Xenon-metmyoglobin equilibrium

1. INTRODUCTION Xenon and the other inert gases were known to I\ave physiological aatktty long before Bartlett unequtvocably established chemic;ll activity for xenon. In fact, the biology of Inert gases was reviewed by Rinfret and Doebbler (1961), a year before any chemistry (other than c1athrate.s) was known. Probeb!y the most striking phyr io1ogice.l effect of xenon Is narcoets. Nlen a,ld Cross (1951), for exampl$, performed surgical operations on human betis using xenon es en anesthelx. Not so striking, but more easUy studied, is the teteraction between xenon and tbr respnatory pi@nents. Here the Int&x?action IS with a we!! chwactoriaed .nolecu,e rhtch sugTests a more understandable svstem. The literature evidence co~&%ning the interectio6 between tnert gases and resptratory pipmeet6 shows some important tnconets+tctee. WhUe there se?ms to be agreement concerning the reversibiiity of the ioteraction. there 1s qwstion conccmin~ whether the re&ion is a ncdubtttty or a stnlchiometric interaCtion. Mortimer and Bauer (1’%30,, Cone (1980, snd (in part] Schwaborn 0985) preeent evidence for a Henry’s law sdubllity of rare gases In resptratory ptgments. That is, when the quadtty of gas dissolved Is plotted against the partid pressure of the dissolvin$ !~a@, a linear plot gotolngthrouk:, the origin is obtahed,

(1962)

it1

P= kx. In eq. (I: > 1s the partial

pressure of the gas, k and X Is the mole. Sract&n ‘of the gas dissolved in tbo liquid phase. In awarent opposttton to the Henry’e law evtdence ate r&rtc analyiis. speelfic site of attachment for xenon on sperm whple metmyoglobin was reported by Schoenborn

is a ~ro~ortionalih constant two

o&X-ray

f0P dnenlinnttonr

at law.

et al. (19Sd), while faxr specific si!es of attachment were reported for horse hcmoglobm by Sehoenborn (1965). This stolchiomctric Were+ tion should give a non-lfnear prresure vereos sdubilltv nlot rather than fdlowim Hemv’s law. should be noted, hwvever, that the &bUlty studies were carried at at relatively low preeSUUP.SS(generally I at.7 DC less) and the ;rpparent obPcrvance 01 Henry’r. law does not rule out the posslblltty of campand formation so long as tbe equllihrium favors deeomposttion of the compM (i.e., the nrajority of the sites unoccuplsd). Since the X-ray work seems to locate spcetflc sita of attachment etVt%? Iormulas of MbXa f&lb t metmywlobl.) &I HbXed (fib = meth&&gl&O), a x&terminattoa of the sdubility of xenon in the

It

A

2. METHODS The experiment6

were

carried

cut utiltztd~

P

149 manXo,d

co”elleted

stnillL!ess steo, (1 themanifold wtle:

of

InIn

diameter). Attached to 1) a xenon tank, 21 a vacuum pwnp, 9) a bowdon gacge (“Duragauge” measuring up la 2 atm), and 4) a 20 ml stainless steei reaction vessel. The gauge lva8 cali’.rated with a mercury mxmxneter. The ne~essarv manifold volumes -were ctiibrated by filling lh6 reaction vessel with water and weigh@ and then determinira the volume of the rsmamder of the manifold with nitrogen, utlWing the cas laws. The chemicals ksed were: 1) xwon, Baker reagent grade containing less than j ppm oxjgen and, 2) horse heart melmyoglobin, 2 x cry&. obtained from Cal Bio Chem. Solutions were prepared by dissolving a weighed amamt oi metmy slobin il. a known quanttty oi dHiUed water. The solutions were about 10% metmy@obin. The equilibration *esse1 was attached to the mani!otd and place? in d constant temperature bath at WC. Xenon was then intr&,ced int,, the manifold aFd after thermal eqvitibration the pi’es&~e was measured Since the volume of the manifold was known, the quUant!ty of xenon cwlti be cakul:xted Th? man,fold was then opened to tbe equi Ib?at:on vebsa!. After 24 RP, the new equilibrium pressure was measured tmd the amount of X.WJR m the aquxxhphase ‘“ae cakulied. The xenon acbxdly assoc6ted ;vilh my~iobin was &?ermi,,ed by subtracting the-xenon dissclved in water irbm the total In tb.at abase. The quniityof xenon that brterarb, with horse metmyoglohin at WC was determined al various xeltOn,Weasurcs betmeen Yaeuum and 2 atm. ‘The meam~resmente were conducted a, OOC because we were limited to pressures below 2 atm. Since the degree of interactian increases wtth decreasing +empsrature, more InteractIon for a given pressure can be Blatned by lowerln~ the temperature. Becurse of ireeziw of the sclvent and the formation 01 the Xenon hydrate, the lowest practical temperature was ow.

3. RESULTS

AND DBCUSSION

At pnseuree above 0.5 atm s$nUicant devtations from Henry’s law were observed. Tbefie result@ are shown In I&?. 1. As dlscussed above, pr~lcus work has shown only D linear plot for data each an shorn in f&f. I. This, we feel, is because ~revtms workerr, did not obtain results &We th. IttRtlpl It”&. portionof the cWW. A plot ~blisbed by Schoenborn (1905) I”dlcates that at 370 the 1blea.r portinn Of a plot eucb aa In it& 1 fur horse hemoglobin watld ex-

eSddnttalty

,.. 3-.[Mb=1 PWxe

If this expreaslon cor,vxtty descrd,es the Lnteraction, then a pl”t of the ratio [MbXe,,[Mb] versue the pressure of xenon should give a straight line @in& through the ori& with its slope equal to K. Fig. 2 Illustrates such a tkat. As can be seen, the~epolnts approxtmate a siralghi line. The least squares line drawn through these points goes essentially through the origin. From the dope of thts line, R. l.07 aim-f. The results of cur study tndlcate that, in ccntrast to prevloue edubllity studies and Ln agreement with X-ray shldlos, xenon and horse metmycglohln lnteraet in a 1 : 1 mole ratio.

REFERENCES

1w

9. MAESTAS

(ZuUcn. S.C.. irnd E.G.Groas, 1951, Theanestbetle prwerllr?s of xenon in animnle and human befnge, ~4th addItIonal observations on krypton. Science 113. 580. Mortlmer. R.G. and N.Bauer. 1960, the afflnlty of leg%lobfn and nlher heme protein8 far gaseous nltrwen, hydrogen and argon, J. Phys. Chem. 84, 387.

and G. J. EWING funfret. A. P, and G. F.Debbler. 1861, Phyaiolo%cal and bfhchemieal effects and applications. fn: G.A. Cwk. Argon, hrllum and the ram gases. Vol. II !fntwscience Publishers, New York) pp. 737-764. Sohoeenborn. B.P., 196S, Binding of xenon to horse haemoglohin. rwure 208. 760. Schoenbont. B.P.. KC. Wataw and J.C.Kendrew. 1965. Rltilng of xenon to wefm whale myr&bln. Nature 207,

28.