4 • ERYTHROCYTES 265 



b. Molecular Constituents of the Erythrocytes 



Erythrocytes consist approximately of two-thirds water and one- 

 third dry residue, which is mainly composed of the red blood pig- 

 ment, haemoglobin, and salts. It is interesting to note that potassium 

 predominates over sodium as cation of the salts. Small amounts of 

 protein foreign to haemoglobin and of lipids constitute the ery- 

 throcyte membrane. 



Haemoglobin. The red blood pigment is a chromoprotein, Hke 

 chloroplastin in green leaves; yet the Hnk between chromogen and 

 protein is closer than in chlorophyll and the blood pigment therefore 

 emerges as protein from the stroma in haemolysis. 



Haemochromogen is a labile porphyrin compound which, outside 

 the organism, is transformed into the more stable haematin. The 

 composition of this compound is C34H3204N4FeCl and it is closely 

 akin to chlorophyll (Gr.\nick, 1948). The main differences are that 

 in the centre of the porphin ring there is, instead of magnesium, tri- 

 valent iron, the third valency of which imparts a saHne nature to the 

 compound usually neutralized by the anion chlorine; and the absence 

 both of the phytol chain and the iso-cyclic ring of the C atoms 6-9-10 

 (see Fig. 127, p. 249). As a result of the missing phytol chain the 

 haematin appears to be morphologically more compressed and less 

 markedly polar than chlorophyll. The protein carrier, to which the 

 haemochromogen is attached is called "globin". 



The haemoglobin molecule is of a thickset rod-like shape with 

 57 A diameter and 34 A height (Perutz, 1948). On the basis of the 

 iron content its molecular weight is computed at 16,000 to 17,000 

 (K.A.RRER, 1941), while the reading in the uhracentrifuge is 69,000, i.e., 

 about four times the value (Svedberg's law of multiples, see p. 141). 

 Stromatin. Jorpes (1932) states that approximately 4% of the total 

 protein content of the erythrocytes consists of a protein foreign to 

 haemoglobin, which is contained in the erythrocyte sheath and is 

 therefore described as stromatin. According to Winkler and Bungen- 

 BERG DE Jong (1941), its I.E.P. is at p^ 5.2. Analysis of the hae- 

 molyzed membrane of erythrocytes shows that there is 80 °o of 

 stromatin and 20 °o of lipids. 



Phospholipids. The bulk of the hpids consist of phosphatides, notably 

 lecithin (Fig. 93, p. 138), besides which there are insignificant 

 amounts of cephaiin and sphingomyelin. The I.E.P. of the phospho- 



