300 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



dissociation curves obtained under similar conditions of pH and tempera- 

 ture. These conditions have not yet been satisfactorily fulfilled, since a 

 change in the ^^Oa alters the pH of the blood and the pH of a solution of 

 haemoglobin in different ways, and the dissociation curve in each case is 

 displaced in a particular manner depending on the pH. The alteration in 

 affinity with temperature also differs, for solutions of haemoglobin, accord- 

 ing to the conditions and the nature of the haemoglobin. Whilst awaiting 

 more precise results it is still possible to arrive at some interesting con- 

 clusions using the results published in the literature. 



Of the known haemoglobins the one with the greatest affinity for oxygen 

 is certainly that in the perienteric fluid of Ascaris lumbricoides. Ascaris 

 possesses two haemoglobins, one in the body wall and the other in the 

 perienteric fluid. These two haemoglobins differ from each other in their 

 absorption spectra, which are both different from that of pig haemoglobin. 

 The haemoglobins of Ascaris have a great affinity for oxygen : sodium 

 hydrosulphite only reduces them very slowly and exposure to vacuum at 

 20° does not reduce them completely. 



The haemoglobin of Gastrophilus larva has a lower affinity for oxygen. 

 At 39° the ^50 of a concentrated solution containing 1 x 10~^g atoms of 

 iron per litre has a value of 4*9mm; whilst the ^50 of a dilute solution 

 containing 0"84 X 10~^g atoms of iron per litre is 0"02mm Hg. The further 

 complication we encounter here, namely the change in affinity with the 

 degree of dilution, also occurs with the different mammalian haemoglobins. 



Fox has compared the various values of /)5o, at 10° and 17°, and in the 

 absence of COg, for the undiluted bloods of Chironomus larva, of Arenicola 

 and Planorbis, and for the slightly diluted bloods of Tubifex, Daphnia and 

 Ceriodaphnia. He obtained the results collected in Table XVII. 



In this list, the haemoglobins of the Chironomus and of Tubifex have the 

 highest affinity and that of Daphnia the lowest. We may compare the values 

 in the second column with the p^o value of 27mm for human blood with a 



