562 LECTURE XXIV. 



It shows one absorption band in the green, and another in the orange-red, 

 between the C and D lines. If hydrogen sulphide acts upon hemoglobin 

 in the presence of oxygen, the hemoglobin is completely decomposed, so 

 that the product formed, as well as its derivatives, no longer shows a char- 

 acteristic absorption spectrum. 



Quite different from the above compounds is carbohemoglobin, in which 

 carbon dioxide is present, but combined at a different place in the molecule 

 from that occupied by the oxygen in oxyhemoglobin. In fact, carbon 

 dioxide and oxygen are taken up by hemoglobin quite independently of 

 one another. The carbonic acid is evidently combined with globin, while 

 oxygen combines with 'hemochromogen. 



Methemoglobin 1 also occupies a unique position. It corresponds to 

 oxyhemoglobin in its composition, and differs from it merely in holding 

 the oxygen in a firmer state of combination. It may be formed from the 

 latter on standing, or be prepared by the action of various agents, such 

 as iodine, chlorates, permanganates, nitrites, nitrates, palladium hydride, 

 pyrogallol, quinol, or ozone. The formation of methemoglobin has also 

 been observed by the action of aniline, toluidine, acetanilide, acetopheneti- 

 dine, and glycerol upon oxyhemoglobin. Methemoglobin may be formed 

 even in the circulating blood, when it comes in contact with substances 

 such as amyl nitrite, nitrobenzene and antifebrin. 



The oxygen cannot be removed from methemoglobin by reducing the 

 partial pressure of this gas. At present it is not known just how this 

 transformation of oxyhemoglobin into methemoglobin is effected. It 

 has been established positively that both compounds contain the same 

 amounts of oxygen. Reducing agents tend to convert methemoglobin 

 back into oxyhemoglobin. Methemoglobin crystallizes in brownish-red 

 needles, prisms, and also in six-sided plates. It is most readily prepared 

 by adding potassium ferricyanide solution to a solution of oxyhemoglobin 

 and then, after cooling to C., adding one-fourth its volume of alcohol. 

 In acid solutions methemoglobin shows an absorption spectrum of a band 

 in the orange-red between the C and D lines, and a second paler band in 

 the blue between the G and F lines. Besides these absorption bands, 

 the acid solutions show two other bands in the same place as the bands 

 which characterize the spectrum of oxyhemoglobin. It seems probable 

 that these last two bands are not characteristic of methemoglobin, but 

 are due to the presence of some oxyhemoglobin as impurity. In alkaline 

 solutions methemoglobin shows three lines, one on either side of the D line, 

 and one near the E line. The spectrum of methemoglobins is, in fact, 

 very similar to that of hematin. 



1 Hoppe-Seyler: Zentr. med. Wissensch. 1863, No. 28. G. Hiifner: Z. physiol. 

 Chem. 8, 366 (1884). Hiifner and Otto: ibid. 7, 65 (1882-83). A. Jaderholm: Z. 

 Biol. 16, 1 (1880); 20, 419 (1884). R. von Zeyneck: Arch. Anat. Physiol. 1899, 460. 



