The kinetics of hœmoglobin IV- General methods and theoretical basis for the reactions with carbon monoxide

Abstract

IV. The oxygen-hemoglobin reaction is reviewed from the physi-cochemical standpoint with a view to the theoretical consideration of the CO reactions. Under certain conditions the velocity equation for the combination of CO with (reduced) Hb is identical on the Hufner theory and on the intermediate compound hypothesis of Adair, but is quite different on the basis of Hill''s equation. The displacements of O2 with CO, and vice versa, from combination with Hb follow simple equations as regards their equilibrium point and their velocities. This is difficult to reconcile with the Hill equation; but it can be brought into line with the Adair hypothesis.[long dash]V. Under similar conditions of pH, temp. and concn. of reagents, CO combines with Hb about 10 times more slowly than O2 does. The greater affinity of CO for Hb lies in the fact that COHb dissociates far more slowly than does O2Hb. 3 quantitative tests suggest that the kinetics of the reaction conform to Adair''s hypothesis rather than to Hill''s equation. During the early stages of the reaction, the rate is given roughly by the equation d[COHb]/dt = l[image][CO] [Hb]. The value of l[image] is only slightly affected by pH, and not appreciably by light; its temp. coefficient (Q10) is about 1.8. A new and more accurate method of measuring the rate of combination of CO with Hb is outlined.[long dash]VI. During the first half of the reaction where the reverse reaction rate is negligible, the velocity of O2 displacement from O2Hb by CO is expressible by d[COBb]/dt = m[image][00][0,Hb]/[02] for a wide range of concns. of each reagent. Several independent methods gave concordant results for this reaction. The exptl results accord with the hypothesis of Adair rather than with Hill''s equation. The velocity of CO displacement from COHb by means of O2 is given by -d[COBb]/dt w[C0Hb][long dash]m[image][CO][O2Hb]/[O2] where m is calculated from expts. at 2 values of [O2]. Within experimental error, m''/m = M the equilibrium constant of the reaction CO + O2Hb?=* O2 + COHb. The relatively slight effect of pH on m and to[image] was unexpected. Its theoretical implications are discussed.[long dash]VII. The rate of combination of CO with Hb freshly reduced from O2Hb was measured by mixing the latter with Na2S2O4 solution and then 0.1-0.2 sec. later mixing the 1st mixture with a 3d solution containing CO. At pH 10 and room temp. the rate of reaction with "new" Hb was about twice as fast as with Hb more than 2 sec. old. The effect disappeared at 33[degree] or at pH 6.6; it was apparently not caused by the Na2S2O4. "Quick" equilibrium between O2 and Hb was studied by driving a dilute solution of O2Hb into a mixing chamber where it met an equal volume of O2-free water. The emerging fluid was examined after varying intervals of 0.5 sec. and upwards with the reversion spectroscope. At pH 10 and about 6.3 the % of O2Hb dropped from about 90 to a value between 50 and 60. At pH 10 there was no further change on standing; but at pH 6.3 there was a further drop to 20% O2Hb. This was not due to inactivation of the Hb. The effect of acid on the affinity of 02 for Hb may be due in part to some slow secondaiy process.