MINERAL METABOLISM 393 



Siiui- |)l.mis (oniaiii iniuli less Na+ than K+ and ihc ic([uirements 

 ol animals arc approximately ihc same lor the two ions, vegetarians 

 suffer sodium deficiencies unless provided with supplementary salt. 

 Growth re(|uirements lor rats, chicks, pigs, and calves amount to 

 about 0.1 to 0.2 per cent of the ration with the exact amount depend- 

 ing on the species and other factors in the diet. Recommended al- 

 lowances and normal intakes both provide a safety factor with enough 

 salt to make 0.5 per cent of dried ration. This amounts to about 

 0.2 per cent Na+. 



Excessive quantities are excreted by the kidneys as the chloride. 

 Chicks seem to be rather sensitive, showing edema on diets contain- 

 ing 3 per cent salt. Other animals seem to tolerate much more. 

 Na+, C1-, and to some extent I" have been added to human foods 

 as condiments and as preservatives. Such general use makes defi- 

 ciencies rare except in certain diseases and as a temporary result of 

 prolonged and profuse sweating. , 



Sulfate 



Although not normally discussed as an aspect of mineral metab- 

 olism, sulfur is required by all living cells. Much of this element oc- 

 curs in the cellular amino acids, peptides, and proteins and is more 

 properly covered under protein metabolism (Chapter 20). The sulfur 

 in these compounds is of the organic sulfide or mercaptan type with 

 a few derived compounds like taurine and a few heterocyclic sub- 

 stances like thiamine. Another group of biological materials not 

 closely related to any of these is based on sulfate. 



Many microorganisms assimilate sulfate ion and reduce it to inter- 

 mediates converted into all the sulfur compounds needed by the 

 organism. The higher plants operate the same way and are believed 

 to take more sulfate from the soil than they do phosphate. Higher 

 animals contain sulfate ion and excrete sulfur in this form, but early 

 workers did not know that inorganic sulfate was important as such 

 to mammals and birds. 



However, embryonic heart, bone, muscle, and artery tissues take 

 up S04= and use it in tissue components. Of these components a 

 major one is chondroitin sulfate (page 70), which serves with pro- 

 teins as the cement substance of bone and cartilage. Since this mate- 

 rial (also called chondroitin sulfuric acid although in the animal it 

 would be present in salt form) is abundant in animals, the sulfate re- 

 quirement during growth must be rather large. Perhaps much of the 

 need is met by sulfate formed metabolically by oxidation of sulfide or 



