjood and Imai, 1999; Imai, personal communication).
For fetal Hb solution, only a few accurate OEC data are available (Tyuma et al., 1973). However, it is generally thought that there is no difference in shape between the fetal Hb OEC and the adult Hb OEC (Allen et al., 1953). To construct the fetal Hb OECs, therefore, the OECs for adult Hb solution obtained under various experimental conditions were employed by changing their position but keeping their shape unchanged. The OEC for fetal blood was constructed from the standard OEC for adult blood, as well.
The normal blood PO2 values used for calculations in the present study are as follows: maternal arterial PO2 at rest, 100 torr; maternal uterine venous PO2, 40 torr; fetal umbilical venous PO2(arterialzed fetal blood), 35 torr; fetal umbilical arterial PO2(“venous blood” coming from the fetus to the placenta), 15 torr. The P50 values for fetal and maternal blood are 20 and 26 torr (Battaglia and Meschia 1986; Dejours 1975)), respectively. In the placental circulation, “arterial blood” and “venous blood” mean the blood flowing through the umbilical vein and the umbilical artery, respectively.
All computations were performed on a personal computer (Model PC-VC500, Nippon Electric Co., Tokyo, Japan) using MS-FORTRAN.
RESULTS AND DISCUSSION
Contribution of the Bohr effect to oxygen transport by human blood
Fig. 1 illustrates the contribution of the Bohr shift to the release and uptake of O2 in human blood. The OEC of fetal Hb (P50=20 torr, A) lies to the left of that of maternal Hb (P50=26 torr, C). B and D represent the OECs right-shifted by the Bohr effect. The additional amount of O2 delivered to tissues by fetal blood as a result of the Bohr shift was estimated from a decrease in O2 saturation at PO2 of 15 torr (ΔS(15A–15B) (①)). Hence, the total amount of O2 transported to fetal tissues by fetal blood was ΔS(35A–15B). At the placenta, the additional amount of O2 loading to fetal Hb by the Bohr shift was ΔS(35A–35B) (②). In maternal blood, the additional amount of O2 released from maternal Hb by the Bohr shift was ΔS(40C–40D) (③), and the additional amount of O2loading to the Hb caused by the Bohr shift was ΔS(100C–100D) (④).
Fig. 1
Graphical expression of additional amounts of O2 unloading and loading caused by the Bohr shift in human fetal and maternal blood, respectively. Solid line A stands for fetal arterial blood, and broken line B for fetal venous blood. Solid line C stands for maternal arterial blood, and broken line D for maternal uterine venous blood. ΔS(15A–15B) (①) and ΔS(40C–40D) (③) represent the additional amount of O2 released from fetal blood (P50=20 torr) and maternal blood (P50=26 torr), respectively, as a result of the Bohr shift. ΔS(35A–35B) (②) and ΔS(100C–100D) (④) represent the Bohr shift-dependent additional amount of O2 loaded to fetal and maternal blood, respectively. ΔS(35A–15B) represents the total O2 delivered by fetal blood to fetal tissues in the presence of the Bohr shift. The human adult standard OEC measured under physiological conditions by Mohammed Mawjood and Imai (1999) was used for generating the necessary OECs.
Influence of the position of the OEC on the effectiveness of the Bohr shift in fetal umbilical venous and arterial blood
Fig. 2 shows the theoretically derived effects of P50 on O2 saturation of Hb (S(PO2), dashed lines) and the effectiveness of the Bohr shift (dS(PO2)/dP50, solid lines) in the fetal placental circulation. The degree of the effectiveness of the Bohr shift in the “venous blood” and “arterial blood” was estimated from the slope of the S(15) vs. P50 plot and S(35) vs. P50 plot, respectively. The slope (dS(PO2)/dP50) is usually negative, because S(PO2) decreases with increases in P50. The –dS(PO2)/dP50 vs. P50 curves are bell-shaped having a single maximum. The highest effectiveness of the Bohr shift occurred at P50 of 12 torr in “venous blood” and at P50 of 28 torr in “arterial blood”. At these P50 values, the Bohr shift can achieve maximal action. Hence, the P50 values of 12 and 28 torr are the optimal P50 for the effectiveness of the Bohr shift for fetal umbilical “venous blood” and “arterial blood”, respectively.
Fig. 2
Oxygen saturation of Hb as a function of P50 and the effectiveness of the Bohr shift of fetal umbilical blood in the placental circulation. The dashed lines represent the O2 saturation of Hb (S(PO2)) in fetal blood with PO2 of 15 and 35 torr. The solid lines represent the effectiveness of the Bohr shift in fetal umbilical “venous blood”, –dS(15)/dP50, and that of fetal umbilical “arterial blood”, –dS(35)/dP50. Double-headed arrow indicates the physiological P50 of fetal blood. Calculation was carried out using the human adult standard OEC.
As shown by the double-headed arrow in Fig. 2, the effectiveness of the Bohr shift in fetal blood with physiological P50 value at the site of O2 release (–dS(15)/dP50 (20)) is slightly higher than that at the site of O2 uptake (–dS(35)/dP50 (20)), but it is of interest that there is only a small difference between –dS(35)/dP50 (20) and that of the maximal –dS(35)/dP50 value. Here, the number in parentheses following “P50” expresses the P50 value at which its differential is taken. The difference in the effectiveness of the Bohr shift at the site of O2 release and O2 uptake was also pointed out in the human adult venous and arterial blood in our previous paper (Itoh et al., 2001). The effectiveness of the Bohr shift in adult venous blood (–dS(40)/dP50 (27)) was about seven times more efficient than that of the arterial blood with physiological P50 of adult Hb (–dS(100)/dP50 (27)), implying that the effectiveness of the Bohr shift at the site of O2 loading is less important. In the lungs, the O2 uptake seems to be ensured by stable high alveolar O2 pressure. In contrast to adult blood, the effectiveness of the Bohr shift of fetal blood at the site of O2 loading is as important as that at the site of O2 release, because the PO2 and PCO2 environment of the fetal blood are extremely different from that of alveolar gas.
Together with the previous conclusion that the position of the OEC for fetal Hb is optimized for O2 delivery (Sold, 1982; Willford et al., 1982; Kobayashi et al., 1996), the present result indicates that the position of the fetal Hb seems to be well adapted to maintaining the effectiveness of the Bohr shift at high levels at both O2 loading and release sites. The higher effectiveness of the Bohr shift at the site of O2 release relative to that at the O2 uptake may be adequate for preventing the accumulation of proton and CO2 in the fetal tissues. Further, as will be described later, the equal or higher effectiveness of the Bohr shift in fetal blood than in maternal blood at the placenta (Fig. 4) will be also adequate for O2 and CO2 exchange across the placental membrane. Because the Bohr coefficient of fetal Hb has been reported to be almost equal value for adult Hb (Bohr coefficient=−0.51 for fetal Hb and −0.48 for adult Hb, Mann and Romney, 1968; Severinghaus, 1966), the equal or higher effectiveness of the Bohr shift may be permitted to receive the maximum benefit from the double Bohr effect.
Influence of cooperativity on the effectiveness of the Bohr shift in fetal blood at the PO2 of umbilical venous and arterial blood
The influence of cooperativity on the effectiveness of the Bohr shift at PO2 values of 15 and 35 torr was theoretically derived from a Hill's equation using the physiological P50 (20 torr) that covered a wide range of n values from 1 to 4 and from experimentally obtained OECs of human adult Hb solution (Fig. 3). Dashed lines represent the effectiveness of the Bohr shift calculated from the Hill equation. The effectiveness of the Bohr shift of “venous blood” –dS(15)/dP50 (20), increased with an increase in n value and reached its highest value at n=5.0. Further increases in n value reduced the effectiveness of the Bohr shift. Similar trends are observed in fetal umbilical “arterial blood” (–dS(35)/dP50 (20)). It is interesting that the highest effectiveness of the Bohr shift was observed at a relatively low n value (2.9). The crosses and closed circles represent the effectiveness of the Bohr shift calculated from the experimentally obtained OECs of adult Hb solutions and the human adult standard OEC. The effectiveness of the Bohr shift calculated from the OECs of human adult Hb solution are slightly lower than those calculated using the Hill equation.
Fig. 3
Influence of cooperativity (n) on the effectiveness of the Bohr shift in fetal umbilical blood. The effectiveness of the Bohr shift in the umbilical “venous blood”, –dS(15)/dP50 (20), and that in the umbilical “arterial blood”, –dS(35)/dP50 (20), were plotted against n. The quantity, –dS(PO2)/dP50, represented by dashed lines were calculated using a Hill's equation. Crosses represent the –dS(PO2)/dP50 (20) values obtained from the OECs of human adult Hb solution taken from Imai (1982), Imai and Yonetani (1975), Imaizumi et al. (1982) and Tyuma et al. (1973). Closed circles represent the–dS(PO2)/dP50 (20) values calculated from the human adult standard OEC.
Comparison of the influence of P50 on the effectiveness of the Bohr shift in fetal umbilical “arterial blood” with that in maternal uterine venous blood
The importance of the Bohr shift in gas exchange at the placental membrane was evaluated by comparing the effectiveness of the Bohr shift in fetal blood, –dS(35)/dP50, with that in maternal uterine venous blood, –dS(40)/dP50.
Fig. 4 shows theoretically derived effects of P50 on the effectiveness of the Bohr shift in maternal uterine venous blood, –dS(40)/dP50, at physiological n value (2.65). As already described, the P50 value of 30 torr, that gives the highest –dS(40)/dP50 value, is relatively close to the physiological P50 value of maternal blood (26 torr). It is also important to note that the effectiveness of the Bohr shift in fetal umbilical “arterial blood”, –dS(35)/dP50 (20), and that in maternal uterine venous blood, –dS(40)/dP50 (26), are nearly equal.
Fig. 4
P50-dependences of oxygen saturation of Hb and the effectiveness of the Bohr shift in fetal and maternal blood. Dashed lines express P50-dependences of oxygen saturation of maternal uterine venous blood at PO2 of 40 torr and fetal “arterial blood” in the umbilical vein at PO2 of 35 torr. Solid lines express P50-dependences of–dS(35)/dP50 (20) (fetal blood) and –dS(40)/dP50 (26) (maternal blood). Open and closed arrows indicate the physiological P50 values for fetal and maternal blood, respectively.
Influence of cooperativity on the effectiveness of the Bohr shift in maternal blood
Fig. 5 illustrates the theoretically derived effect of cooperativity (n) on the effectiveness of the Bohr shift in maternal uterine venous blood, –dS(40)/dP50 (26), calculated in the range of n values from 1 to 4. The –dS(40)/dP50 (26) value increases with increases in n value, reaches its highest value at n of 4.0., and decreases on further increases in n. The effectiveness of the Bohr shift of maternal uterine venous blood, –dS(40)/dP50 (26) values, calculated from the OECs of human adult Hb were slightly lower than that obtained from the Hill equation. For comparison, the effectiveness of the Bohr shift in fetal umbilical “arterial blood”, –dS(35)/dP50 (20) is also drawn in the figure. At n value of 3.0, both the maternal and fetal values are nearly equal and relatively high.
Fig. 5
Influence of cooperativity (n) on the effectiveness of the Bohr shift in fetal and maternal blood. The effectiveness of the Bohr shift in fetal umbilical “arterial blood”, –dS(35)/dP50 (20), and that in maternal uterine venous blood, –dS(40)/dP50 (26) are plotted against n. The dashed lines represent the –dS(PO2)/dP50 values calculated from the Hill equation. The –dS(35)/dP50 (20) (—) and –dS(40)/dP50 (26) (|) values were calculated from the same OECs for human Hb solutions as those used in Fig. 3. The –dS(35)/dP50 (20) (▴) and –dS(40)/dP50 (26) (▾) values were calculated from the human adult standard OEC.
Correlation between the effectiveness of the Bohr shift in fetal umbilical “arterial blood” and that in maternal uterine venous blood
The correlation between the effectiveness of the Bohr shift in fetal umbilical “arterial blood”, –dS(35)/dP50 (20), and that in maternal uterine venous blood, –dS(40)/dP50 (26), is shown in Fig. 6. The bold dotted line is the result obtained from the Hill equation. The plus signs and closed circles represent the effectiveness of the Bohr shift calculated using the human Hb OECs. Both quantities increased with increases in n, reaching nearly equal highest values at a relatively low n value (about 3.0). Further increases in n value resulted in marked decreases in the –dS(35)/dP50 (20) value. It is interesting that high effectiveness of the Bohr shift is observed at a relatively low n value (from 2.5 to 3.5).
Fig. 6
Correlation between the effectiveness of the Bohr shift in fetal and maternal blood at various n values. The effectiveness of the Bohr shift in fetal umbilical “arterial blood” (–dS(35)/dP50 (20)) was plotted against that in maternal uterine venous blood (–dS(40)/dP50(26)). Bold dotted line was constructed from the two lines in Fig. 5, and numbers attached to this line represent the value of n. Symbols (+) indicate the –dS(PO2)/dP50 values obtained from OEC data sets of human adult Hb solutions. Symbols (▾ and ▴) are the same as those in Fig. 5. Straight thin dotted line represents the relation: –dS(35)/dP50 (20)=–dS(40)/dP50 (26).
The effect of cooperativity on the O2 transport efficiency of fetal blood
Fig. 7 shows the amount of O2 transported by fetal blood (ΔS(35A–15B)) calculated at various n values. This amount gradually increased with increases in n, indicating that there is no optimal cooperativity for O2 transport efficiency.
Fig. 7
Influence of cooperativity on the efficiency of O2 transport by fetal blood at physiological P50. Dotted line represents ΔS(35A–15B)(20) value calculated from the Hill equation. The closed circle represents the ΔS(35A–15B)(20) value calculated from the human adult standard OEC.
The amino acid substitution of Ser-143 of fetus Hb γ chain for His-143 of adult Hb β chain causes a reduction of the acid Bohr effect for fetal Hb (Perutz et al., 1980). However, this functional difference occurs at a low pH range which is far from the physiological condition.
It is well known that the high oxygen affinity of fetal blood relative to that of adult blood is ascribed to the weakened interaction of fetal Hb with 2,3-diphosphoglycerate (DPG). The amino acid substitution described above causes a weakening of the interaction of DPG with fetal Hb since His-143 makes up part of the DPG binding site of adult Hb. Although fetal Hb has a lower oxygen affinity than adult Hb in the absence of DPG, the former shows a higher affinity than the latter in the red cell where DPG is present (Tyuma and Shimizu, 1970). The difference in allosteric properites between fetal Hb and adult Hb does not affect our present results of analysis since the effectiveness of the Bohr shift was measured by the factor, dS(PO2)/dP50, that was calculated from whole blood OECs. The role of DPG was to produce the difference in oxygen affinity between the fetal blood and the adult blood.
Conclusions
Although the principal role of fetal Hb is in the transport of O2 from the placenta to peripheral tissues, the Bohr shift is also important to enhance the O2 and CO2 exchange between fetal blood and maternal one across the placental membrane. From the results obtained in the present study, it is concluded that the positions of the OECs of fetal and maternal Hbs are mutually adjusted to receive a maximal benefit from the double Bohr effect, and that the relatively low cooperativity of tetrameric human Hb (n=2.65) is adequate for nearly maximizing the effectiveness of the Bohr shift in O2 and CO2 exchange processes across the placental membrane.
