470 



ANIMAL BIOCHEMISTRY 



jjoitaiit components ol the animal itscll both as the blood pigment 

 hemoglobin and as respiratory jjroteins like catalase. Since red cells, 

 tor example, have a relatively limited life, some provision must be 

 made ior disposing of the hemoglobin liberated. Thus animals have 

 both external and internal supplies of such materials to excrete or to 

 make use of according to their needs and capabilities. 



Most animals anci birds appear to synthesize all the porphyrin they 

 need for hemoglobin from glycine and succinyl coenzyme A. They 

 do not utilize porphyrins from their diets to a significant extent. 

 Isotope studies indicate that all the atoms of protoporphyrin come 

 from glycine and succinate, revealing the very close connection be- 

 tween reactions already discussed and porphyrin synthesis. 



HgNCHgCOO" + "OOCCH2CH2CO— CoA ^~OOCCH2CH2COCHCOO + CoA 



NH3+ 



a-ainino-/3-ketoadipate 



glycine 



succinyl coenzyme A 



-OOCCH2 

 +H3NCH2 



CHaCHzCOO^ 



CH. 



-co.> 



"OOCCH2CH2COCH2NH3"*' 



5 -aminolevulinic acid 



hemoglobin 



CH=CH2 



protoporphyrin 



After an average life of aboiu 125 days, erythrocytes are destroyed 

 in human beings by cells in the spleen, liver, and bone marrow. The 

 overall process leads to return of the globin to the amino acid pool 

 and to excretion of degradation products of the porphyrin. Iron is 

 carried by the plasma to the iron-storage system for later use. The 

 porphyrin is converted to bile pigments which accompany the bile 

 into the intestine and are excreted in the feces. Small quantities of 

 bile pigments appear in the urine, amounting to 1 to 2 mg. daily 



