202 PRACTICAL PHYSIOLOGY 



It has further been found that animal nucleic acid differs in certain details 

 from plant nucleic acid. 



Animal Nucleic Acid. Plant Nucleic Acid. 



Phosphoric acid Phosphoric acid 



Carbohydrate = a hexose Carbohydrate = a pentose 



2 Purine bases -! ^ an ! ne 2 Purine bases ( Guanine 



2 Pyrimidine bases 2 Pynmidine bases | 



It would seem also to be conclusively proven that there is a combination 

 between the phosphoric acid and the carbohydrate and between the carbo- 

 hydrate and a purine base. Such compounds of carbohydrate, phosphoric 

 acid and purine base are known as mononucleotides, and as there are four 

 such groupings in nucleic acid it is regarded as a tetranucleotide. 



Nucleo-proteins and nucleins are insoluble in water, also in dilute acid 

 solutions, but are soluble in dilute alkalis and strong acids. They can be 

 precipitated from their alkaline solutions by acidification with acetic acid. 

 Phosphoric acid is not liberated on hydrolysis with alkali (differentiation from 

 phospho-proteins, q.v.), but it is yielded on hydrolysis with acid. 



In order to prepare nucleo-protein the following method may be 

 used : 



A cellular organ, such as the thymus or pancreas, is minced and 

 macerated overnight with water made faintly alkaline with caustic 

 soda or ammonia. The extract is then strained through muslin, 

 litmus added,- and then weak acetic acid. When the reaction 

 becomes faintly acid, a copious precipitate of nucleo-protein occurs. 

 The nucleo-protein is filtered off and dissolved in weak alkali (1 per 

 cent, sodium carbonate). (See also p. 325.) 



EXPERIMENT. Some of this alkaline solution is supplied : 



(1) Add acetic acid white precipitate soluble with difficulty in 

 excess. (Cf. mucin, which is insoluble, and caseinogen, which is 

 readily soluble.) 



(2) Perform the protein colour tests. 



(3) Ascertain how it is " salted out." 



Derivatives of Proteins. When proteins are hydrolysed either by 

 means of inorganic reagents like hydrochloric acid or by proteolytic 

 ferments a series of changes take place with the formation of 

 increasingly simple substances before the ultimate amino acid or 

 peptide form is reached. The order from complex to simple is 

 meta-protein, proteose, peptone, polypeptides, amino acids. 



(a) Meta-proteins. When a protein is treated either with acid 

 or alkali at about 60 C. it goes into solution. This solution does 

 not coagulate on boiling, but there is the formation of a precipitate 

 when the solution is carefully neutralised. This precipitate is 

 again soluble in excess either of acid or alkali. If the precipitate 

 on neutralisation is filtered off and suspended in water coagulation 

 will occur on boiling. The coagulum is no longer soluble, for example, 

 in dilute acid. Acid meta-proteins differ from alkali meta-proteins 

 in that they are precipitated from solution on saturation with 

 sodium chloride. Both meta-proteins are precipitated on full 

 saturation with magnesium sulphate and half saturation with 



